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Electric-Field-Induced Lock-and-Key Interactions between Colloidal Spheres and Bowls
Abstract
To realize new and directed self-assembly (SA) pathways, the focus in colloid science and nanoscience has shifted from spherical particles and interactions to increasingly more complex shapes and interparticle potentials. This field is fueled by recent breakthroughs in particle synthesis, such as particles with complementary shapes that allow for specific lock-and-key interactions induced by depletants. Here, we show that electric fields form an alternative route for directing the SA of convex and concave colloids, with the additional advantage that the system now becomes switchable by external conditions. Both experimental and theoretical results are presented that validate the electric-field-induced assembly mechanism and show that even irreversibly bound composites can be generated by tuning the force balance. The successful isolation of the irreversible composite particles, in combination with generalization to different materials, shows that the current mechanism provides a versatile new path not only toward complex-particle synthesis but also toward directed self-assembly.
... The dimpled patchy particles could for example be used in depletion-guide assembly procedures to form higher order colloidal structures. [36][37][38] Another interesting class of colloids that can be assembled into well-designed structures via depletion forces are those consisting of both rough and smooth domains. 39 Because smooth surfaces have larger overlap volumes than rough surfaces, at appropriate depletant concentrations only the smooth lobes become attractive, generating directional interactions between the partially rough particles. ...
Hypothesis
Self-assembly using anisotropic colloidal building blocks may lead to superstructures similar to those found in molecular systems yet can have unique optical, electronic, and structural properties. To widen the spectrum of achievable superstructures and related properties, significant effort was devoted to the synthesis of new types of colloidal particles. Despite these efforts, the preparation of anisotropic colloids carrying chemically orthogonal anchor groups on distinct surface patches remains an elusive challenge.
Experiments
We report a simple yet effective method for synthesizing patchy particles via seed-mediated heterogeneous nucleation. Key to this procedure is the use of 3-(trimethoxysilyl)propyl methacrylate (TPM) or 3-(trimethoxysilyl)propyl acrylate (TMSPA), which can form patches on a variety of functional polymer seeds via a nucleation and growth mechanism.
Findings
A family of anisotropic colloids with tunable numbers of patches and patch arrangements were prepared. By continuously feeding TPM or TMSPA the geometry of the colloids could be adjusted accurately. Furthermore, the patches could be reshaped by selectively polymerizing and/or solvating the individual colloidal compartments. Relying on the chemically distinct properties of the TPM/TMSPA and seed-derived domains, both types of patches could be functionalized independently. Combining detailed control over the patch chemistry and geometry opens new avenues for colloidal self-assembly.
... A third route to increase the number or patches could be the use of more deformable particles, such as silica shells. 105,106 Centrifugation techniques such as in refs 107−109 may be applied in future research to separate particles of different patch numbers. In a glassy system, the volume fraction and number of contacts among colloids may even be slightly higher than in a colloidal crystal, 110 so colloidal glasses could be used to increase the patch number and/or to obtain patchy particles with more than 12 patches. ...
We report methods to synthesize sub-micron and micron-sized patchy silica particles with fluorescently labeled hemispherical titania protrusions, as well as routes to efficiently characterize these particles and self-assemble these particles into non-close-packed structures. The synthesis methods expand upon earlier work in literature, in which silica particles packed in a colloidal crystal were surface-patterned with a silane coupling agent. Here, hemispherical amorphous titania protrusions were successfully labeled with fluorescent dyes, allowing for imaging by confocal microscopy and super-resolution techniques. Confocal microscopy was exploited to experimentally determine the numbers of protrusions per particle over large numbers of particles for good statistical significance, and these distributions were compared to simulations predicting the number of patches as a function of core particle polydispersity and maximum separation between the particle surfaces. We self-assembled these patchy particles into open percolating gel networks by exploiting solvophobic attractions between the protrusions.
... T he shape of colloidal particles plays a critical role in biomedicine (shape influence on cellular internalization and vascular dynamics for drug delivery), 1 phagocytosis, 2 catalysis (facet-dependent electrocatalytic activity and durability), 3 emulsion stabilization (e.g., Pickering−Ramsden emulsions) 4 and, importantly, in the assembly of colloidal structures for new functional materials. 5−10 Particles with anisotropic shapes 11−19 have been used to create complex colloidal structures, including lock-and-key assemblies, 20,21 staggered linear chains, 22 and cubic crystals, 23 which are either difficult or impossible to access using spherical colloids. Controlled transformations of colloid particle morphologies represent a pathway to creating ordered structures and reconfigurable materials on demand. ...
A facile method has been discovered that enables switching of non-cross-linked poly(styrene) (Nx-PS) colloidal particle morphologies, including spherical and anisotropic convex–convex, plano–convex, and concave–convex. The anisotropic morphologies can be achieved readily from spherical Nx-PS particles through control of annealing time at elevated temperatures or surfactant concentration. These anisotropic morphologies can be reversed to spherical at lower temperatures, and reversible morphology switching between any pair of anisotropic morphologies also can be attained. This approach to particle morphology switching establishes a straightforward method for the synthesis of designer anisotropic particles using commercially available spherical particles as starting materials.
... As an example, in their pioneering work, Sacanna et al. (14) have adapted the concept of a lock-and-key mechanism originally proposed by Fisher (12,13) for colloidal mixtures of complementary shapes, driven by the use of an additional depletion interaction. Although most studies focus on the use of depletion interactions to direct this specific self-assembly (14)(15)(16)(17), alternative approaches have been considered (18)(19)(20)(21). ...
We have seen a considerable effort in colloid sciences to copy Nature’s successful strategies to fabricate complex functional structures through self-assembly. This includes attempts to design colloidal building blocks and their intermolecular interactions, such as creating the colloidal analogs of directional molecular interactions, molecular recognition, host-guest systems, and specific binding. We show that we can use oppositely charged thermoresponsive particles with complementary shapes, such as spherical and bowl-shaped particles, to implement an externally controllable lock-and-key self-assembly mechanism. The use of tunable electrostatic interactions combined with the temperature-dependent size and shape and van der Waals interactions of these building blocks provides an exquisite control over the selectivity and specificity of the interactions and self-assembly process. The dynamic nature of the mechanism allows for reversibly cycling through various structures that range from weakly structured dense liquids to well-defined molecule-shaped clusters with different configurations through variations in temperature and ionic strength. We link this complex and dynamic self-assembly behavior to the relevant molecular interactions, such as screened Coulomb and van der Waals forces and the geometrical complementarity of the two building blocks, and discuss our findings in the context of the concepts of adaptive chemistry recently introduced to molecular systems.
