Dynamics and structure formation in field-driven suspensions and emulsions

Electric and magnetic fields are powerful tools commonly used to manipulate multiphase soft materials, with applications such as electrocoalescence, electro- and magnetorheology, and directed self-assembly. Imposition of an external field induces microscale interactions between drops or particles suspended in fluid, resulting in relative motion of the constituent objects. These interactions give rise to the development of structures much larger than the individual drop or particle size, which can have significant impact on the macroscale properties of complex fluids, such as viscosity or conductivity. To engineer controllable, reconfigurable materials using external fields, the effect of the microscale interactions on the resulting structure and dynamics of the suspensions needs to be quantified. In this thesis, we build a platform for the analysis of emulsions of drops in electric fields, and use that platform to understand field-driven pattern formation in a more general context. The impact of this work is the prediction of emulsion behavior in electric fields, and a new understanding of the process-ability of suspensions of dipolar particles.

We first derive an expression, using the asymptotic method of reflections, for the pairwise interaction of drops in a uniform electric field. The interplay between electrohydrodynamic (EHD) flow and the dielectrophoretic force is determined to control the nature of the interaction between a drop pair, demonstrated with both theory and experiments. Next, we simulate the behavior of large-scale emulsions of drops under a uniform electric field, governed by the pair interactions from our asymptotic model. The EHD flow can vastly alter the formation of chains commonly observed with DEP alone, and emulsions can ungergo a variety of behaviors, such as accelerated chain formation, the formation of dynamic, wavy sheets, and the active motion of drop pairs, all accessible due to the presence of EHD interactions. We then extend our focus to suspensions of polarizable particles under electric or magnetic fields. We introduce a generalized model for dipolar interactions, and show that this model recovers the assembly of drops into zig-zag patterns that span orthogonal to the field axis, an experimentally observed behavior not captured with the classical dipole-dipole model. Generally, we show that the manipulation of the pairwise interactions of constituent objects in soft materials can give rise to a variety of tunable, macroscale responses. Using theory, experiments, and computation, we make significant progress toward the development of designer materials using external fields.