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Schematic illustration of the effect of wettability changes that occur in the continuous phase only, going from a weak interaction between continuous phase and solid material (high contact angle for the continuous phase, and high surface free energy that is illustrated in the length of the blue arrow) to increasingly more favorable interactions, which lead to lower contact angles, and ultimately in a change in polarity on the left, with a contact angle < 90° for the continuous phase.
Source publication
In microfluidics and other microstructured devices, wettability changes, as a result of component interactions with the solid wall, can have dramatic effects. In emulsion separation and emulsification applications, the desired behavior can even be completely lost. Wettability changes also occur in one phase systems, but the effect is much more far-...
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Citations
... The system must be carefully selected to generate an emulsion with desired physicochemical features, e.g., size and stability. Schroën and co-workers [7] pointed out that in emulsion separation and emulsification applications (e.g. direct membrane emulsification -DME, cross-flow membrane emulsification -CFME, and premix membrane emulsification), the desired behavior can even be completely lost owing to wettability changes in the device as an effect of component interactions with the solid wall [8]. ...
... There are a lot of developments that allow for 3D printing of devices, production in paper or in thin film, and these techniques have also been reviewed recently [52,53], and even commercialized, which brings microfluidics within reach of many. Irrespective of the production method used, it is always important for multiphase systems to monitor wettability changes [54,55] that may occur because of the adsorption of components at the device surface, which influence its operation greatly. ...
Food design is often done based on a trial and error basis, using structure properties as an indicator of product quality. Although this has led to many good products in the market, this ‘cook and look’ approach could benefit from insights into dynamic processes as they occur during food formation, storage, and digestion. Currently microfluidic devices are being developed to allow these types of observations, and here we show the latest examples in the field of emulsions and foams, including effects that occur during digestion. We expect that these techniques will supply a stepping stone to thorough understanding at various length and time scales that are all instrumental in designing high quality food products, and ultimately creating foods with health benefits.
... The protein concentration now has negligible influence on t fill (Fig. 3B), except for slightly shorter filling time observed in a protein-free system (0% wt., Fig. SI 3). On one hand, this indicates that protein adsorption towards the meniscus (inside the pore) is negligible during these short times; on the other hand, any protein adsorption taking place on the channel walls possibly renders the glass surface more hydrophilic (leading to a lower contact angle) [33,34] and thus slightly increases the DP pore . Hence, only in a completely protein-free system the higher contact angle results in a distinctly shorter t fill . ...
Hypothesis
The interplay of interface evolution and surfactant adsorption determines the formation and stabilization of bubbles, and can be controlled by the liquid phase properties.
Experiments
We studied bubble formation in an Edge-based Droplet GEneration (EDGE) microfluidic device at relevant length and time scale, allowing investigation of sub-events in a single bubble formation cycle. We vary the properties of the continuous phase that contains whey proteins and study a range of trans-pore pressures (Pd∗).
Findings
The shallow pores highlight the crucial role of the Laplace pressure and dynamic adsorption of proteins to the meniscus. Bubble formation is divided into two regimes by the Laplace pressure of the bare meniscus inside the pore. At Pd∗<1400 mbar, pre-adsorption of proteins is required to lower the Laplace pressure; the bubble formation frequency f0 increases with increasing protein concentration and is hardly affected by velocity and viscosity. At Pd∗≥1400 mbar, bubble formation immediately occurs upon applying pressure, and f0 mainly decreases with increasing viscosity. In both regimes, the initial bubble size d0 mainly increases with the viscosity (η1/3). Bubble coalescence is only observed at Pd∗≥1400 mbar and can be effectively suppressed by raising protein concentration and viscosity within certain boundaries, yet ultimately this is at the cost of higher polydispersity of the bubbles. Our insights into the formation dynamics of micrometer-sized bubbles at time scales down to tens of microseconds can be used for effective control of bubble formation and stabilization in practical applications.
... In addition to the transmembrane pressure, the wettability of the membrane also affects the activation of membrane pores. Over the years, membrane emulsification produces oil in water (o/w) emulsions by hydrophilic microporous membrane or prepares water in oil (w/o) emulsions through hydrophobic microporous membrane in most cases [16]. The process of emulsion components contacting membrane pores is affected by the interfacial tension between the membrane and emulsion components to form different droplets [17]. ...
