Emulsions are mixtures of immiscible fluids, which typically exhibit a thermodynamically unstable droplets dispersed within a continuous body of fluid. Though surfactants have traditionally been used to achieve kinetically stable emulsions, colloids are increasingly being utilized. Today, solid-stabilized emulsions, or Pickering emulsions, are ubiquitous within the consumer market, often found in food formulations and many personal care products. However, to expand their utilization in many emerging technological applications, such as drug delivery, oil recovery and composite materials, a greater fundamental understanding of the role particles play in mediating the microstructure and mechanical behavior of Pickering emulsions is needed. To this end, I have studied a variety of solid-stabilized emulsions spanning a wide range of droplet concentrations. First, the microstructure of high internal phase emulsions is examined utilizing confocal microscopy. These concentrated systems are composed of highly faceted droplets separated by thin films of the continuous fluid. By varying the size of the stabilizing particles used to create a HIPE, it is revealed that larger particles provide greater stability against droplet coalescence, as they more readily adopt a bridged particle (particle which resides on two droplet interfaces) monolayer configuration within the thin films, in lieu of particle bilayers. Second, combining confocal microscopy with rheological measurements I have explored the connection between the microstructure and rheology of solid-stabilized emulsions, and the influence particles have on these properties. In concentrated emulsions, particle excluded volume is shown to strongly impact the microstructural transition from spherical to highly faceted packed droplets, which is coupled to variations in sample rigidity. Pickering emulsion gels are dilute emulsions that feature a tenuous network of faceted droplets, held together by bridged monolayers of particles. Here, particle loading governs the degree of particle bridging within the droplet network, yielding a non-monotonic dependence of the zero-shear elastic modulus. Lastly, I have begun counter-rotation shear experiments, monitored with confocal microscopy to allow direct observation of the impact particles have on droplets deformation and rupture under shear. My fundamental results provide greater insight into the effect particles have on the microstructure and rheology of Pickering emulsions at various compositions and different process conditions.

Emulsions are a group of mixtures made up of two or more immiscible liquids. In 1907, Pickering first described the phenomenon that sufficiently fine-grained solid particles could act as emulsifiers. The solid particles are strongly adsorbed at the liquid-liquid interface to retard droplets coalescence which separates the emulsion into organic and aqueous phase. For a surfactant molecule, the interfacial attachment energy is several kBT which keeps a dynamic equilibrium between interfaces and host solvent, whereas colloidal particles need to overcome hundreds or thousands of kBT to detach from the interface. In various fields, such as food, pharmaceutical, cosmetics and personal care products, particle-stabilized emulsions have received significant attention for their potential applications.

After exposing to power ultrasound, particle-stabilized emulsion system could transit into highly stable gel-like emulsions. The gel-like behavior in these systems is a unique result of the percolating network of droplets that are attached together by a cohesive network of solid particles. The continuous droplet linkage found in particle-stabilized emulsion gels show long-term mechanical stability and resistance to gravitational effects which improve the possibility of improving gel structural life. In addition, the possibility of recoverability for droplet bridging and studies on corresponding microscopic origins and rheological behavior are shown in this work. At macroscopic level, droplet bridging system show stronger structural stability and prolonged mechanical stability.

With a stress-controlled rheometer with cone-and plate geometry, the rheological properties of the emulsion systems were measured. Oscillatory measurement concerning G', the in-phase storage modulus, and G'', the out-of-phase loss modulus were taken into consideration in rheological behavior. The higher the frequency is, the lower the modulus crossover strain is; the larger the ratio (aqueous/ organic) is, the higher the strain of crossover modulus is. In the second time strain sweep tests within entire destroyed samples, it shows a unique rheological behavior comparing to original structure. For a group of ratio of aqueous/organic η in colloidal gels, zero shear modulus G'0 and shear stress τy indicate a signature tendency. At small stress range, Pickering emulsion gels exhibit "creep ringing" vibrating behavior; at large enough stress, the strain increases with increasing stress.

Emulsions are a class of disperse systems consisting of two immiscible liquids. The liquid droplets (the disperse phase) are dispersed in a liquid medium (the continuous phase). In order to disperse two immiscible liquids, a third component is required, namely the emulsifier. The choice of emulsifier is crucial not only for the formation of the emulsion but also for its long term stability. Pickering emulsions are emulsions of any type, either oil-in-water (o/w), water-in-oil (w/o), or even multiple, stabilized by solid particles in place of surfactants. Pickering emulsions are named after S.U. Pickering whose paper is considered the first report of o/w emulsions stabilized by solid particles adsorbed at the surface of oil droplets. The ‘surfactant-free’ character of these emulsions makes them attractive to several applications fields, in particular cosmetic and pharmaceutical applications where surfactants often show adverse effects.

