The microstructure of Pickering emulsion gels features a tenuous network of faceted droplets, bridged together by shared monolayers of particles. In this investigation, we use standard oscillatory rheometry in conjunction with confocal microscopy to gain a more comprehensive understanding of the role particle bridged interfaces have on the rheology of Pickering emulsion gels. The zero-shear elastic modulus of Pickering emulsion gels shows a non-monotonic dependence on particle loading, with three separate regimes of power-law and linear gel strengthening, and subsequent gel weakening. The transition from power-law to linear scaling is found to coincide with a peak in the volume fraction of particles that participate in bridging, which we indirectly calculate using measureable quantities, and the transition to gel weakening is shown to result from a loss in network connectivity at high particle loadings. These observations are explained via a simple representation of how Pickering emulsion gels arise from an initial population of partially-covered droplets. Based on these considerations, we propose a combined variable related to the initial droplet coverage, to be used in reporting and rationalizing the rheology of Pickering emulsion gels. We demonstrate the applicability of this variable with Pickering emulsions prepared at variable fluid ratios and with different-sized colloidal particles. The results of our investigation have important implications for many technological applications that utilize solid stabilized multi-phase emulsions and require a priori knowledge or engineering of their flow characteristics.

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 electrode's 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.

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.

We introduce a custom-built stress-controlled shear cell coupled to a confocal microscope for direct visualization of constant-stress shear deformation in soft materials. The torque generator is a cylindrical Taylor–Couette system with a Newtonian fluid between a rotating inner bob and a free-to-move outer cup. A spindle/cone assembly is coaxially coupled to the cup and transfers the torque exerted by the fluid to the sample of interest in a cone-and-plate geometry. We demonstrate the performance of our device in both steady-state and transient experiments with different viscoelastic materials. Our apparatus can conduct unidirectional constant-stress experiments as accurately as most commercial rheometers, with the capability to directly visualize the flow field using tracer particles. Further, our step-stress experiments on viscoelastic materials are devoid of creep ringing, which is an advantageous aspect of our torque generation mechanism. We believe that the device presented here could serve as a powerful and cost-effective tool to investigate the microstructural determinants of nonlinear rheology in complex fluids.

Polystyrene colloids exhibit unique bimodal dynamics at the air–water interface in the presence of PEG polymers. We attribute this to agglomeration of the polymer into polymer-rich and polymer-sparse domains at the interface. Colloids adsorbed to polymer-depleted regions retain mobility and self-assemble into two-dimensional crystals with square symmetry, which is ascribed to an undulated contact line on the particles and the droplet's negative Gaussian curvature. To our knowledge, this is the first example of thermodynamically favoured two-dimensional square crystals formed by means of interfacial colloid assembly.

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.

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 mixture’s 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.

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.

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.

We investigate the link between the microstructure, dynamics, and rheological properties in dense (ϕ = 0.3) mixtures of charge-stabilized colloidal silica and oppositely charged poly(ethylene imine) polymer in a mixed DMSO/H2O 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 gel’s 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.

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 electrode's 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.