Typically in the EHD patterning process, the thin liquid film is considered as either a perfect dielectric (PD) or a leaky dielectric (LD). 1–3 The LS analysis shows that the lateral size of pillars decreases by increasing the applied voltage ( V), mean initial film thickness ( h 0), electric permittivity of the film ( ε) and its conductivity ( σ). 1 The maximum wavelength for the growth of instabilities, ( λ) which characterizes the center-to-center distance of pillars in a hexagonal pattern, is predicted by LS analysis. Initial linear stages in the growth of instabilities were investigated through linear stability (LS) analysis. λ is the wavelength of growing instabilities at the interface. 1 2-D Schematic representation of (a) thin liquid film sandwiched between two electrodes and a 3-D schematic of the upper electrode with (b) strip-like (c) square block protrusions. When the electrostatic force overcomes these damping forces, instabilities grow and pillars (raised columnar structures) form at the interface. In contrast, capillary and viscous forces tend to dampen the instabilities. 19 In the EHD patterning process, thicker regions of the film are subjected to a higher electrostatic force compared to thinner regions, thus the electrostatic force increases the instabilities. As a consequence, a net electrostatic force acts at the interface due to the Maxwell stress. The disparity of electrical properties of the film and the bounding fluid (the material which fills the gap between the film and the upper electrode) results in a different electric field in each layer. 17,18 In this process, a thin liquid film is sandwiched between two electrodes and an electric field is applied to the film in the transverse direction (see Fig. 1 Introduction Electrohydrodynamic (EHD) destabilization of molten polymer or liquid films has received extensive attention over the past few decades as a unique and interesting approach for creating micron- and submicron-sized features for soft lithography 1–16 and coatings. The structure size in PD films is reduced by a factor of 4 when they are replaced with IL films, which results in nano-sized features with well-ordered patterns over the domain. This is attributed to better control of the characteristic spatial lengths by applying a heterogeneous electric field by patterned electrodes. Replacing the flat electrode with the patterned one is found to result in more compact and well-ordered structures particularly when an electrode with square block protrusions is used. The validity of our simulation technique is determined from close agreement between the simulation results of a PD film and the experimental results in the literature. Finite differences in the spatial directions using an adaptive time step ODE solver are used to solve the 2-D nonlinear thin film equation. The 3-D spatiotemporal evolution of a thin IL film interface under homogenous and heterogeneous electric fields is numerically simulated. Commonly used perfect dielectric (PD) films are replaced with ionic conductive films to reduce the lateral length scales to a sub-micron level in the EHD pattering process. The influence of electrostatic heterogeneity on the electric-field-induced destabilization of thin ionic liquid (IL) films is investigated to control spatial ordering and to reduce the lateral dimension of structures forming on the films.
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