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Surface and interfacial structures induced by electrohydrodynamic instabilities
Subjecting a liquid/liquid interface to an electrohydrodynamic pressure enhances fluctuations of a characteristic wavelength, leading to an instability and eventually the formation of well-defined columnar structures. Extending the linear stability analysis of a single fluid interface to a liquid/liquid bilayer produced generalized results applicable to any interface. Countering the electrohydrodynamic pressure is the Laplace pressure, which is dictated by the surface energy or the interfacial energy. Consequently, the characteristic length scale for the bilayer instability is reduced. Results presented for different polymer bilayers under a wide range of experimental conditions show quantitative agreement with the generalized theory with no adjustable parameters. Data over four orders of magnitude in reduced wavelength and field strength can be described by the theory. External electric fields are also used to amplify interfacial fluctuations in a air/polymer/polymer system where one polymer dewets the other. Two different hydrodynamic regimes are found as a function of electric field strength. For weak fields, heterogeneous nucleation can lead to the formation of holes before the electrostatically driven instability sets in and the dewetting kinetics are not influenced by the electric field. Stronger electric fields lead to a spinodal instability that causes the formation of polymer columns on top of the second polymer. In addition, the analysis of the polymer-polymer interface during the early stage of the instability indicates a slip boundary condition for the upper layer on the lower fluid substrate. Columnar structures with a characteristic hexagonal order increase in diameter at a rate dictated by a balance of the forces exerted on the surface of the columns and on the underlying reservoir. If the reservoir is exhausted, or if dewetting occurs, the columns are isolated and growth is arrested, kinetically trapping the size of the columns. An alternative way to control structure formation at the surface of a thin liquid film is presented by creating topographical patterns comprised of stripes with well defined widths. It is seen that undulations beneath a stripe pattern lead to the formation of columns. The width of the stripes is seen to markedly alter the wavelength of lateral growth of the fluctuations.
Lin, Zhiqun, "Surface and interfacial structures induced by electrohydrodynamic instabilities" (2003). Doctoral Dissertations Available from Proquest. AAI3078703.