Colloid science has recently grown substantially owing to the innovative use of silane coupling agents (SCAs), especially 3-trimethoxysilylpropyl methacrylate (TPM). SCAs were previously used mainly as modifying agents, but their ability to form droplets and condense onto pre-existing structures has enabled their use as a versatile and powerful tool to create novel anisotropic colloids with increasing complexity. In this Review, we highlight the advances in complex colloid synthesis facilitated by the use of TPM and show how this has driven remarkable new applications. The focus is on TPM as the current state-of-the-art in colloid science, but we also discuss other silanes and their potential to make an impact. We outline the remarkable properties of TPM colloids and their synthesis strategies, and discuss areas of soft matter science that have benefited from TPM and other SCAs.
We explore the statistics of assembling soft-matter building blocks to investigate the uptake and encapsulation of cargo particles by carriers engulfing their load. While the such carrier-cargo complexes are important for many applications out of equilibrium, such as drug delivery and synthetic cell encapsulation, we uncover here the basic statistical physics in minimal hard-core-like models for particle uptake. Introducing an exactly solvable equilibrium model in one dimension, we demonstrate that the formation of carrier-cargo complexes can be largely tuned by both the cargo concentration and the carriers' interior size. These findings are intuitively explained by interpreting the internal free space (partition function) of the cargo inside a carrier as its engulfment strength, which can be mapped to an external control parameter (chemical potential) of an additional effective particle species. Assuming a hard carrier membrane, such a mapping can be exactly applied to account for multiple cargo uptake involving various carrier or cargo species and even attractive uptake mechanisms, while soft interactions require certain approximations. We further argue that the Boltzmann occupation law identified within our approach is broken when particle uptake is governed by nonequilibrium forces. Speculating on alternative occupation laws using effective parameters, we put forward a Bose-Einstein-like phase transition associated with polydisperse carrier properties.
Bonding simple building blocks to create crystalline materials with design has been sophisticated in the molecular world, but this is still very challenging for anisotropic nanoparticles or colloids, because the particle arrangements, including position and orientation, cannot be manipulated as expected. Here biconcave polystyrene (PS) discs to present a shape self‐recognition route are used, which can control both the position and orientation of particles during self‐assembly by directional colloidal forces. An unusual but very challenging two‐dimensional (2D) open superstructure—tetratic crystal (TC)—is achieved. The optical properties of the 2D TCs are studied by the finite difference time domain method, showing that the PS/Ag binary TC can be used to modulate the polarization state of the incident light, for example, converting the linearly polarized light into left‐handed or right‐handed circularly polarized light. This work paves an important way for self‐assembling many unprecedented crystalline materials.
Integrating nanoscale building blocks of low dimensionality (0D; i.e., spheres) into higher dimensional structures endows them and their corresponding materials with emergent properties non-existent or only weakly existent in the individual building blocks. Constructing 1D chains, 2D arrays and 3D superlattices using nanoparticles and colloids therefore continues to be one of the grand goals in colloid and nanomaterial science. Amongst these higher order structures, 1D colloidal chains are of particular interest, as they possess unique anisotropic properties. In recent years, the most relevant advances in 1D colloidal chain research have been made in novel synthetic methodologies and applications. In this review, we first address a comprehensive description of the research progress concerning various synthetic strategies developed to construct 1D colloidal chains. Following this, we highlight the amplified and emergent properties of the resulting materials, originating from the assembly of the individual building blocks and their collective behavior, and discuss relevant applications in advanced materials. In the discussion of synthetic strategies, properties, and applications, particular attention will be paid to overarching concepts, fresh trends, and potential areas of future research. We believe that this comprehensive review will be a driver to guide the interdisciplinary field of 1D colloidal chains, where nanomaterial synthesis, self-assembly, physical property studies, and material applications meet, to a higher level, and open up new research opportunities at the interface of classical disciplines.
We describe a general procedure for the large-scale fabrication of bowl-shaped colloidal particles using an emulsion templating technique. Following this method, single polymeric seed particles become located on individual oil droplet surfaces. The polymer phase is subsequently plasticized using an appropriate solvent. In this critical step, the compliant seed is deformed by surface tension, with the droplet serving as a templating surface. Solvent evaporation freezes the desired particle shape and the oil is subsequently removed by alcohol dissolution. The resulting uniformly-shaped colloidal particles were studied using scanning electron and optical microscopy. By adjusting the droplet size and the seed particle diameter, we demonstrate that the final particle shape can be controlled precisely, from shallow lenses to deep bowls. We also show that the colloid's uniformity and abundant quantity allowed the depletion-mediated assembly of flexible colloidal chains and clusters.
Due to the extremely tiny size, micro‐nanorobots have been promised in performing tasks at micro‐nanoscale, which is impossible for macroscale robots. However, the massive fabrication of micro‐nanorobots is of crucial challenge and has been attracting extensive attentions among the academics. Active colloids can be self‐assembled into ordered structures, and offer a promising solution to this largescale manufacture challenge. Here, we will summarize the development and mechanisms of different assembly technologies in the past two decades, and introduce the applications of micro‐nanorobots using self‐assembly strategies in biomedical and physical fields. For clarification, the self‐assembly mechanisms are presented in three categories: chemical, physical and biological. The manufacture of micro‐nanorobots through self‐assembly not only solves the problem of large‐scale manufacturing, but also increases the reconfigurability of structure and improves their adaptability to the environment, which promises further development as microsurgeons and the application of micro‐nanorobots in other fields, such as environmental restoration and micro‐manipulation.
Under external pressure compression, various kinds of artificial microcapsules can undergo buckling induced deformation and catastrophic rupturing failure, which need to be understood for their diverse practical applications. For this, many theories and numerical simulations have recently emerged, leading to some intriguing but often debatable predictions and scaling laws. However, experimental testing of these predictions is very limited, due to challenges in realizing prescribed buckling pathways and in situ monitoring of the buckling procedure. Herein, we reported the buckling behaviors of well-defined spherical polydopamine (PDA) capsules with tunable sizes and homogeneous nanoscale shells. A simple but controlled solvent evaporation was implemented inside a home-made optical chamber to induce PDA capsules to buckle by following a prescribed pathway toward targeted shapes that are only dictated by the inherent material properties of the capsules. In addition, the buckling speed was slowed down to the timescale of minutes, which can prevent buckling from being trapped at some metastable intermediate states as well as facilitate in situ optical monitoring of the whole buckling procedure in slow motion. In this way, several classic buckling behaviors were clearly observed, including spinodal-like sudden appearance of dimples above critical pressures, transition of the indentation rim from the axisymmetric to polygonal shape, evolving of multi-indented buckling into single indented buckling following Oswald ripening. These observations are qualitatively comparable with recent predictions from numerical results. Furthermore, some novel buckling phenomena are reported for the first time, which might stimulate further theories and numerical simulations.
