UCLA

Dense colloidal emulsions represent a captivating category of soft materials, boasting a vast range of practical applications in industry and consumer goods. To design and tailor the mechanical properties of concentrated emulsions for specific applications, a comprehensive quantitative understanding of emulsion rheology is essential. In practice, colloidal emulsions are not typically monodisperse with nearly hard repulsive interactions between droplets; instead, they often exhibit depletion attractive interactions induced by excess ionic surfactant molecules, other additives, or even polydispersity. In this dissertation, we outline the advancements we have achieved in quantitatively describing the linear plateau elastic shear modulus, G′p, of depletion attractive emulsions and extremely bidisperse colloidal emulsions, coupled with their optical transport and dynamic properties. Our investigation also encompasses the unjamming behavior and the exploration of potential molecular-probing platforms for microrheology, employing a combination of experimental, analytical, and computational approaches with a primary focus on oil-in-water (O/W) nanoemulsions. Our first focus is on the quantitative microrheology of attractive emulsions. We employ diffusing wave spectroscopy (DWS) microrheology analysis for quantifying the rheological properties, particularly G′p, of depletion-induced attractive emulsions having various attractive potential at contact |Ud|; and at each given |Ud|, we investigate G′p over a wide range of droplet volume fraction, φ. We show that on top of correcting for collective light scattering effects present in highly scattering concentrated colloidal systems through an empirically determined average structure factor, it is necessary to apply an effective scattering probe size factor associated with the resulting corrected mean square displacements (MSDs) in the generalized Stoke-Einstein relation (GSER) of passive microrheology, and thereby lead to accurate values of G′p for attractive emulsions. By developing the sophisticated decorated core-shell network (DCSN) model for strongly attractive emulsions and the extended DCSN model for intermediately attractive emulsions, along with our discovery of moderately attractive emulsions, we systematically and self-consistently understand the effective probe size that depends on both φ and the depletion attractive strength. To broaden our investigation of the rheology of attractive emulsions, we explore highly bidisperse mixtures of microscale emulsions and nanoemulsions. In these polydisperse mixtures, nanodroplets form repulsively jammed glasses while simultaneously acting as depletion agents, which can lead to the formation of attractive gels by microscale droplets. We demonstrate that at lower microscale droplet volume fractions, φEM, far below its jamming point, nanodroplets predominantly contribute to the bulk shear elasticity. In this condition, microscale droplets serve as elastic inclusions residing within the jammed matrix of nanodroplets without weakening the system’s elasticity. At higher φEM, near yet below jamming, the nanodroplet-induced depletion attractions cause larger droplets to form percolating gel networks, which contribute to the macroscopic shear rigidity. Motivated by advances in DWS microrheology for a diverse range of colloidal emulsions, we explore the potential of applying molecule-probing techniques to microrheology. Experimentally, we demonstrate the unjamming behavior of customized 19F-laden nanoemulsions, stabilized by non-fluorinated ionic surfactants, using 19F pulsed-field gradient nuclear magnetic resonance (PFG-NMR) measurements. Our findings reveal dramatic changes in NMR magnetization decays at high field-gradient strengths as φ is lowered through dilution. We show that this dramatic change coincides with the loss of low-frequency shear elasticity of the nanoemulsion, which occurs at the φ associated with Lindemann melting criterion. More- over, we present a trajectory-based simulation study that illustrates passive microrheology by analyzing the MSDs of a Brownian probe molecule confined within a droplet undergoing harmonically bound Brownian motion. Our simulation approach highlights the significance of small colloid size and high contrast between the viscosities of the dispersed and continuous phases for extracting accurate MSDs of the droplets, thus yielding quantitative microrheological results.