Leopoldo R. Gómez

National Scientific and Technical Research Council | conicet · IFISUR Instituto de Fisica del Sur

Nonlinear sound is an extreme phenomenon typically observed in solids after violent explosions. But granular media are different. Right when they jam, these fragile and disordered solids exhibit a vanishing rigidity and sound speed, so that even tiny mechanical perturbations form supersonic shocks. Here, we perform simulations in which two-dimensio...

Nucleation and growth is the dominant relaxation mechanism driving first-order phase transitions. In two-dimensional flat systems, nucleation has been applied to a wide range of problems in physics, chemistry and biology. Here we study nucleation and growth of two-dimensional phases lying on curved surfaces and show that curvature modifies both cri...

Uniform triangular crystals are the ground state for particles that interact isotropically in two dimensions. However, when immersed in an external potential, for example, one arising from an electric field, a flow field, or gravity, the resulting phases are significantly distorted in a way reminiscent of conformal transformations of planar lattice...

Unraveling the packing structure of dense assemblies of semiflexible rings is not only fundamental for the dynamical description of polymer rings, but also key to understand biopackaging, such as observed in circular DNA of viruses or genome folding. Here we use X-ray tomography to study the geometrical and topological features of disordered packin...

Here, we study the shock wave response of semicrystalline polymers using coarse-grained molecular dynamics simulations. The crystallinity of the systems was controlled by using block copolymer-like chains with different volume fractions of crystallizable blocks. Our results indicate that the degree of initial crystallinity affects the speed of soun...

We employ molecular dynamics simulations to investigate the shock compression of linear semiflexible polymers. While the propagation velocity of a shock primarily depends on density, both chain rigidity and chain orientation significantly influence the shock width and the final temperature of the system. In general, the shock wave triggers molecula...

During the synthesis of ring polymers, mixtures of ring and linear molecules are regularly obtained. Recent experiments and simulations have shown that even small fractions of linear molecules can considerable modify the shape of rings, affecting its rheological response. Here we study blends of semiflexible linear and ring filaments, by using diso...

We introduce an innovative instrument designed to investigate fluid-induced fractures under mixed loading conditions, including uniaxial tension and shear stress, in gels and similar soft materials. Equipped with sensors for measuring force, torque, and fluid pressure, the device is tailored for compatibility with x-ray tomography scanners, enablin...

The entangled structure of polymeric materials is often described as resembling a bowl of spaghetti, swarms of earthworms, or snakes. These analogies not only illustrate the concept, but form the foundation of polymer physics. However, the similarity between these macroscopic, athermal systems and polymers in terms of topology remains uncertain. To...

Here we use simulations and theory to show that, close to the jamming point, an arbitrary initial distortion of a granular media induces the formation of forward and backward non-linear finite amplitude waves. There are two regimes in the evolution of these waves (near field and far field). Initially, non-linear interactions between forward and bac...

While the propagation of shocks through monoatomic liquids and solids is now well understood, the response of macromolecular systems to shock compression remains far less studied. Here we use molecular dynamics simulations to study the shock compression of diblock copolymers assembled in a lamellae morphology, which may display outstanding ballisti...

Here, we build and characterize a single-stage gas-gun microparticle accelerator, where a pressurized gas expands and launches particles on a target. The microparticles in the range of 60–250 μm are accelerated by the expansion of pressurized nitrogen. By using a high-speed camera, we study how the velocity distribution of accelerated particles is...

Here we study how curvature affects the structure of two-dimensional crystals growing on spheres. The mechanism of crystal growth is described by means of a Landau model in curved space that accounts for the excess of strain on crystal bonds caused by the substrate's curvature (packing frustration). In curved space elastic energy penalization stron...

The spontaneous formation of a crystal phase is one of the most common and beautiful pattern formation mechanisms in nature. Different instabilities in the crystal interface may lead to the growth of ramified structures, known as dendritic crystal growth. In this work, we use a Phase Field Model and numerical simulations to study 2D dendritic growt...