The low percentage of pores at which droplets are formed results in lower production efficiency, which is one of the main issues hampering the application of microporous membrane emulsification on a large scale. The effect of microporous membrane pore activation on the droplet’s formation and emulsion performance was systemically investigated by changing both transmembrane pressures and membrane wettability during membrane emulsification. It indicated that the high transmembrane pressure increased the pore activation ratio (the percentage of pores at which droplets are formed) by the growing flux. These induced the enhancements of emulsification efficiency and the degradation of emulsion performance. Different wettability of membrane inflow and outflow influenced the pore activation speed (the speed which droplets are formed at pores). It indicated that the wettability of membrane outflow had a key effect on droplet formation and the high pore activation speed avoided splashing for high emulsion performance.
... Wettability of the microfluidic systems needs to be guarded very carefully, which can be done through surface modification to prevent adsorption of surface active components, or in some case by an in-situ layer formed that warrants appropriate wettability (as demonstrated for proteins in combination with so-called EDGE emulsification devices) [32,33]. ...
Microfluidics have been extensively used to investigate bubbles and droplets, and make uniform emulsions and foams; the benefit being that very monodisperse products are more stable. Besides, when used for encapsulate production, their size codetermines the payload, and allows accurate dosing of the component of interest. In this review, we specifically highlight the perspective of microfluidic production of microgel capsules for anti-obesity strategies that activate the intestinal brakes via controlled delivery of nutrients to the small intestine. We show various microgel capsule structures, their release characteristics, and discuss first results in human trials. We wrap up with recent progress made in up-scaling, which in our view is the key to bring the technology toward application in the food field.
... To use the microfluidic tensiometer effectively, a few recommendations are: 1) to carry out the measurements with narrow pores, with which we expect the inflow of the continuous phase and thus the wetting issues to dampen to some extent; 2) to modify the channel surface to prevent wettability changes 150 . We expect the EDGE tensiometer to find a wide range of applications, e.g., as high-throughput screening tools for emulsifiers that can be evaluated at time scales as emulsifier adsorption would occur during large-scale processing, and under similar conditions. ...
... The protein concentration now has negligible influence on t fill (Figure 4.3B), except for a slightly shorter filling time observedin a protein-free system (0% wt.,Figure A4.3). On one hand, this indicates that protein adsorption towards the meniscus (inside the pore) is negligible during these short times; on the other hand, any protein adsorption taking place on the channel walls possibly renders the glass surface more hydrophilic (leading to a lower contact angle)91,150 and thus slightly increases the ∆P pore . Hence, only in a completely protein-free system the higher contact angleresults in a distinctly shorter t fill . ...
... Previous studies on idealized membrane pore geometries [11] as well as fully resolved membranes pointed out that the highest shear stresses occur for a short time at the wall, while lower shear stresses occur at the liquid/liquid interface and their absolute value is higher [11]. Schroën et al. [12] pointed out the importance of wettability and surfactant interactions with interfaces. They concluded, that most surface interactions increase the contact angle towards 90°, either through surfactant or the oil that is used. ...
The membrane emulsification process obtains the advantage to formulate liquid/liquid systems in a low shear stress process. Whereas the emulsification process and the influence of the membrane structure and geometry on the final product are well investigated, the liquid deformation and liquid dispersion process inside the membrane structure are quite unknown. In the present study, the droplet breakup mechanism in the vicinity of a solid wall is numerically investigated with OpenFoam using the Volume of Fluid Method. Different wetting conditions on the membrane wall have been implemented. The results show a relation between wetting properties and fluid dynamic disturbances’ resp. instabilities that may grow to larger instabilities, resulting in droplet breakup inside the membrane. These results can be used to find the sweet spot of capillary driven droplet breakup with minimum shear force to handle shear sensitive media while keeping a defined mono disperse emulsion.
... On the one hand, this facilitates droplet formation in microfluidic devices, as surfactants adsorb to the oil-water interface. On the other hand, this may complicate droplet formation due to interactions with the construction material(s), leading to wettability changes that are generally undesirable [29,68]. In the section below, we highlight how microfluidic devices can be used to monitor fast processes at small length scales, such as adsorption or coalescence taking place during and just after droplet formation. ...
