UCLA

Emulsions are an interesting class of soft materials and have a wide range of practical applications in industry and in consumer goods. To design and tailor the mechanical properties of concentrated emulsions at high droplet volume fractions for specific applications and products, it is helpful to have a good quantitative understanding of emulsion rheology. In this dissertation, we describe the improvements that we have made in the quantitative description of the linear plateau elastic shear modulus, G'p, of jammed monodisperse colloidal emulsions that are stabilized by ionic surfactant molecules.

We have created an improved analytical model, which accurately describes the G'p of jammed monodisperse micro- and nano-scale emulsions. We incorporate entropic, electrostatic, and interfacial (EEI) contributions into a quasi-equilibrium free energy while retaining key aspects of random jamming of deformable droplets, and we calculate G'p via free energy minimization. This analytical EEI model successfully describes the empirically measured volume-fraction dependent G'p() for microscale emulsions and nanoemulsions without any ad hoc adjustments to the empirically measured  and with very few adjustable parameters that appear to be universal. In addition, we use this EEI model to identify different -regimes of jamming caused by electrostatic repulsions and droplet interfacial deformations.

Using jammed monodisperse emulsions as model system, we have improved diffusing wave spectroscopy (DWS) microrheology analysis for quantifying the rheological properties of dense colloidal systems, particularly G'p() of jammed repulsive emulsions. We show that we can correct for collective light scattering effects present in highly scattering concentrated colloidal systems through an empirically determined average structure factor and thereby obtain corrected mean square displacements (MSDs), which lead to accurate values of G'p through the generalized Stokes-Einstein relationship (GSER) of passive microrheology. This advance enables accurate optical microrheology measurements of concentrated emulsions over a wide range of frequencies beyond the capabilities of traditional mechanical rheometers. This approach of correcting DWS MSDs for collective scattering is general and can be applied to other types of highly scattering concentrated colloidal dispersions, not just emulsions.

Motivated by advances in DWS microrheology for repulsive emulsions, we perform DWS microrheology studies on depletion-induced attractive emulsions near and below the jamming volume fraction of hard spheres. By adapting the analytical approach developed for repulsive emulsions, we show that in some limits DWS microrheology of attractive emulsions can be extracted and compare accurately with macroscopic mechanical measurements. We reveal systematic features in an excess MSD that is present only for the attractive emulsions, and we attribute this excess MSD to additional dynamics of clusters of droplets that are only loosely attached to the main stress-bearing struts of the main gel network of droplets. More theoretical attention is needed in attractive emulsion systems in order to determine how to analyze DWS MSDs and predict the excess MSDs. Interestingly, these measured excess MSDs can be fit using an empirical modified bound Brownian particle equation that we created to describe these extra fluctuations in the DWS signals. This application of DWS microrheology to attractive emulsions herein serves as a basis from which additional DWS microrheology studies of attractive soft colloidal systems can be performed and analyzed.