Experimental data on thin films of cylinder-forming block copolymers (BC) -- free-standing BC membranes as well as supported BC films -- strongly suggest that the local orientation of the BC patterns is coupled to the geometry in which the patterns are embedded. We analyze this phenomenon using general symmetry considerations and numerical self-con...

We use Monte Carlo simulations to study the finite temperature behavior of vortices in the XY- model for tangent vector order on curved backgrounds. Contrary to naive expectations, we show that the underlying geometry does not affect the proliferation of vortices with temperature respect to what is observed on a flat surface. Long-range order in th...

The order-disorder transition of a sphere-forming block copolymer thin film was numerically studied through a Cahn-Hilliard model. Simulations show that the fundamental mechanisms of pattern formation are spinodal decomposition and nucleation and growth. The range of validity of each relaxation process is controlled by the spinodal and order-disord...

In this work we study equilibrium and non-equilibrium structures of smectic block copolymer thin films deposited on a topographically patterned substrate. A Brazovskii free energy model is employed to analyze the coupling between the smectic texture and the local mean curvature of the substrate. The substrate’s curvature produces out-of-plane defor...

In this work we study the processes of defect formation and coarsening of two-dimensional (2D) curved crystal structures. These processes are found to strongly deviate from their counterparts in flat systems. In curved backgrounds the process of defect formation is deeply affected by the curvature, and at the onset of a phase transition the early d...

Block copolymers constitute a fascinating group of polymeric materials belonging to the "soft matter" family. These materials are formed by blocks of polymeric chains from different monomers. In most cases, these blocks are thermodynamically incompatible, but the covalent bonds between them impose specific arrangements, generating particular morpho...

We study the dynamics of coarsening in a cylinder-forming polystyrene-block-poly(ethylene-alt-propylene) block copolymer thin film deposited on a topographically patterned substrate. Thermal annealing leads to highly ordered arrays of polystyrene cylinders embedded in the poly(ethylene-alt-propylene) matrix, lying in-plane and oriented perpendicula...

The interaction of a solitary wave with an interface formed by two strongly nonlinear noncohesive granular lattices displays rich behavior, characterized by the breakdown of continuum equations of motion in the vicinity of the interface. By treating the solitary wave as a quasiparticle with an effective mass, we construct an intuitive (energy- and...

We study the evolution from a liquid to a crystal phase in two-dimensional curved space. At early times, while crystal seeds grow preferentially in regions of low curvature, the lattice frustration produced in regions with high curvature is rapidly relaxed through isolated defects. Further relaxation involves a mechanism of crystal growth and defec...

Disordered packings of soft grains are fragile mechanical systems that loose rigidity upon lowering the external pressure towards zero. At zero pressure, we find that any infinitesimal strain-impulse propagates initially as a non-linear solitary wave progressively attenuated by disorder. We demonstrate that the particle fluctuations generated by th...

The confining pressure P is perhaps the most important parameter controlling the properties of granular matter. Strongly compressed granular media are, in many respects, simple solids in which elastic perturbations travel as ordinary phonons. However, the speed of sound in granular aggregates continuously decreases as the confining pressure decreas...

In this work we have studied through a Landau's free energy functional approach the dynamic and equilibrium configurations of two-dimensional hexagonal systems constrained to lie on a substrate with sinusoidal topography. Similarly to previous studies, we have found a strong coupling between defects and geometry, where the regions with the highest...

We use a Brazovskii model to numerically investigate the defect structures and coarsening process of spherical shells of asymmetric block copolymers. It was found that the configurations of defects are dictated by the ratio between the radius of the spherical shell and the average lattice constant. For small system sizes most configurations of defe...

A general Landau's free energy functional is used to study the dynamics of crystallization during liquid-solid Spinodal Decomposition (SD). The strong length scale selectivity imposed during the early stage of SD induces the appearance of small precursors for crystallization with icosahedral order. These precursors grow in densely packed clusters o...

