Morphological Evolution and Hysteresis in External Electric Field induced Instability in Viscoelastic Thin Films

Self-organized instability patterns in thin films are of great interest in diverse scientific and technological applications. We investigated the surface instability in a viscoelastic film engendered by externally applied electric field perpendicular to the film surface in a capacitor geometry. Here the main destabilizing force is electrostatic pressure and the restoring effects arising out of stretching of the crosslinked elastomeric film. The external field polarizes the air-polymer interface and the film surface destabilizes itself to align in the direction of the applied field to minimize the energy forming different morphologies. We report here complete bonding-debonding sequence and morphological transformation. During bonding, the patterns appear only when the field strength (U) exceeds critical voltage (U_C), which depends on the shear modulus, gap spacing, and film thickness etc. With an increase in U, initially columns appear which further evolves into labyrinths and holes before the film surface comes in complete conformal contact with the top electrode. During debonding, the patterns appear following exactly opposite morphological sequence. Though the patterns are completely switchable by electric field we have observed hysteresis during de-bonding.