Large Amplitude Oscillatory Shear Rheology of Pickering Emulsions
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.