Self-assembly of nanomaterials with desired material properties requires assembly control from nanometer to millimeter scales. Here, hierarchical C60 and C70 films are assembled from the nanoscale to macroscale by combining co-solvent precipitation with electric-field directed assembly. Fullerene molecular crystals are grown via seeded co-solvent precipitation with mixed organic solvents and antisolvent 2-propanol. Rods of varying aspect ratios (1.7, 2 and 3.7) are prepared as a function of injection volume, fullerene type and solution concentration. Electric fields are applied to colloidal fullerene rods confined to two dimensions. The applied field induces dipolar forces, dielectrophoretic forces, and electrohydrodynamic flows. Frequency-dependent phase transitions occur at the critical Maxwell–Wagner crossover frequency, where the effective polarizability of the particles in the medium is substantially reduced, resulting in changes in structure. Partial order phases form as a function of field strength, frequency, and confinement including tetratic, smectic, centered rectangular and string fluids.
Charged fluorescent bowl‐shaped colloids consisting of a polystyrene core surrounded by a poly(N‐isopropylmethacrylamide) shell are obtained by nanoengineering spherical composite microgels. The phase diagram of these soft bowl‐shaped colloids interacting through long‐range Yukawa‐type interactions is investigated using confocal laser scanning microscopy. The bowl‐shaped structure leads to marked differences in phase‐behavior compared to their spherical counterpart. With increasing number density, a transition from a fluid to a plastic crystal phase, with freely rotating particles, followed by a glass‐like state is observed. It is found that the anisotropic bowl shape frustrates crystallization and slows down crystallization kinetics and causes the glass‐like transition to shift to a significantly lower volume fraction than for the spheres. Quantitative analysis of the positional and orientational order demonstrates that the plastic crystal phase exhibits quasi‐long range translational order and orientational disorder, while in the disordered glass‐like phase the long‐range translational order vanishes and short‐range rotational order appears, dictated by the specific bowl shape. It is further shown that the different structural transitions are characterized by decoupling of the translational and orientational dynamics.
Hierarchical C60 colloidal films are assembled from nanoscale to macroscale. Fullerene molecular crystals are grown via seeded cosolvent precipitation with mixed solvent [tetrahydronaphthalene (THN)/trimethylpyridine (TMP)] and antisolvent 2-propanol. The fullerene solutions are aged under illumination, which due to the presence of TMP reduces the free monomer concentration through fullerene aggregation into nanoparticles. The nanoparticles seed the growth of monodisperse fullerene colloids on injection into the antisolvent. Diverse colloidal morphologies are prepared as a function of injection volume and fullerene solution concentration. The high fullerene solubility of THN enables C60 colloids to be prepared in quantities sufficient for assembly (5 × 10(8) ). Electric fields are applied to colloidal C60 platelets confined to two dimensions. The particles assemble under dipolar forces, dielectrophoretic forces, and electrohydrodynamic flows. Frequency-dependent phase transitions occur at the critical Maxwell-Wagner crossover frequency, where the effective polarizability of the particles in the medium is substantially reduced. Structures form as a function of field strength, frequency, and confinement including hexagonal, oblique, string fluid, coexistent hexagonal-rhombic, and tetratic.
A novel lyotropic liquid-crystal (LC) based assembly strategy is developed for the first time, to fabricate composite films of vanadium pentoxide (V2O5) nanobelts and graphene oxide (GO) sheets, with highly oriented layered structures. It is found that similar lamellar LC phases can be simply established by V2O5 nanobelts alone or by a mixture of V2O5 nanobelts and GO nanosheets in their aqueous dispersions. More importantly, the LC phases can be retained with any proportion of V2O5 nanobelts and GO, which allows facile optimization of the ratio of each component in the resulting films. Named VrGO, composite films manifest high electrical conductivity, good mechanical stability, and excellent flexibility, which allow them to be utilized as high performance electrodes in flexible energy storage devices. As demonstrated in this work, the VrGO films containing 67 wt% V2O5 exhibit excellent capacitance of 166 F g−1 at 10 A g−1; superior to those of the previously reported composites of V2O5 and nanocarbon. Moreover, the VrGO film in flexible lithium ion batteries delivers a high capacity of 215 mAh g−1 at 0.1 A g−1; comparable to the best V2O5 based cathode materials.
Fortschritte auf dem Gebiet der kolloidalen Selbstassemblierung hängen stark von der Entwicklung neuer Verfahren zur Herstellung anisotroper Partikel mit räumlich definierten Interaktionsmustern ab, um die Bildung definierter Überstrukturen zu ermöglichen. Hier beschreiben wir erstmalig die Verwendung von direktem 3D-Laserschreiben zur Herstellung einheitlicher Populationen von anisotropen, kegelförmigen Mikropartikeln, die zur Selbstassemblierung über Schlüssel/Schloss-Formerkennung in der Lage sind. Triebkraft dieser Assemblierung sind Depletions-Wechselwirkungen, welche die kolloidalen Partikel zu linearen, suprakolloidalen Polymeren wachsen lassen. Die resultierenden suprakolloidalen Fibrillen zeigen eine hierarchische Ordnung und bilden nematische, flüssigkristalline Domänen. Ein derartiges Verhalten konnte in Abwesenheit externer Felder bisher nicht beobachtet werden. Die Studie eröffnet neue Möglichkeiten, um mithilfe von direktem Laserschreiben maßgeschneiderte Kolloide herzustellen und ihre Selbstassemblierung zu verstehen.
Progress in colloid self-assembly crucially depends on finding preparation methods for anisotropic particles with recognition motifs to facilitate the formation of superstructures. Here, we demonstrate for the first time that direct 3D laser writing can be used to fabricate uniform populations of anisotropic cone-shaped particles that are suitable for self-assembly through shape recognition. The driving force for the self-assembly of the colloidal particles into linear supracolloidal polymers are depletion forces. The resulting supracolloidal fibrils undergo hierarchical ordering and form nematic liquid-crystalline domains. Such a behavior could so far not be observed in the absence of an electric field. The study opens possibilities for using direct laser writing to prepare designed colloids on demand, and to study their self-assembly.
This study explored the application of localized electric fields for reversible directed self-assembly of colloidal particles in 3 dimensions. Electric field microgradients, arising from the use of micro-patterned electrodes, were utilized to direct the localization and self-assembly of polarizable (charged) particles resulting from a combination of dielectrophoretic and multipolar forces. Deionized dispersions of spherical and ellipsoidal core-shell microgels were employed for investigating their assembly under an external alternating electric field. We demonstrated that the frequency of the field allowed for an exquisite control over the localization of the particles and their self-assembled structures near the electrodes. We extended this approach to concentrated binary dispersions consisting of polarizable and less polarizable composite microgels. Furthermore, we utilized the thermosensitivity of the microgels to adjust the effective volume fraction and the dynamics of the system, which provided the possibility to dynamically "solidify" the assembly of the field-responsive particles by a temperature quench from their initial fluid state into an arrested crystalline state. Reversible solidification enables us to re-write/reconstruct various 3 dimensional assemblies by varying the applied field frequency.