... The interfacial tension played an important role, together with the applied shear force, and the expansion rates during premix membrane emulsification, and very reasonable predictions of the droplet size were obtained. These results show that translation of microfluidic observations to large-scale processes is possible to some degree [68,78]. The bottom graph represents the interfacial tension at the hexadecane-water interface with various SDS concentrations in the aqueous phase, as measured with a microfluidic Y-junction (symbols) and with a droplet volume tensiometer (solid lines). ...
Droplet microfluidics revolutionizes the way experiments and analyses are conducted in many fields of science, based on decades of basic research. Applied sciences are also impacted, opening new perspectives on how we look at complex matter. In particular, food and nutritional sciences still have many research questions unsolved, and conventional laboratory methods are not always suitable to answer them. In this review, we present how microfluidics have been used in these fields to produce and investigate various droplet-based systems, namely simple and double emulsions, microgels, microparticles, and microcapsules with food-grade compositions. We show that droplet microfluidic devices enable unprecedented control over their production and properties, and can be integrated in lab-on-chip platforms for in situ and time-resolved analyses. This approach is illustrated for on-chip measurements of droplet interfacial properties, droplet–droplet coalescence, phase behavior of biopolymer mixtures, and reaction kinetics related to food digestion and nutrient absorption. As a perspective, we present promising developments in the adjacent fields of biochemistry and microbiology, as well as advanced microfluidics–analytical instrument coupling, all of which could be applied to solve research questions at the interface of food and nutritional sciences.
... The researchers also reported a change in wettability of the membrane (a decrease in contact angle) due to repetitive emulsification and cleaning cycles. The change in membrane wettability may occur due to surface interactions between the membrane and the surface-active components present in the processed emulsion [103]. Güell et al [100] suggested using Ohnesorge number, which relates viscous forces to inertial and interfacial forces, to describe a PME process in non-dimensional form. ...
Premix membrane emulsification (PME) is a pressure driven process of droplet breakup, caused by their motion through membrane pores. The process is widely used for high-throughput production of sized-controlled emulsion droplets and microparticles using low energy inputs. The resultant droplet size depends on numerous process, membrane, and formulation factors such as flow velocity in pores, number of extrusions, initial droplet size, internal membrane geometry, wettability of pore walls, and physical properties of emulsion. This paper provides a comprehensive review of different mechanisms of droplet deformation and breakup in membranes with versatile pore morphologies including sintered glass and ceramic filters, SPG and polymeric membranes with sponge-like structures, micro-engineered metallic membranes with ordered straight-through pore arrays, and dynamic membranes composed of unconsolidated particles. Fundamental aspects of droplet motion and breakup in idealized pore networks have also been covered including droplet disruption in T-junctions, channel constrictions, and obstructed channels. The breakup mechanisms due to shear interactions with pore walls and localized shear (direct breaking) or due to interfacial tension effects and Rayleigh-Plateau instability (indirect breaking) were systematically discussed based on recent experimental and numerical studies. Non-dimensional droplet size correlations based on capillary, Weber, and Ohnesorge numbers were also presented.
... In principle, the Y-junction can be used for rapid screening of emulsion components (initial results are available for proteins [81]), and most probably, the obtained data can be used to predict effects occurring in the more classic emulsification devices that were discussed earlier in this review, as has already been done for membrane emulsification to some extent [82]. The droplet formation times are rather similar (as is discussed further in the outlook section) but the mass transfer conditions may be different, and that is still a point of attention. ...
This paper starts with short descriptions of emulsion preparation methods used at large and smaller scales. We give scaling relations as they are generally used, and focus on the central role that interfacial tension plays in these relations. The actual values of the interfacial tension are far from certain given the dynamic behavior of surface-active components, and the lack of measurement methods that can be applied to conditions as they occur during large-scale preparation. Microfluidic techniques are expected to be very instrumental in closing this gap. Reduction of interfacial tension resulting from emulsifier adsorption at the oil-water interface is a complex process that consists of various steps. We discuss them here, and present methods used to probe them. Specifically, methods based on microfluidic tools are of great interest to study short droplet formation times, and also coalescence behavior of droplets. We present the newest insights in this field, which are expected to bring interfacial tension observations to a level that is of direct relevance for the large-scale preparation of emulsions, and that of other multi-phase products.
