We investigate the 3D structure and drying dynamics of complex mixtures of emulsion droplets and colloidal particles, using confocal microscopy. Air invades and rapidly collapses large emulsion droplets, forcing their contents into the surrounding porous particle pack at a rate proportional to the square of the droplet radius. By contrast, small droplets do not collapse, but remain intact and are merely deformed. A simple model coupling the Laplace pressure to Darcy's law correctly estimates both the threshold radius separating these two behaviors, and the rate of large-droplet evacuation. Finally, we use these systems to make novel hierarchical structures.
We present a simple method for accessing the elastic properties of microscopic deformable particles. This method is based on measuring the pressure-induced deformation of soft particles as they are forced through a tapered glass microcapillary. It allows us to determine both the compressive and the shear modulus of a deformable object in one single experiment. Measurements on a model system of polyacrylamide microgel particles exhibit good agreement with measurements on bulk gels of identical composition. Our approach is applicable over a wide range of mechanical properties and should thus be a valuable tool for the characterization of a variety of soft and biological materials.
A microfluidic melt emulsification method for encapsulation and release of actives is presented. Using a water-in-oil-in-water (W-O-W) double emulsion template, solid capsules can be formed by freezing the middle shell phase. Actives encapsulated inside the solid shell can be controllably and rapidly released by applying a temperature trigger to melt the shell. The choice of the shell materials can be chosen to accommodate the storage and release temperatures specific to the applications. In addition, we have also demonstrated the same concept to encapsulate multiple actives in multicompartment capsules, which are promising as multifunctional capsules and microreactors.
In this paper, silicon-based devices with a step structure integrated at the flow-focusing junction were designed, fabricated and characterized for droplet generation. A two-step silicon etching method was demonstrated to create the step structure. During fabrication, undesirable spikes encountered at the step edge were removed by oxygen plasma ashing and silicon isotropic etching. With this method, two types of step profile (flat and triangular profiles) were fabricated. These two profiles were compared for their differences in droplet-generation behavior. The device with the flat-step profile was found to make larger droplets and at a lower frequency compared to the device with the triangular-step profile. Additionally, polydimethylsiloxan e and glass were tested as capping materials for the devices and the impact of their surface characteristics (hydrophobic and hydrophilic) on the type of droplets (water-in-oil or oil-in-water) formed was investigated.
Microfluidic devices can be used to produce highly controlled and monodisperse double or multiple emulsions. The presence of inner drops inside a jet of the middle phase introduces deformations in the jet, which leads to breakup into monodisperse double emulsions. However, the ability to generate double emulsions can be compromised when the interfacial tension between the middle and outer phases is low, leading to flow with high capillary and Weber numbers. In this case, the interface between the fluids is initially deformed by the inner drops but the jet does not break into drops. Instead, the jet becomes highly corrugated, which prevents formation of controlled double emulsions. We show using numerical calculations that the corrugations are caused by the inner drops perturbing the interface and the perturbations are then advected by the flow into complex shapes. (c) 2010 American Institute of Physics. [doi:10.1063/1.3480561]
We fabricate thermo-responsive polymer microgels by combining microfluidic pre-gel emulsification with polymer-analogous gelation. This separates the microgel formation from the polymer synthesis; it combines highly controlled microfluidic templating with the great flexibility of preparative polymer chemistry, allowing each to be controlled independently. We produce monodisperse pre-gel droplets from semidilute solutions of photocrosslinkable poly(N-isopropylacrylamide) precursors. The size and morphology of these droplets can be precisely controlled by the microfluidic emulsification, provided the molecular weight of the precursor is limited. Using polymer-analogous gelation rather than monomer chain-growth gelation yields gels with a higher efficiency of crosslinking and a greater homogeneity on nano- and micrometre scales, as determined by oscillatory shear rheology, static light scattering, and optical microscopy. We also demonstrate the applicability of our method to fabricate microgel particles with well-defined concentrations of functional sites.
Stimuli-responsive polymer microgels can be produced with exquisite control using droplet microfluidics; however, in existing methods, the droplet templating is strongly coupled to the material synthesis, because droplet solidification usually occurs through rapid polymerization immediately after the microfluidic droplet formation. This circumstance limits independent control of the material properties and the morphology of the resultant microgel particles. To overcome this limitation, we produce sensitive polymer microgels from pre-fabricated precursor polymers. We use microfluidic devices to emulsify semidilute solutions of crosslinkable poly(N-isopropylacrylamide) and solidify the drops via polymer-analogous gelation. This approach separates the polymer synthesis from the particle gelation and allows each to be controlled independently, thus enabling us to form monodisperse, thermo-responsive microgel particles with well-controlled composition and functionality. In addition, the microfluidic templating allows us to form complex particle morphologies such as hollow gel shells, anisotropic microgels, or multi-layered microgel capsules. (C) 2010 Elsevier Ltd. All rights reserved.