Using confocal microscopy and first passage time analysis, we measure and predict the rates of formation and breakage of polymer-depletion-induced bonds between lock-and-key colloidal particles and find that an indirect route to bond formation is accessed at a rate comparable to that of the direct formation of these bonds. In the indirect route, the pocket of the lock particle is accessed by nonspecific bonding of the key particle with the lock surface, followed by surface diffusion leading to specific binding in the pocket of the lock. The surprisingly high rate of indirect binding is facilitated by its high entropy relative to that of the pocket. Rate constants for forward and reverse transitions among free, nonspecific, and specific bonds are reported, compared to theoretical values, and used to determine the free energy difference between the nonspecific and specific binding states.
Organization of spherical particles into lattices is typically driven by packing considerations. Although the addition of directional binding can significantly broaden structural diversity, nanoscale implementation remains challenging. Here we investigate the assembly of clusters and lattices in which anisotropic polyhedral blocks coordinate isotropic spherical nanoparticles via shape-induced directional interactions facilitated by DNA recognition. We show that these polyhedral blocks-cubes and octahedrons-when mixed with spheres, promote the assembly of clusters with architecture determined by polyhedron symmetry. Moreover, three-dimensional binary superlattices are formed when DNA shells accommodate the shape disparity between nanoparticle interfaces. The crystallographic symmetry of assembled lattices is determined by the spatial symmetry of the block's facets, while structural order depends on DNA-tuned interactions and particle size ratio. The presented lattice assembly strategy, exploiting shape for defining the global structure and DNA-mediation locally, opens novel possibilities for by-design fabrication of binary lattices.
Current theoretical attempts to understand the reversible formation of stable microtubules and virus shells are generally based on shape-specific building blocks or monomers, where the local curvature of the resulting structure is explicitly built-in via the monomer geometry. Here we demonstrate that even simple ellipsoidal colloids can reversibly self-assemble into regular tubular structures when subjected to an alternating electric
field. Supported by model calculations, we discuss the combined effects of anisotropic shape and field-induced dipolar interactions on the reversible formation of self-assembled structures. Our observations show that the formation of tubular structures through self-assembly requires much less geometrical and interaction specificity than previously thought, and advance our current understanding of the minimal requirements for self-assembly into regular virus-like structures.
http://www.nature.com/ncomms/2014/141120/ncomms6516/full/ncomms6516.html
Particle shape is a critical parameter that plays an important role in self-assembly, for example, in designing targeted complex structures with desired properties. Over the last decades, an unprecedented range of monodisperse nanoparticle systems with control over the shape of the particles have become available. In contrast, the choice of micrometer-sized colloidal building blocks of particles with flat facets, that is, particles with polygonal shapes, is significantly more limited. This can be attributed to the fact that in contrast to nanoparticles, the larger colloids are significantly harder to synthesize as single crystals. It is now shown that a very simple building block, such as a micrometer-sized polymeric spherical colloidal particle, is already enough to fabricate particles with regularly placed flat facets, including completely polygonal shapes with sharp edges. As an illustration that the yields are high enough for further self-assembly studies, the formation of three-dimensional rotator phases of fluorescently labelled, micrometer-sized, and charged rhombic dodecahedron particles was demonstrated. This method for fabricating polyhedral particles opens a new avenue for designing new materials.
We examine the effect of external electric fields on the behavior of colloidal silica rods. We find that the electric fields can be used to induce para-nematic and para-smectic phases, and to reduce the number of defects in smectic phases. At high field strengths, a new crystal structure was observed that consisted of strings of rods ordered in a hexagonal pattern in which neighboring rods were shifted along their length. We also present a simple model to describe this system, which we used in computer simulations to calculate the phase diagram for rods of L/D = 6, with L the end-to-end length of the rods and D the diameter of the rods. Our theoretical predictions for the phase behavior agree well with the experimental observations.
During the last decade the focus in colloid science on self-assembly has
moved from mostly spherical particles and interaction potentials to more
and more complex particle shapes, interactions and conditions. In this
minireview we focus on how external electric fields, which in almost all
cases can be replaced by magnetic particles and fields for similar
effects, are used to manipulate the self-assembly process of ever more
complex colloids. We will illustrate typical results from literature
next to examples of our own work on how electric fields are used to
achieve a broad range of different effects guiding the self-assembly of
colloidal dispersions. In addition, preliminary measurements and
calculations on how electric fields can be used to induce lock-and-key
interactions will be presented as well.
When a crystal melts into a liquid both long-ranged positional and orientational order are lost, and long-time translational and rotational self-diffusion appear. Sometimes, these properties do not change at once, but in stages, allowing states of matter such as liquid crystals or plastic crystals with unique combinations of properties. Plastic crystals/glasses are characterized by long-ranged positional order/frozen-in-disorder but short-ranged orientational order, which is dynamic. Here we show by quantitative three-dimensional studies that charged rod-like colloidal particles form three-dimensional plastic crystals and glasses if their repulsions extend significantly beyond their length. These plastic phases can be reversibly switched to full crystals by an electric field. These new phases provide insight into the role of rotations in phase behaviour and could be useful for photonic applications.
Monodisperse peanut-shaped particles having various chemical compositions, architectures (core–shell and hollow) and properties (optical and magnetic) were prepared from hematite templates. The colloids enable shape programming alone, optical manipulation, or magnetic field approaches to assembly.
The creation of a new material often starts from the design of its constituent building blocks at a smaller scale. From macromolecules to colloidal architectures, to granular systems, the interactions between basic units of matter can dictate the macroscopic behaviour of the resulting engineered material and even regulate its genesis. Information can be imparted to the building units by altering their physical and chemical properties. In particular, the shape of building blocks has a fundamental role at the colloidal scale, as it can govern the self-organization of particles into hierarchical structures and ultimately into the desired material. Herein we report a simple and general approach to generate an entire zoo of new anisotropic colloids. Our method is based on a controlled deformation of multiphase colloidal particles that can be selectively liquified, polymerized, dissolved and functionalized in bulk. We further demonstrate control over the particle functionalization and coating by realizing patchy and Janus colloids.
We describe a facile and flexible approach for synthesizing uniform non-spherical micron sized PMMA (poly(methyl methacrylate)) colloids with well-controlled protrusions. When homogeneously cross-linked PMMA spheres were used as seeds in a swelling process using again a methyl methacrylate monomer, they were found to transform into non-spherical particles with a single or multiple protrusions mainly depending on the cross-link density of the seeds. Alternatively, if core–shell PMMA spheres bearing a highly cross-linked shell around an uncross-linked 'soft' core were employed as seed particles, they always developed just a single protrusion. Precise control over the anisotropy of the particles was achieved by varying the amount and composition of the swelling mixture as well as the concentration of the stabilizer. Subsequently, the phase separation was enhanced and protrusions could be readily polymerized through temperature elevation of the system, yielding PMMA 'snowman'-like or dumbbell-like colloids. Furthermore, these particles could be labeled with fluorescent dyes either before or after the polymerization, and transferred into apolar, refractive index and density matching liquids (cyclohexyl bromide (CHB) and/or decalin), enabling their use in quantitative confocal fluorescence microscopy studies in concentrated systems. Some examples of the use of these particles as a model system for real space analysis are given. These examples include the formation of plastic crystals, a special form of a colloidal crystal where the particles are positionally ordered but orientationally disordered. Additionally, the non-spherical particles could be organized into semi-flexibly bonded colloidal chains aided by an electric field in a polar solvent (formamide). Such chains of anisotropic particles are interesting as polymer analogs and for the preparation of new materials.