We study the process of defect formation during a continuous phase transition in an off-critical system. We focus on the spinodal-assisted nucleation regime, where the continuous amplification of density fluctuations induces the pseudo-nucleation of the equilibrium phase. The collision of the different propagating domains produces a domain structur...

The kinetics of spinodal-assisted crystallization in a region of the phase diagram where the dynamics is controlled by the critical slow down was studied by means of a Cahn-Hilliard model. The length-scale selectivity conducted by the spinodal process led to the formation of a filamentary network of density fluctuations that resemble the scarred st...

We study the dynamics of pattern formation of two-dimensional smectic systems constrained to lie on a substrate with sinusoidal topography. We observe a coupling between defects and geometry that induces the preferential location of positive (negative) defects onto regions with positive (negative) Gaussian curvature. For the curvatures studied here...

For the first time, order–order and order–disorder transitions were detected and characterized in a model diblock copolymer of poly(butadiene-1,3) and poly(dimethylsiloxane) (PB-b-PDMS). This model PB-b-PDMS copolymer was synthesized by the sequential anionic polymerization (high vacuum techniques) of butadiene 1,3 (B) and hexamethylciclotrisiloxan...

During the early stage of spinodal decomposition most of the phase separating systems lead to the formation of incoherent structures with small-range orientational and translational order. In this work we found that in the region near below the spinodal line two-dimensional systems with competing interactions can form hexagonal structures with long...

We study through the Otha-Kawasaki model for diblock copolymers equilibrium and non-equilibrium features of two dimensional smectic phases on curved sinusoidal substrates. At low curvatures defect free patters are found to be stable. In agreement with theoretical predictions of Vitelli and Nelson [], at high curvatures topological defects are obser...

The dynamics of ordering in a 2D hexagonal system was investigated through the Cahn–Hilliard–Cook model. At low thermal noise amplitudes, pinning forces acting on grain boundaries dominate the dynamics and the coarsening evolves logarithmically in time. As noise amplitude increases, fluctuations becomes large enough to unlock dislocations located a...

We look at a grain rotation mechanism in a bidimensional pattern forming system exhibiting an hexagonal phase. This mechanism is believed to be relevant in the coarsening process of a variety of physical systems with short- and long-range competing interactions. We focus on the Cahn-Hilliard model with thermal fluctuations. The grain rotation proce...

The coarsening process in a two-dimensional hexagonal system in the region close to both spinodal and order-order transitions was investigated through the Cahn-Hilliard model. We found a distinctive region of the phase diagram where the pinning of dislocations plays only a minor role and the dynamics is led by the triple points. In this region, we...

The coarsening process in planar patterns has been extensively studied during the last two decades. Although progress has been made in this area, there are still many open questions concerning the basic mechanisms leading the system towards equilibrium. Some of these mechanisms (including curvature driven growth, grain rotation and defect annihilat...

The disorder-order transition of a two dimensional sphere forming block copolymer is studied through the Cahn-Hilliard model at different deeps of quench. The process of microphase separation and kinetic of pattern formation are controlled by the spinodal and order-disorder temperatures. In the spinodal region the deep of quench strongly affect bot...

We study the stress relaxation of model polymer networks containing low contents of star shaped and linear dangling polymers. As compared with their melts, the behavior of star and dangling polymers leads to a dynamic response with unprecedented large relaxation times. By comparing data of star melts with those corresponding to stars and dangling c...

We analyze the influence of low concentrations of star and dangling polymer chains on the stress relaxation process of model polymer networks. Model PDMS networks with well defined structure were obtained by the hydrosylilation reaction, based on the addition of hydrogen silanes from a trifunctional cross- linker to end vinyl groups of alpha- phi p...

The order disorder transition corresponding to the two dimensional hexagonal patterns formed by sphere forming diblock copolymers is studied through the Cahn-Hilliard model by taking into account a long range term in the free energy functional. The growth kinetic of the equilibrium patterns at deep quenches is dominated by spinodal decomposition. A...