We introduce a novel and versatile technique to fabricate monodisperse stimuli-responsive colloidosomes using stimuli-responsive microgel particles its building blocks, aqueous droplets as templates, and microfluidic devices to control the assembly. Our colloidosomes exhibit similar to 80% decrease in volume when actuated; thus, they can be of immense potential in applications that require targeted pulsed-release of active materials. The use of microfluidics allows fabrication of extremely monodisperse colloidosomes, Alternatively, our technique can also be combined with bulk emulsification techniques to produce large quantities of colloidosomes for various applications.
Microfluidic devices enable massive parallelization of sample manipulation and delivery, but a similarly parallelized and integrated optical detection system does not yet exist. Standard large numerical aperture wide field or scanning optical systems are not capable of the large field of view and detection sensitivity required to collect fluorescence from parallel arrays of microfluidic devices. Instead, we present a fluorescence measurement platform based on a microfabricated zone-plate array integrated into a parallelized microfluidic device. The zone-plate array is orientated so that a single high numerical aperture zone plate is aligned to read out the fluorescence from each of 64 output channels of a drop-making device. The parallelization of microfluidics and optics produces an integrated system capable of analysis of nearly 200 000 drops per second.
Microgel particles and capsules which consist of multiple layers can be fabricated using droplet microfluidics, but in existing methods, emulsion templating forms layers of dissimilar polarity. In this paper, we fabricate functional microgel capsules that consist of two miscible yet distinct layers. We use microfluidic devices to template micrometer-sized drops that are loaded with prepolymerized precursors and solidify them through a polymer-analogous reaction. This allows the particle morphology to be controlled and prevents pronounced interpenetration of the different layers despite their miscibility. We use polyacrylamide and poly(N-isopropylacrylamide) precursors to form thermoresponsive core-shell microparticles and demonstrate their utility for encapsulation and controlled release applications.
Micrometer-sized Janus particles of many kinds can be formed using droplet microfluidics, but in existing methods, the microfluidic templating is strongly coupled to the material synthesis, since droplet solidification occurs through rapid polymerization light after droplet formation. This circumstance limits independent control of the material properties and the morphology of the resultant particles In this paper, we demonstrate a microfluidic technique to produce functional Janus miciogels nom prefabricated, cross-linkable precursor polymers This approach separates the polymer synthesis from the particle gelation, thus allowing the microfluidic droplet templating and the functionalization of the matrix polymer to be performed and controlled in two independent steps We use microfluidic devices to emulsify semidilute solutions of cross-linkable, chemically modified or unmodified poly(N-isopropylacrylamide) precursors and solidify the drops via polymer-analogous gelation The resultant microgel particles exhibit two distinguishable halves which contain most of the modified precursors, and the unmodified matrix polymer separates these materials The spatial distribution of the modified precursors across the particles can be controlled by the flow rates during the microfluidic experiments We also form hollow microcapsules with two different sides (Janus shells) using double emulsion droplets as templates, and we produce Janus microgels that are loaded with a fen magnetic additive which allows remote actuation of the microgels
The elastic modulus, G'(p), of concentrated suspensions of thermoresponsive microgel particles highlights transitions between glassy, liquid-like, and gel-like behavior as the temperature is varied Approaching the low critical solution temperature of the particles, T(c) approximate to 29 degrees C, the material undergoes solid-to-liquid transitions even above random close packing and irrespective of particle concentration
Microfluidic devices can be molded easily from PDMS using soft lithography. However, the softness of the resulting microchannels makes it difficult to photolithographically pattern their surface properties, as is needed for applications such as double emulsification. We introduce a new patterning method for PDMS devices, using integrated oxygen reservoirs fabricated simultaneously with the microfluidic channels, which serve as "chemo-masks". Oxygen diffuses through the PDMS to the nearby channel segments and there inhibits functional polymer growth; by placement of the chemo-masks, we thus control the polymerization pattern. This patterning method is simple, scalable, and compatible with a variety of surface chemistries.