Yanking the chain: A general method for the preparation of colloidal analogues of polymer chains was developed. The flexibility of these chains can be tuned by applying electric fields in combination with their subjection to simple linkage-forming procedures.
Electrorheological (ER) fluids are a class of materials whose rheological properties are controllable by the application of an electric field. A dielectric electrorheological (DER) fluid is the simplest type of ER fluid, in which the material components follow a linear electrostatic response. We review and discuss the progress of the studies on physics of this type of material. A first-principles theory of DER fluids, along with relevant experimental verifications, are presented in some detail. In particular, the properties presented include static equilibrium structure, shear modulus, static yield stress and its variation with applied electric field frequency, and structure-induced dielectric nonlinearity.
Self-assembly and alignment of anisotropic colloidal particles are important processes that can be influenced by external electric fields. However, dielectric nanoparticles are generally hard to align this way because of their small size and low polarizability. In this work, we employ the coupled dipole method to show that the minimum size parameter for which a particle may be aligned using an external electric field depends on the dimension ratio that defines the exact shape of the particle. We show, for rods, platelets, bowls, and dumbbells, that the optimal dimension ratio (the dimension ratio for which the size parameter that first allows alignment is minimal) depends on a nontrivial competition between particle bulkiness and anisotropy because more bulkiness implies more polarizable substance and thus higher polarizability, while more anisotropy implies a larger (relative) difference in polarizability.
We employ the coupled dipole method to calculate the polarizability tensor of various anisotropic dielectric clusters of polarizable atoms, such as cuboid-, bowl-, and dumbbell-shaped nanoparticles. Starting from a Hamiltonian of a many-atom system, we investigate how this tensor depends on the size and shape of the cluster. We use the polarizability tensor to calculate the energy difference associated with turning a nanocluster from its least to its most favorable orientation in a homogeneous static electric field, and we determine the cluster dimension for which this energy difference exceeds the thermal energy such that particle alignment by the field is possible. Finally, we study in detail the (local) polarizability of a cubic-shaped cluster and present results indicating that, when retardation is ignored, a bulk polarizability cannot be reached by scaling up the system.
Non-uniform size distribution can be observed in the chemical synthesis of many types of nanoparticles. To obtain uniformly distributed nanoparticles, it is necessary to separate them by size. Several size sorting methods have been developed, including size exclusion chromatography, [1] filtration, [2,3] electrophoresis, [4,5] and solvent/antisolvent selective precipitation. [6] Recently, direct size separation of nanoparticles in the complete liquid phase through centrifugation has been proven to be a more effective method due to its high efficiency, capability of scalable production, and free of nanoparticle aggregation. [7-9].
New functional materials can in principle be created using colloids that self-assemble into a desired structure by means of a programmable recognition and binding scheme. This idea has been explored by attaching 'programmed' DNA strands to nanometre- and micrometre- sized particles and then using DNA hybridization to direct the placement of the particles in the final assembly. Here we demonstrate an alternative recognition mechanism for directing the assembly of composite structures, based on particles with complementary shapes. Our system, which uses Fischer's lock-and-key principle, employs colloidal spheres as keys and monodisperse colloidal particles with a spherical cavity as locks that bind spontaneously and reversibly via the depletion interaction. The lock-and-key binding is specific because it is controlled by how closely the size of a spherical colloidal key particle matches the radius of the spherical cavity of the lock particle. The strength of the binding can be further tuned by adjusting the solution composition or temperature. The composite assemblies have the unique feature of having flexible bonds, allowing us to produce flexible dimeric, trimeric and tetrameric colloidal molecules as well as more complex colloidal polymers. We expect that this lock-and-key recognition mechanism will find wider use as a means of programming and directing colloidal self-assembly.
We investigate the assembly of colloidal ellipsoids in ac electric fields. Polystyrene latex ellipsoids with aspect ratios 3.0, 4.3, and 7.6 orient with the applied field and, at sufficient field strengths, interact to form particle chains at an angle with respect to the field. The characteristic chain angle decreases with increasing aspect ratio. The angled chains combine laterally to form an open centered rectangular two-dimensional structures belonging to the c2mm plane group. This chaining and assembly behavior is explained based on calculations of the particle pair interactions explicitly accounting for the electric field and shape of the ellipsoids.
The assembly of complex structures out of simple colloidal building blocks is of practical interest for building materials with unique optical properties (for example photonic crystals and DNA biosensors) and is of fundamental importance in improving our understanding of self-assembly processes occurring on molecular to macroscopic length scales. Here we demonstrate a self-assembly principle that is capable of organizing a diverse set of colloidal particles into highly reproducible, rotationally symmetric arrangements. The structures are assembled using the magnetostatic interaction between effectively diamagnetic and paramagnetic particles within a magnetized ferrofluid. The resulting multipolar geometries resemble electrostatic charge configurations such as axial quadrupoles ('Saturn rings'), axial octupoles ('flowers'), linear quadrupoles (poles) and mixed multipole arrangements ('two tone'), which represent just a few examples of the type of structure that can be built using this technique.
Phase diagrams of hard and soft spheres with a fixed dipole moment are determined by calculating the Helmholtz free energy using simulations. The pair potential is given by a dipole-dipole interaction plus a hard-core and a repulsive Yukawa potential for soft spheres. Our system models colloids in an external electric or magnetic field, with hard spheres corresponding to uncharged and soft spheres to charged colloids. The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres exhibits, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and it agrees well with the experimental phase diagram [Nature (London) 421, 513 (2003)]. Our results show that bulk hcp, bct, and bco crystals can be realized experimentally by applying an external field.
We study the phase behavior of hard and soft spheres with a fixed dipole moment using Monte Carlo simulations. The spheres interact via a pair potential that is a sum of a hard-core Yukawa (or screened-Coulomb) repulsion and a dipole-dipole interaction. The system can be used to model colloids in an external electric or magnetic field. Two cases are considered: (i) colloids without charge (or dipolar hard spheres) and (ii) colloids with charge (or dipolar soft spheres). The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres shows, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and is in agreement with the experimental phase diagram [Nature (London) 421, 513 (2003)]. In both cases, the fluid phase is inhomogeneous but we find no evidence of a gas-liquid phase separation. The validity of the dipole approximation is verified by a multipole moment expansion.