Typically in Pickering emulsions, the droplet interfaces are stabilized by the presence of particles. These particles can modify the flow properties or rheology of the emulsions by modifying the interaction between the droplets. Hence an understanding of emulsion flow properties is of fundamental interest and of value for many applications. This research work is focused on studying and analyzing the Large Amplitude Oscillatory Shear (LAOS) Rheology of Pickering emulsions stabilized by Poly(methyl methacrylate) or PMMA particles. With the help of the MITlaos framework, intra-cycle nonlinearities such as strain stiffening/softening and shear thickening/thinning have been studied for these Pickering emulsions. Firstly, the repeatability aspect of the experimental protocol has been studied followed by the LAOS analyses at three different strain percentages. The analyses have been compared for the slip v/s no-slip cases as well and the elastic and viscous nonlinearities have been quantified. Finally, the prospect of analyzing the LAOS Rheology at two different volume fractions of particles have been performed. In these experiments, elastic and viscous nonlinearities are not observed at low strains but they become more prominent as the strain percentages are increased. Through the use of Chebyshev decomposition of the stress response and alternate moduli to quantify the nonlinearities, we gain physical insight about the sample behavior as it is subjected from low to higher strains/strain-rates. These physical insights can be obscured by conventional rheology characterization test protocols like Small Amplitude Oscillatory Shear (SAOS) techniques.

Another aspect of the thesis work is aimed to combine rheology with imaging using a 20X objective lens on the Discovery Hybrid Rheometer (TA-Instruments, DHR-3). This imaging allowed me to correlate between the droplet deformations and the large amplitude rheology. At low strains, the droplet deformations are reversible, as observed from the comparison of the images taken before and after the sample is subjected to shear. At large strains, however, the droplet deformations are irreversible. As the emulsion yields from being ‘solid-like’ to ‘liquid-like’ in response to the increasing strain, the permanent droplet deformations become kore prominent. The observations of the imaging studies using the DHR are complemented by the results of the LAOS analyses.

Next-generation energy storage materials aim to combine the best characteristics of batteries and supercapacitors, producing devices that concurrently deliver high energy and power densities. In order to achieve this goal, the kinetics of ion and electron transport within the electrodes must be enhanced while maintaining a large volume fraction of electrolytically active material for energy storage. In this regard, the idealized electrode microstructure has been envisioned as a three-dimensional, co-continuous arrangement of percolating domains that allow for large interfacial contact between the constituent phases and low-resistance paths for ion and electron transport. In this dissertation, I report a novel method to produce this unique microstructure through the use of soft matter templates derived from bicontinuous interfacially jammed emulsion gels (bijels). These non-equilibrium soft materials inherently have a co-continuous microstructure, where two interpenetrating fluid domains are kinetically arrested through jamming of particles at the fluid-fluid interface during spinodal decomposition. The neutrally-wetting colloids adsorb to the fluid interface upon phase separation and become jammed when the interfacial area is sufficiently reduced to just accommodate them. The characteristic domain size, which establishes both the pore diameter and the internal surface area, is controlled solely through the overall colloid volume fraction and can be tuned over a wide range. Through simple chemical post processing steps, bijels are converted to electrolytically active composites. Combined, the morphological control afforded during bijel formation and our chemical processing steps allow for independent tailoring of the microstructural parameters that govern the electrode’s electrochemical performance. In this work, I present composite electrodes that I have synthesized through this route and demonstrate how their salient electrochemical characteristics are dictated by these morphological parameters, allowing for concurrent delivery of high energy and power, bridging the gap between modern batteries and supercapacitors.

Bicontinuous interfacially jammed emulsion gels, or bijels, is a class of soft material uniquely characterized by its co-continuous and interpenetrating fluid domains. These novel materials are formed by jamming nano-scale colloidal particles at the interface between two fluids undergoing spinodal decomposition. As such, the internal microstructure of the resulting bijel bears a striking resemblance to the morphology of spinodal decomposition. This unique morphology exhibits attractive transport potentials that are desired in the development of functional porous materials such as bio-implants, electrochemical devices, catalytic devices, separation membranes, and structural supports. However, there is a lack of understanding of the mechanisms that impart stability in bijels, which significantly limits the large-scale production of bijels and their derivatives. To this end, I have performed extensive rheological characterizations to examine the roles of particles and the interface within bijels to elucidate the origin of mechanical stability. First, by testing bijels at different temperatures and particle volume fractions, it was revealed that interfacial jamming alone is sufficient to impart stability in bijels. Second, bijels were deformed near their yield limits to explore the connection between particle jamming and the continuous interface which percolates the sample. In the transition to yielding, bijels uniquely exhibit a self-rejuvenating quality where shear-induced stretching of the interface is reversible by interfacial compaction. Incorporating approximate particle dynamics under dilute conditions into a simple rheological model produces stress responses that closely mimic the nonlinear rheological response measured from bijels. Lastly, I have developed a bijel templating method that bypasses a significant limiting factor of existing protocols, specifically the addition of monomer to bijels which disrupts the stability of the particle-laden interface. Bijel templating was simplified by forming bijels using partially miscible mixtures of solvent and monomer and directly polymerizing them into porous polymeric scaffolds for further processing. My results provide insights into the fundamental physics of bijels and pave ways for mass production of bijel-derivatives.