In multiphase flow in confined geometries an elementary event concerns the interaction of a droplet with an obstacle. As a model of this configuration we study the collision of a droplet with a circular post that spans a significant fraction of the cross-section of a microfluidic channel. We demonstrate that there exist conditions for which a drop moves completely around the obstacle without breaking, while for the same geometry but higher speeds the drop breaks. Therefore, we identify a critical value of the capillary number above which a drop will break. We explain the results with a one-dimensional model characterizing the flow in the narrow gaps on either side of the obstacle, which identifies a surface-tension-driven instability associated with a variation in the permeability in the flow direction. The model captures the major features of the experimental observations. Copyright (C) EPLA, 2010
Producing polymeric or hybrid microfluidic devices operating at high temperatures with reduced or no water evaporation is a challenge for many on-chip applications including polymerase chain reaction (PCR). We study sample evaporation in polymeric and hybrid devices, realized by glass microchannels for avoiding water diffusion toward the elastomer used for chip fabrication. The method dramatically reduces water evaporation in PCR devices that are found to exhibit optimal stability and effective operation under oscillating-flow. This approach maintains the flexibility, ease of fabrication, and low cost of disposable chips, and can be extended to other high-temperature microfluidic biochemical reactors. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3481776]
Over several billion years, cyanobacteria and plants have evolved highly organized photosynthetic systems to shuttle both electronic and chemical species for the efficient oxidation of water(1). In a similar manner to reaction centres in natural photosystems, molecular(2) and metal oxide(3) catalysts have been used to photochemically oxidize water. However, the various approaches involving the molecular design of ligands(4), surface modification(5) and immobilization(6,7) still have limitations in terms of catalytic efficiency and sustainability. Here, we demonstrate a biologically templated nanostructure for visible light-driven water oxidation that uses a genetically engineered M13 virus scaffold to mediate the co-assembly of zinc porphyrins (photosensitizer) and iridium oxide hydrosol clusters (catalyst). Porous polymer microgels are used as an immobilization matrix to improve the structural durability of the assembled nanostructures and to allow the materials to be recycled. Our results suggest that the biotemplated nanoscale assembly of functional components is a promising route to significantly improved photocatalytic water-splitting systems.
Intermediate filaments are common structural elements found in abundance in all metazoan cells, where they form networks that contribute to the elasticity. Here, we report measurements of the linear and nonlinear viscoelasticity of networks of two distinct intermediate filaments, vimentin and neurofilaments. Both exhibit predominantly elastic behavior with strong nonlinear strain stiffening. We demonstrate that divalent ions behave as effective cross-linkers for both networks, and that the elasticity of these networks is consistent with the theory for that of semiflexible polymers.
We study the geometry of biopolymer networks and effects of the geometry on bulk mechanical properties. It is shown numerically that the physical network geometry can be quantified statistically and regenerated from its statistical description, so that the regenerated network exhibits the same network mechanics as the physical network in the elastic regime. A collagen-I biopolymer network is used for validation. The method enables parametric studies of the network geometry, whose parameters are often difficult to vary independently in experiments.
Intermediate filament networks in the cytoplasm and nucleus are critical for the mechanical integrity of metazoan cells. However, the mechanism of crosslinking in these networks and the origins of their mechanical properties are not understood. Here, we study the elastic behavior of in vitro networks of the intermediate filament protein vimentin. Rheological experiments reveal that vimentin networks stiffen with increasing concentrations of Ca(2+) and Mg(2+), showing that divalent cations act as crosslinkers. We quantitatively describe the elastic response of vimentin networks over five decades of applied stress using a theory that treats the divalent cations as crosslinkers: at low stress, the behavior is entropic in origin, and increasing stress pulls out thermal fluctuations from single filaments, giving rise to a nonlinear response; at high stress, enthalpic stretching of individual filaments significantly modifies the nonlinearity. We investigate the elastic properties of networks formed by a series of protein variants with stepwise tail truncations and find that the last 11 amino acids of the C-terminal tail domain mediate crosslinking by divalent ions. We determined the single-filament persistence length, l(P) approximate to 0.5 mu m, and Young's modulus, Y approximate to 9 MPa; both are consistent with literature values. Our results provide insight into a crosslinking mechanism for vimentin networks and suggest that divalent ions may help regulate the cytoskeletal structure and mechanical properties of cells. (C) 2010 Elsevier Ltd. All rights reserved.