Due to its high sensitivity to changes in the environmental dielectric constant, localized surface plasmon resonance (LSPR) is utilized for various sensing materials. However, because of this high sensitivity, it is difficult to detect a specific substance among mixed materials. Here, we enhanced the size selectivity using a metal nanostructure. By preparing a cup-like metal nanostructure that is the same size as a virus, this cup-like structure may be able to capture a virus, and thus the LSPR characteristics would change. This phenomenon would not occur for larger materials, such as animal cells, i.e., this cup-like structure may have selectivity for material size. In this paper, we fabricated metal nanocups out of two-dimensional colloidal crystals by exploiting the difference of interfacial energy at a dielectric/metal interface. The size selectivity and plasmon characteristics of the fabricated metal nanocups were investigated with model viruses, i.e., virus-like polystyrene (PSt) nanoparticles with almost the same dielectric constant and size as a virus. As a result, the transmission spectra show a 20-nm red shift in the visible light range before and after adsorption of the model virus. Field emission scanning electron microscopy (FE-SEM), optical microscope, UV-Vis spectrometry and contact angle measurement results are also discussed.
We report several facile, surfactant-free methods to prepare monodisperse polydimethylsiloxane (PDMS) droplets in the size range 3-8 μm in water. These methods, of which the pros and cons are discussed, are extensions of a procedure described before by our group which focused on smaller droplet sizes. The PDMS oil droplets are formed by ammonia catalyzed hydrolysis and condensation of the monomer dimethyldiethoxysilane (DMDES) in water. One of the methods entails a seeded growth procedure in which other oils, such as lower molecular weight hydrocarbons, were found to be able to swell the PDMS droplets if their solubility in water was higher than that of the seed droplets. This way, larger droplets with mixed composition could be prepared. It also turned out to be possible to load the monodisperse droplets with an oil soluble dye. The droplets could be coated with an elastic, partially permeable, shell formed by cross-linking the PDMS with tetraethoxysilane (TES) in the presence of poly(vinylpyrrolidone) (PVP) that provided colloidal stability. Besides, the liquid interior of these shells could be changed by solvent exchange.
This article describes a facile and promising method to prepare uniform, non-spherical colloidal particles
with shape and chemical anisotropies. We first synthesize polystyrene (PS) colloids with a concave multidimple
structure, through a unique combination of two-stage dispersion polymerizations and a
programmed feeding of cross-linker during the synthesis. These concave particles, after a seeded
polymerization process, can be swelled out to precisely controlled shapes by varying the amounts of the
swelling chemicals. A myriad of concave- and convex-shaped colloidal particles, including hemispheres,
peanuts and bowls are obtained. In addition, we explore the potential applications of the synthesized
anisotropic colloids by looking at the field-driven directional self-assembly, the optical hiding effect and
their use as colloidal surfactants.
We present a simple method for preparing large quantities of micrometer-sized core–shell silica rods, via needle-like akaganéite (β-FeOOH) template particles. The preparation of the akaganéite needles by aging ferric chloride solutions at elevated temperature is explored in order to find improved conditions for the formation of well-dispersed high-aspect-ratio needles. The influence of reaction conditions on irreversible bundling of the needles, yield, and the particle size distribution are reported and discussed. Subsequent coating of the template particles with silica leads to core–shell rods with an aspect ratio (A = L/D) that decreases with shell thickness. This facilitates control over the final aspect ratio and is accompanied by a decrease in the polydispersity. The composition of these core–shell particles offers various possibilities in terms of modifying the particle properties, which we demonstrate by preparing fluorescent particles and hollow silica rods while outlining other interesting options. We also include some optical microscopy images illustrating particle behavior and ordering under gravity, in a homogeneous magnetic field and at the interfaces of phase-separated binary liquids.
We introduce a new experimental method to encapsulate and release oils and fluorescent molecules, into preformed elastic colloidal microcapsules of PDMS filled siloxane shells, which are cross-linked with tetraethoxysilane. The method uses controlled buckling, where the volume of the capsule is reduced by dissolving the PDMS oil inside the capsule by surfactant micelles. This results in a change in the morphology of the capsule that depends on the ratio of shell thickness to total particle radius (d⁄Rt). Microcapsules of d⁄Rt in the range 0.007-0.05 formed microbowls upon decreasing the inner volume. The amount of oil released or dissolved by the micelles can be directly related to the concentration of surfactant. By tuning the amount of oil released we can make microbowls of variable depth. In addition, we demonstrate that the microbowls can be further used to load different oils like silicone oil, hydrocarbons, and apolar dyes. The elasticity of the capsule wall and the left-over PDMS oil inside the capsule provide the principal driving forces by which one can promote the uptake of different oils, including dissolved dye molecules.
Polymer particles with controlled internal architecture are currently under development for a number of emerging applications. In compartmentalized particles, well-defined pockets of distinct materials can be designed that can give rise to a set of orthogonal (i.e., dissimilar) properties within the same particle. While this aspect appears crucial, when multifunctional particles for sensing, imaging or drug delivery are sought after, their experimental realization has only recently been explored in broader terms. In this review, we highlight current progress related to the design and fabrication of multicompartmental particles and discuss potential benefits and experimental challenges associated with different synthetic routes.
Chains of micrometer-size colloidal particles have been self-assembled that are flexible, mechanically stable, and observable in optical microscopy. The chains sometimes have more than 30 particles, and we call them "polloidal chains". A key aspect of the work is the careful modeling of the interparticle forces between partially flattened polystyrene spheres. This modeling helped us to identify a narrow window of system conditions that produce interparticle physical bonds with a bond energy greater than 15kT, as well as a gap of fluid between particles that enables freely rotating bonds and flexible chains. The formation of the chains is well-modeled using linear condensation growth from classical polymer theory, suggesting that the chains might be used experimentally as large-scale, relatively slow moving models for polymer chains.
Due to the small difference between the refractive indices of water and silica, the addition of salt decreases the turbidity of silica sols. The decrease of turbidity must not be attributed to an influence of the electrolyte on the hydration repulsive forces between silica particles. The electrolyte also weakens the attractive Van Der Waals forces.
By measuring the orientation of colloidal doublets suspended in water under the opposing alignment forces of gravity and electrophoresis, we observed tangential forces between nontouching particles. We found a new phenomenon of time-dependent switching between ``slipping'' (no tangential force) and ``sticking'' (tangential forces producing rigid-body behavior of the doublet) for heterodoublets of silica/polystyrene. Tangential forces between particle surfaces across a fluid gap and their transient nature are important to the dynamics of colloidal suspensions and the evolution of floc morphology.
