Browsing by Subject "Electrohydrodynamics"

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    Electrohydrodynamic and Thermocapillary Effects on Thin-Film Flows
    (2016-01) Corbett, Andrew
    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.
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    Stability of microscale fluid interfaces: a study of fluid flows near soft substrates and pattern formation under electrostatic fields.
    (2009-12) Roberts, Scott Alan
    Surfaces having microscale features are rapidly being developed for applications ranging from microelectronics to biomaterials. In many cases, flowing fluids interact with or are used to create these features. Despite its importance, a fundamental understanding of fluid behavior in these situations is generally lacking. In this thesis, several problems are examined to advance that understanding. First, a linear stability analysis of flow of power-law fluids adjacent to deformable solids is presented. Fluid rheology significantly affects the conditions for instability and may serve as a mechanism for enhancing mixing within microfluidic devices. Second, the use of normally oriented electrostatic fields to create regular topographical patterns on the surfaces of thin polymer films is considered. Linear stability analysis and one-dimensional nonlinear simulations demonstrate how AC electrostatic fields may be used to control the width and height of the pillar-like structures that are formed by this process. Two-dimensional nonlinear simulations are carried out to determine how AC fields affect the arrangement of these pillars into their final patterns. As an extension, the use of trilayer films to create unique pillar-like structures is studied using linear stability analysis and nonlinear simulations. Then, linear stability analysis is applied to study how tangentially oriented electrostatic fields may also be used to create topographical patterns, and the effect of gravity on stability is considered. Finally, a model addressing the stability of viscoelastic fluids under electric fields is proposed. All of these results shed light on how the stability of microscale fluid interfaces can be exploited to improve emerging technological applications.