This thesis presents an engineered approach to create bicontinuous interfacially jammed emulsion gels (bijels), achieved through the kinetic arrest of temperature-induced spinodal decomposition within partially miscible binary fluids. The subsequent incorporation of these spinodal structures into 3D porous polymers forms the basis of bijel templated materials (BTMs), which offer distinct advantages over traditional porous materials. These advantages encompass tunable domain size, enhanced structural integrity, continuous morphology, and deformability. Initial methods to solidify bicontinuous architectures into interpenetrating porous scaffolds faced limitations in selecting suitable polymers, creating different geometries, and elongated processing times. To address these challenges, a new technique called intrinsically polymerizable bijels (IPBs) was developed, where one of the bicontinuous phases is a monomer. The aim of this research is to create a degradable BTM, serving as a bicontinuous mold, allowing for filling the void space with a polymerizable material followed by matrix degradation. Successful synthesis of disulfide diacrylate was achieved high yields where partial miscibility of 1,2-pentanediol was found to have a transition temperature below room temperature. By experimentally determining the coexistence curve based on fluids miscibility, both the critical mass fraction and critical transition temperature were identified. Neutrally wetting nanoparticles were synthesized, and their surface was functionalized with various silanizing agents to optimize consistent results for proper bijel formation. Fine-tuning of polymerization and quenching conditions led to the production of a degradable porous BTM sheet. This process introduces a bijel templated invertible mold (BTIM) to expand the spectrum of functional materials with spinodal microstructures, with potential applications as porous coatings to enhance material and surface functionalization and interactions with its biological interface

Increasing demands for energy storage and conversion has fueled research and development of next-generation electrochemical devices, including batteries, supercapacitors, and catalysts. Viability of technology beyond the proof-of-concept is dependent on the morphology and topology of the electrode, which should be carefully designed to maximize material utilization during operation. Despite the close relationship between microstructure and operational efficiency, there does not currently exist a single configuration that can be broadly applied across electrochemically active materials. The ideal microstructure is expected to have co-continuous and interpenetrating domains that have high interfacial area and present minimal resistance to ionic and electronic transport.

In the following dissertation, I present a technique to create such a structure through bicontinuous interfacially jammed emulsion gels (bijels), which are generated via spinodal decomposition and therefore confer characteristic microstructural qualities to derivative materials. Domain size distribution, interfacial curvature, tortuosity, and self-similarity are discussed in detail and compared quantitatively to alternative microstructures that have been proposed for electrochemical devices. These qualities are shown to influence material utilization in two specific applications, energy storage in zinc electrodes, and electrocatalytic water splitting for hydrogen generation.

Maintaining electronic conductivity in electrodes has been shown previously to delay capacity loss during repeated charge-discharge cycling. In the case of zinc anodes, converting the metallic material to the semiconducting zinc oxide creates heterogeneities in current distribution that inspire material reconfiguration and premature cell failure. Spinodal-like electrodes mitigate these effects by improving electronic accessibility of the active material and maintaining conduction pathways to a high degree of discharge compared to electrodes built with randomly sized features.

Homogeneous activity and co-continuity in an electrode are also advantageous in the electrocatalytic separation of water into constituent hydrogen and oxygen gases. Product desorption from the electrode is necessary to continue the reaction, but microstructural impediments to efficient removal result in the underutilization of active surfaces. Bijel templated electrodes improve gas transport through the same microstructural qualities discussed above and are shown to reduce energy losses associated with this inefficiency.

Biomaterials are engaged ubiquitously to regenerate or replace damaged or diseased tissues. Numerous processing techniques aim to impart interconnected, porous structures within biomaterials to support cell delivery, direct tissue growth, and increase the acceptance of foreign materials in the body. Many processing techniques lack predictable control of scaffold architecture, and rapid prototyping methods are often limited by time-consuming, layer-by-layer fabrication of micro-features appropriate for biomaterials applications. Further, scaffold architecture is implicated in the body’s innate ability to isolate foreign substances making mitigation of this foreign body response (FBR) essential to ensuring the longevity of implanted biomaterials and devices. Bicontinuous interfacially jammed emulsion gels (bijels) offer a robust, self-assembly-based platform for synthesizing a new class of morphologically distinct biomaterials. Bijels form via kinetic arrest of temperature-driven spinodal decomposition in partially miscible binary liquid systems. These non-equilibrium soft materials comprise co-continuous, fully percolating, non-constricting liquid domains separated by a nanoparticle monolayer. In this dissertation, fluid incompatibility in bijels is exploited to process biocompatible precursors to form hydrogel scaffolds displaying the morphological characteristics of the parent bijel template. Bijel-derived materials are first used to generate structurally unique, fibrin-loaded polyethylene glycol hydrogel composites to demonstrate a new, robust cell delivery system. Next, bijel-derived materials are investigated as tissue integrating implants with high vascularization and FBR mitigation potential stemming from their uniquely arranged pore morphology, presenting a new paradigm for designing long-lasting biomaterials.