(352f) On the Reversibility and Chaos in Microscopic Fluid Systems

Soft amorphous materials such as foams, emulsions, granular systems, or colloidal suspensions often display complex flow properties. These complex materials have many applications in chemical, biological and industrial processes. The central quantity of interest for determining complex fluids rheology is the spatial distribution of the particles. Under imposed flow, the ability of the microstructure to rearrange to accommodate flow and interparticle forces determines its macroscopic rheological response. The many-body, shear-history-dependent nature of the microstructure renders the prediction of the complex material’s dynamics and rheology highly nontrivial. Hence a fundamental understanding of the microscopic rearrangements and specifically the transition from hydrodynamic reversible to irreversible behavior is of central importance.

In this study we explore the transition from reversible to chaotic behavior in the oscillatory shear flow of water-in-oil emulsions. The emulsion is flown through a microchannel and is forced to rearrange due to a central constriction in the channel. We study the motion of the individual droplets and their neighbors in order to determine their ability to retain their original position after several cycles of oscillations.

We have found that while at the Stokes flow limit, the emulsion exhibit behaviors that vary from complete reversible to complete irreversible depending on the volume fraction, velocity and strain amplitude. We provide the first direct visualization of this phenomenon. This work is an important step in understanding the microscopic rearrangements of droplets and particles near jamming.