Monodisperse nonspherical poly (methyl methacrylate) (PMMA) particles where a central core particle had grown two extra "lobes", or protrusions, placed opposite each other were successfully synthesized by swelling and subsequent polymerization of cross-linked PMMA spheres with methyl methacrylate and the cross-linker ethylene glycol dimethacrylate. The use of large (similar to 3 mu m) seed particles allowed for real-time monitoring of the swelling and deswelling of the cross-linked particles with optical microscopy. First, a large number of small droplets of swelling monomers formed simultaneously on the surface of the seed particles, and then fused together until under certain conditions two protrusions remained on opposite sides of the seed particles. The yield of such particles could be made up to 90% with a polydispersity of 7.0%. Stirring accelerated the transfer of the swelling monomers to the seed particles. Stirring was also found to induce self-assembly of the swollen seed particles into a wide variety of n-mers, consisting of a certain number, n, of swollen seed particles. The formation of these structures is guided by the minimization of the interfacial free energy between the seed particles, liquid protrusions and aqueous phase, but stirring time and geometrical factors influence it as well. By inducing polymerization the structures could be made permanent. Some control over the topology as well as overall size of the clusters was achieved by varying the stirring time before polymerization. 3D models of possible particle structures were used to identify all projections of the structures obtained by scanning electron microscopy. These models also revealed that the seed particles inside the central coalesced body were slightly compressed after polymerization. By extending the synthesis of the monodisperse particles with n = 1 to (slightly) different monomers and/or different cores, an important class of patchy particles could be realized.
We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing water-soluble polymers as depletion agents. We have found that under certain conditions the cubes self-organize into crystals with a simple cubic symmetry, which is set by the size of the depletant. The dynamic of crystal nucleation and growth is investigated, monitoring the samples in time by optical microscopy. Furthermore, by using temperature sensitive microgel particles as depletant it is possible to fine tune depletion interactions to induce crystal melting. Assisting crystallization with an alternating electric field improves the uniformity of the cubic pattern allowing the preparation of macroscopic (almost defect-free) crystals that show visible Bragg colors.
We measure the aggregation kinetics of model, aqueous polymer latices of varying particle size as a function of added NaCl using dynamic light scattering to evaluate the available theoretical models for predicting the aggregation rate (stability ratio) and critical coagulation concentration (CCC). Our focus is on dilute colloidal suspensions of fixed surface chemistry but varying particle size. A master curve for the growth of the aggregate size is observed, with an intermediate regime that follows predictions for diffusion-limited cluster−cluster aggregation. Theoretical predictions based on DLVO theory are found to be in quantitative agreement for all but the largest particle size, when the particle surface potentials are determined by matching the experimentally determined CCCs. Thus, we conclude that for sufficiently smooth, nearly monodisperse particles such as those investigated here, DLVO theory can provide accurate predictions of colloidal stability for the range of parameters explored here down to truly atomic dimensions. The particle potential determined from phase analysis light scattering measurements of the zeta potential overpredicted colloidal stability but can be brought into agreement by assigning a Stern layer thickness equal to the hydrodynamic size of the counterion.
A method of simultaneous field- and flow-directed assembly of anisotropic titania (TiO2) nanoparticle films from a colloidal suspension is presented. Titania particles are oriented by an alternating (ac) electric field as they simultaneously advect towards a drying front due to evaporation of the solvent. At high field frequencies (ν > ∼25 kHz) and field strengths (E > 300 V cm−1), the particles orient with their major axis along the field direction. As the front recedes, a uniform film with thicknesses of 1–10 µm is deposited on the substrate. The films exhibit a large birefringence (Δn ≈ 0.15) and high packing fraction (ϕ = 0.75 ± 0.08), due to the orientation of the particles. When the frequency is lowered, the particle orientation undergoes a parallel–random–perpendicular transition with respect to the field direction. The orientation dependence on field frequency and strength is explained by the polarizability of ellipsoidal particles using an interfacial polarization model. Particle orientation in the films also leads to anisotropic mechanical properties, which are manifested in their cracking patterns. In all, it is demonstrated that the field-directed assembly of anisotropic particles provides a powerful means for tailoring nanoparticle film properties in situ during the deposition process.
Recent experiments have demonstrated that nanoparticles with dipoles can self-assemble into interesting one-dimensional and two-dimensional nanostructures. In particular, nanocubes with dipoles are found to form straight nanowires and nanorings with potential applications for nanodevices. In this paper, we use a minimal model to study dipole-induced self-assembly of nanocubes with varying dipole directions, dipole strengths and both with and without face-face attractions arising from dispersive or solvophobic interactions. We reproduce the structures observed in experiments and illustrate that the self-assembled morphologies are dictated by the head-to-tail alignment of the dipoles, the orientation of the dipoles within the cubes, and the face-to-face packing of the nanocubes. Our results show how the self-assembly of dipolar nanocubes differs from that of dipolar spheres in which the only anisotropy is the dipole itself and how system parameters can be manipulated to control the assembled morphologies and the phase behavior. Our simulation model, which uses the plane separating algorithm for efficient detection of nanoparticle overlaps, can be utilized to investigate the self-assembly of other smooth, convex polyhedral-shaped nanoparticles to facilitate novel nanomaterials design.
The synthesis and characterization of monodisperse, micrometer-sized, hollow particles obtained by encapsulation of emulsion droplet in solid shells followed by dissolution of the cores was discussed. Monodisperse, stable, oil-in-water emulsion droplets were prepared using surfactant by hydrolysis and condensation of the difunctional silane monomer dimethyldiethoxysilane (DMDES). The amount of hydrolyzed monomer which contributed to the shell thickness was less than for the microsphere for capsule-like particles. Analysis shows that while capsules deformed elastically like a rubber ball, the microballoons did not show such a deformation.
An attractive force appears between particles suspended in solutions of macromolecules when there is neither direct interaction between two particles nor energetic interaction between particles and solute macromolecules. The magnitude of this force is of the order of the osmotic pressure of the solution of macromolecules and the range is of the order of the diameter of macromolecules. This force is calculated as a function of concentration, shape, and charge of macromolecules, and it is shown that it becomes stronger in solutions of chain macromolecules or of macromolecules of dissymmetrical shape than in solutions of rigid spherical macromolecules at the same net concentration. If macromolecules have charge, the force can be greatly intensified. In every case numerical estimation is made, and it is found that actually this kind of force can have a remarkable influence on the state of suspended particles. Numerical examples of the critical concentration of particles at their macroscopic aggregation are given. Finally a short description is added on the effect of energetic interaction between particles and macromolecules.
Assembly and directed orientation of anisotropic particles with an external ac electric field in a range from 1 kHz to 2 MHz were studied for asymmetric composite dumbbells incorporating a silica, titania, or titania/silica (titania:silica = 75:25 vol %) sphere. The asymmetric composite dumbbells, which were composed of a polymethylmethacrylate (PMMA)-coated sphere (core-shell part) and a polystyrene (PSt) lobe, were synthesized with a soap-free emulsion polymerization to prepare PMMA-coated inorganic spheres and another soap-free emulsion polymerization to form a polystyrene (PSt) lobe from the PMMA-coated inorganic spheres. The composite dumbbells dispersed in water were directly observed with optical microscopy. The dumbbells incorporating a silica sphere oriented parallel to an electric field in the whole frequency range and they formed a pearl chain structure at a high frequency of 2 MHz. The titania-incorporated dumbbells formed chain structures, in which they contacted their core-shell parts and oriented perpendicularly to a low-frequency (kHz) field, whereas they oriented parallel to a high-frequency (MHz) field. Since the alignment of dumbbells in the chains depends not only on the interparticle forces but also on the torque that the induced dipoles in the dumbbells experience in the electric field, the orientation of dumbbells perpendicular to the electric field was the case dominated by the interparticle force, whereas the other orientation was the case dominated by the torque. The present experiments show that the incorporation of inorganic dumbbells is an effective way to control the assembled structure and orientation with an electric field.
