Stability of microscale fluid interfaces: a study of fluid flows near soft substrates and pattern formation under electrostatic fields.

University of Minnesota Ph.D. dissertation. December 2009. Major: Chemical Engineering. Advisor: Satish Kumar. 1 computer file (PDF); xv, 165 pages, appendices A-B.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