Ali Mohraz

Characteristics of Pickering Emulsion Gels Formed by Droplet Bridging

We experimentally characterize the microstructure and rheology of a carefully designed mixture of immiscible fluids and near-neutral-wetting colloidal particles. Particle bridging across two fluid interfaces provides a route to highly stable gel-like emulsions at volume fractions of the dispersed phase well below the random close-packing limit for spheres. We investigate the microstructural origins of this behavior by confocal microscopy and reveal a percolating network of colloidal particles that serves as a cohesive scaffold, bridging together droplets of the dispersed phase. Remarkably, the mixtures salient rheological characteristics are governed predominantly by the solids loading and can be tailored irrespective of the droplet volume fraction. The identification of this rheological hallmark could provide a means toward the improved design of modern products that utilize solid-stabilized interfaces.

Orientation and rupture of fractal colloidal gels during start-up of steady shear flow

The transient structural evolution of polystyrene colloidal gels with fractal structure is quantified during start-up of steady shear flow by time-resolved small-angle light scattering and rheometry. Three distinct regimes are identified in the velocity-gradient plane: structural orientation, network breakup, and cluster densification. Structural anisotropy in the first regime is a universal function of applied strain. Flow cessation in this regime shows a lack of structural relaxation for Pe⪡1, where Pe is the Peclet number. In the second regime, the anisotropy attains a maximum value before monotonically decreasing. The volume fraction dependence of the critical strain for maximum anisotropy follows the scaling: 1+0.6γc,r∼ϕ(1−x)(3−D). Here x and D are the backbone and cluster fractal dimensions, respectively. This scaling agrees with the simple model of a gel network that ruptures after the cluster backbone is extended affinely to its full length. Rheological measurements demonstrate that the maximum an...

Hierarchically porous silver monoliths from colloidal bicontinuous interfacially jammed emulsion gels.

Silver monoliths with interconnected hierarchical pore networks and three-dimensional (3D) bicontinuous morphology are synthesized from a colloidal bicontinuous interfacially jammed emulsion gel (bijel) via reduction of silver ions within a nanoporous cross-linked polymer template. The pore sizes may be tuned independently and range from tens of nanometers to over a hundred micrometers. The method is straightforward as well as flexible and can pave the way to a host of hierarchical materials for current technologies.

Microstructural response of dilute colloidal gels to nonlinear shear deformation

The time-resolved microstructural response of dilute, depletion-induced colloidal gels prepared in a density and refractive index matched solvent, to nonlinear shear deformation was investigated in 3D by fast scanning confocal microscopy in a custom-built cone-and-plate shear cell. Two sets of experiments were performed by manipulating the connectivity of the gel network with the stationary plate, thereby changing the flow boundary conditions. The gel structure evolves from its quiescent state via local rearrangement, rupture, and densification, first to a highly anisotropic network oriented near the extensional component of the shear flow field, and eventually to a mixture comprised of dense clusters and large voids. The transitions between these stages are highly sensitive to the boundary condition at the stationary plate. Our findings indirectly support the notion of soft pivot points along the backbone of dilute colloidal gels with centrosymmetric interactions, and will have important implications for the nonlinear rheology of colloidal gels and other structured fluids.

Bijel reinforcement by droplet bridging: a route to bicontinuous materials with large domains

Bijels are non-equilibrium solid-stabilized emulsions with bicontinuous arrangement of the constituent fluid phases. These multiphase materials spontaneously form through arrested spinodal decomposition in mixtures of partially miscible liquids and neutrally wetting colloids. Here, we present a new solid-stabilized emulsion with an overall bicontinuous morphology similar to a bijel, but with one continuous phase containing a network of colloid-bridged droplets. This dual morphology is the result of combined spinodal decomposition and nucleation and growth in a binary liquid mixture containing colloidal particles with off-neutral wetting properties and partial affinity for one liquid phase. The rheology of these systems, which we call bridged bijels, is nearly identical to their simple bijel counterparts, with a unique exponential dependence of the zero-shear elastic modulus on the colloid volume fraction. However, partitioning of the colloids between the spinodal surface and the fluid domains delays the onset of structural arrest, providing access to domain sizes much larger than available in simple bijels without loss of mechanical stability. This ability greatly expands the potential technological applications of these unique materials. In addition, our findings reveal new strategies for tuning the rheology of bijels and outline new directions for future fundamental research on this unique class of soft materials.

Relationship between microstructure, dynamics, and rheology in polymer-bridging colloidal gels.