Frequently we experience the existance of adhesive forces between small particles. It seems natural to ascribe this adhesion for a large part to London-v.d. Waals forces. To obtain general information concerning their order of magnitude the London-v. d. Waals interaction between two spherical particles is computed as a function of the diameters and the distance separating them. A table is calculated which enables numerical application of the formulae derived. Besides approximations are added, which may be used when the distance between the particles is small. In a separate section it is investigated how the results must be modified, when both particles are immersed in a liquid. Here we are led to the important conclusion that even in that case London-v. d. Waals forces generally cause an attraction.
We present unusual microstructures formed by patchy metallodielectric particles under high frequency alternating current (AC) electric fields. These particles assemble into two-directional percolated networks and lattices of unusual symmetry by interactions pre-programmed by their patch size and orientation. Electrostatic simulation results indicate that the assembly pattern of the patchy particles is guided by quadrupolar and multipolar interactions.
Although the experimental study of spherical colloids has been extensive, similar studies on rodlike particles are rare because suitable model systems are scarcely available. To fulfill this need, we present the synthesis of monodisperse rodlike silica colloids with tunable dimensions. Rods were produced with diameters of 200 nm and greater and lengths up to 10 μm, resulting in aspect ratios from 1 to ∼25. The growth mechanism of these rods involves emulsion droplets inside which silica condensation takes place. Due to an anisotropic supply of reactants, the nucleus grows to one side only, resulting in rod formation. In concentrated dispersions, these rods self-assemble in liquid crystal phases, which can be studied quantitatively on the single particle level in three-dimensional real-space using confocal microscopy. Isotropic, paranematic, and smectic phases were observed for this system.
A facile and flexible synthesis for colloidal molecules with well-controlled shape and tunable patchiness is presented. Cross-linked polystyrene spheres with a liquid protrusion were found to assemble into colloidal molecules by coalescence of the liquid protrusions. Similarly, cross-linked poly(methyl methacrylate) particles carrying a wetting layer assembled into colloidal molecules by coalescence of the wetting layer. Driven by surface energy, a liquid droplet on which the solid spheres are attached is formed. Subsequent polymerization of the liquid yields a wide variety of colloidal molecules as well as colloidosomes with tunable patchiness. Precise control over the topology of the particles has been achieved by changing the amount and nature of the swelling monomer as well as the wetting angle between the liquid and the seed particles. The overall cluster size can be controlled by the seed size as well as the swelling ratio. Use of different swelling monomers and/or particles allows for chemical diversity of the patches and the center. For low swelling ratios assemblies of small numbers of seeds resemble clusters that minimize the second moment of the mass distribution. Assemblies comprised of a large number of colloids are similar to colloidosomes exhibiting elastic strain relief by scar formation.
(Figure Presented) Sowing the seeds: Binary colloidal particles of controlled morphology, including line segments, triangles, tetrahedra, octahedra, and square antiprisms (see picture) were obtained by an emulsion polymerization of styrene in the presence of silica seeds. These morphologies result from the minimization of an energy term that is the sum of two forces - an attraction towards the center and two-body particle repulsions that balance the attractive force.
We demonstrate an approach using temperature-dependent hydrogel depletants to thermoreversibly tune colloidal attraction and interfacial colloidal crystallization. Total internal reflection and video microscopy are used to measure temperature-dependent depletion potentials between approximately 2 microm silica colloids and surfaces as mediated by approximately 0.2 microm poly-N-isopropylacrylamide (PNIPAM) hydrogel particles. Measured depletion potentials are modeled using the Asakura-Oosawa theory while treating PNIPAM depletants as swellable hard spheres. Monte Carlo simulations using the measured potentials predict reversible, quasi-2D crystallization and melting at approximately 27 degrees C in quantitative agreement with video microscopy images of measured microstructures (i.e., radial distribution functions) over the temperature range of interest (20-29 degrees C). Additional measurements of short-time self-diffusivities display excellent agreement with predicted diffusivities by considering multibody hydrodynamic interactions and using a swellable hard sphere model for the PNIPAM solution viscosity. Our findings demonstrate the ability to quantitatively measure, model, and manipulate kT-scale depletion attraction and phase behavior as a means of formally engineering interfacial colloidal crystallization.
When small numbers of colloidal microspheres are attached to the surfaces of liquid emulsion droplets, removing fluid from
the droplets leads to packings of spheres that minimize the second moment of the mass distribution. The structures of the
packings range from sphere doublets, triangles, and tetrahedra to exotic polyhedra not found in infinite lattice packings,
molecules, or minimum–potential energy clusters. The emulsion system presents a route to produce newcolloidal structures and
a means to study howdifferent physical constraints affect symmetry in small parcels of matter.
The development of model materials and image processing methods to directly visualize and quantify colloidal rod assembly by means of confocal laser scanning microscopy (CLSM) is reported. Monodisperse fluorescent colloidal rods are prepared by the uniaxial extensional deformation of sterically stabilized microspheres at elevated temperatures. The particles are stably dispersed in refractive index matching mixed organic solvents for CLSM. An image processing algorithm is developed to detect rod backbones and extract particle centroids and orientation angles from the CLSM image volumes. By means of these methods we quantify the distribution of rod orientation angles in self-assembled structures of rods formed by sedimentation. We find the observations to be consistent with aspect-ratio-dependent jamming and orientational order/disorder transition in the rod sediments.
Nanosized hexagonal gibbsite seeds are grown from a mixture of dissolved alumina alkoxides at 85 degrees C. Centrifugation reduces the polydispersity by 30%. The seeds can be grown further by adding them to a fresh alkoxide mixture and heating it. This procedure was repeated several times to obtain particles of 570 nm +/- 11% diameter and a thickness of 47 +/- 23%. No indications of a size limit were observed. The thus obtained particles may form easily a columnar phase. Individual gibbsite particles in solution can be seen by confocal microscope.
A revolution in novel nanoparticles and colloidal building blocks has been enabled by recent breakthroughs in particle synthesis. These new particles are poised to become the 'atoms' and 'molecules' of tomorrow's materials if they can be successfully assembled into useful structures. Here, we discuss the recent progress made in the synthesis of nanocrystals and colloidal particles and draw analogies between these new particulate building blocks and better-studied molecules and supramolecular objects. We argue for a conceptual framework for these new building blocks based on anisotropy attributes and discuss the prognosis for future progress in exploiting anisotropy for materials design and assembly.