Electrohydrodynamic and Thermocapillary Effects on Thin-Film Flows

Abstract

Controlling thin liquid film flows is a problem that has implications for technologies such as microelectronics and microfluidics. As these types of devices continue to become both smaller and more complex, our ability to manipulate liquids at small length scales will become increasingly critical. In this thesis we study several problems which advance our understanding of how electric and temperature fields can be harnessed to manipulate thin liquid film flows. First, we study how the combined application of normal electric and temperature fields can be used for the patterning of thin polymeric liquid films using a linear stability analysis and nonlinear simulations. For perfect dielectric liquids we find that thermocapillary forces arising from the temperature gradient dominate the patterning process, rendering the electrohydrodynamic forces nearly negligible. For leaky dielectric liquids, charge which accumulates at the liquid-air interface generates shear stresses which contribute significantly to the patterning process by reducing feature size and patterning time. Inclusion of viscoelasticity in our model shows that rheology affects the rate of patterning but not the length scale of the pattern. Second, we use nonlinear simulations to examine electrohydrodynamic and thermocapillary effects on gravity-driven droplet spreading. We find that in perfect dielectric liquids, the electric field modifies the liquid-air interface but will not alter the long-time spreading rate. However in leaky dielectric liquids, the buildup of surface charge can greatly alter the long-time spreading dynamics by causing separation of the droplet into a series of smaller droplets. In both cases, thermocapillary forces imposed by cooling the film from below can negate the effects of the electric field. We also find that partially wetting liquids are more susceptible to droplet separation in both perfect and leaky dielectric liquids. Finally, we conduct a linear stability analysis to study electrohydrodynamic and thermocapillary effects on the gravity-driven spreading of thin liquid films. We find that both electric and temperature fields can be used to stabilize the advancing contact line of the liquid film to transverse perturbations. We perform an energy analysis and find complex interactions between the traveling wave solution and the perturbations which shed light on the mechanism behind this stabilization.