We investigate the link between the microstructure, dynamics, and rheological properties in dense (phi = 0.3) mixtures of charge-stabilized colloidal silica and oppositely charged poly(ethylene imine) polymer in a mixed DMSO/H(2)O solvent. Over a finite range of polymer concentrations, the addition of polymer results in the formation of sample-spanning, self-supporting gel networks. As the polymer concentration is increased, a reentrant rheological transition is observed where the gels elastic modulus and yield stress initially increase and subsequently drop. The dynamic and microstructural changes associated with this transition are resolved using quantitative confocal microscopy. Within the initial regime, a biphasic system consisting of a mixture of arrested and diffusive particles is observed. We segregate the particles with high accuracy into mobile and arrested populations based on their dynamics. The addition of polymer in this regime systematically decreases the proportion of free particles, until all the particles are arrested. Concurrent with this transition, the elastic modulus and yield stress go through their corresponding maxima. However, over the range of polymer concentrations studied, the reentrant transition to weak gels is not captured by the particle dynamics but is instead accompanied by subtle changes in the microstructure of the arrested phase. We discuss two possible scenarios for this behavior in view of the strength of interparticle bonds.

Microstructural tunability of co-continuous bijel-derived electrodes to provide high energy and power densities

Emerging demands for national security, transportation, distributed power, and portable systems call for energy storage and conversion technologies that can simultaneously deliver large power and energy densities. To this end, here we report three-dimensional Ni/Ni(OH)2 composite electrodes derived from a new class of multi-phase soft materials with uniform, co-continuous, and tunable internal microdomains. These remarkable morphological attributes combined with our facile chemical processing techniques allow the electrodes salient morphological parameters to be independently tuned for rapid ion transport and a large volumetric energy storage capacity. Through microstructural design and optimization, our composite electrodes can simultaneously deliver energy densities equal to that of batteries and power densities equivalent to or greater than that of the best supercapacitors, bridging the gap between these modern technologies. Our synthesis procedure is robust and can be extended to a myriad of other chemistries for next generation energy storage materials.

Steady shear microstructure in dilute colloid–polymer mixtures

The shear-induced microstructure in dilute colloid–polymer mixtures, where the presence of polymer induces a tuneable attractive interaction between the colloids, is investigated using quantitative confocal microscopy, over a wide parameter space including the shear rate, the polymer concentration, and two different polymer molecular weights corresponding to polymer/colloid size ratios of approximately 0.02 and 0.05. Overall, the imposition of low shear rates radically transforms the relatively uniform quiescent structure into one marked by long-range heterogeneities and pronounced segregation of dense clusters and voids. Increasing the rate of deformation effects a consistent decrease in the average cluster size and a gradual transition towards a more homogeneous structure through the redistribution of voids. Interestingly, at high shear rates, the suspension microstructure for the large molecular weight polymer is nearly insensitive to the polymer concentration, and primarily determined by the shear rate alone. The prominent microstructural features of these shear-induced transformations are quantified in detail and discussed in light of the competition between interparticle attraction and microscopic shear forces.

Modulating colloidal adsorption on a two-dimensional protein crystal.

The geometric and physicochemical properties of the protein streptavidin make it a useful building block in the construction and manipulation of nanoscale structures and devices. However, one requirement in exploiting streptavidin for bottom-up assembly is the capability to modulate protein-nanoparticle interactions. This work examines the effects of pH and the biotin-streptavidin interaction on the adsorption of colloidal gold onto a two-dimensional streptavidin crystal. Particle deposition was carried out below (pH 6), at (pH 7), and above (pH 8) the proteins isoelectric point with both biotinylated and nonbiotinylated nanoparticles. Particle surface coverage depends on deposition time and pH, and increases by 1.4-10 times when biotin is incorporated onto the particle surface. This coverage is highest for both particle types at pH 6 and decreases monotonically with increasing pH. Calculations of interparticle potentials based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory demonstrate that this trend in surface coverage is most likely due to alterations in particle-surface electrostatic interactions and not a result of changes in interparticle electrostatic repulsion. Furthermore, post-adsorption alterations in pH demonstrate that electrostatically adsorbed particles can be selectively desorbed from the surface. Evaluation of the nonspecifically adsorbed fraction of biotinylated particles indicates that the receptor-ligand adsorption mechanism gives a higher rate of attachment to the substrate than nonspecific, electrostatic adsorption. This results in faster adsorption kinetics and higher coverages for biotinylated particles relative to the nonbiotinylated case.

Expanding functionality of recombinant human collagen through engineered non-native cysteines.

Collagen is the most abundant protein in extracellular matrices and is commonly used as a tissue engineering scaffold. However, collagen and other biopolymers from native sources can exhibit limitations when tuning mechanical and biological properties. Cysteines do not naturally occur within the triple-helical region of any native collagen. We utilized a novel modular synthesis strategy to fabricate variants of recombinant human collagen that contained 2, 4, or 8 non-native cysteines at precisely defined locations within each biopolymer. This bottom-up approach introduced capabilities using sulfhydryl chemistry to form hydrogels and immobilize bioactive factors. Collagen variants retained their triple-helical structure and supported cellular adhesion. Hydrogels were characterized using rheology, and the storage moduli were comparable to fibrillar collagen gels at similar concentrations. Furthermore, the introduced cysteines functioned as anchoring sites, with TGF-β1-conjugated collagens promoting myofibroblast differentiation. This approach demonstrates the feasibility to produce custom-designed collagens with chemical functionality not available from native sources.