Electric-field-induced instabilities in thin bilayers composed of either purely viscous or purely elastic films resting on a solid substrate are studied. In contrast to the electric-field-induced instability in a single elastic film, the length scale of the instability for elastic bilayers can be tuned by changing the ratios of the shear moduli, thicknesses, and dielectric permittivities of the films. Linear stability analysis is employed to uncover the variations in the wavelength. The instabilities of the viscous bilayers follow different modes of interfacial evolution: either in-phase bending or antiphase squeezing. Linear and nonlinear analyses show that the mode type can be switched by changing the dielectric permittivities of the films. Nonlinear simulations find a number of intriguing interfacial morphologies: (a) an embedded upper layer in an array of lower layer columns, (b) upper layer columns encapsulated by lower layer beakers, (c) lower layer columns covered by the upper layer liquid resulting in concentric core-shell columns, (d) droplets of upper liquid on a largely undisturbed lower layer, and (f) evolution of two different wavelengths at the two interfaces of the bilayer. The simulated morphology types (a), (b) and (d) have been seen previously in experiments. The effect of the film viscosities on the evolution of the instability and final morphologies is also discussed.

A nonlinear analysis is presented to identify the conditions for the formation of ordered patterns consisting of arrays of core-shell columnar structures in thin viscous bilayers by applying a spatially varying electric field. The influence of the electric field strength, film thickness, and topography of the electrode patterns on the order and morphology of the mesostructures is thus investigated. The material with the higher permittivity in the bilayer forms the core, the other one forming the shell. Bilayers under a uniform electric field show only a hexagonal order, whereas a patterned electrode can impose other more complex ordered arrangements, which upon etching of one of the phases produces three-dimensional imbedded hollow structures. Simulations also capture the onset of a phase-inversion phenomenon where the upper layer descends into the lower layer and the latter rises upward to form simple columns without the core-shell structure. When the permittivity of the lower layer is higher (or lower), the columns formed are spatially synchronized with the electrode protrusions (or depressions).

We study by nonlinear simulations the electric field induced pattern formation in a thin viscous film resting on a topographically or chemically patterned substrate. The thin film microstructures can be aligned to the substrate patterns within a window of parameters where the spinodal length scale of the field induced instability is close to the substrate periodicity. We investigate systematically the change in the film morphology and order when (i) the substrate pattern periodicity is varied at a constant film thickness and (ii) the film thickness is varied at a constant substrate periodicity. Simulations show two distinct pathway of evolution when the substrate-topography changes from protrusions to cavities. The isolated substrate defects generate locally ordered ripplelike structures distinct from the structures on a periodically patterned substrate. In the latter case, film morphology is governed by a competition between the pattern periodicity and the length scale of instability. Relating the thin film morphologies to the underlying substrate pattern has implications for field induced patterning and robustness of inter-interface pattern transfer, e.g., coding-decoding of information printed on a substrate.

We report fabrication of ordered polymeric nanodomains and control of their morphology and size by self-organized intensified dewetting of ultrathin polymer films which are selectively exposed to small doses of electron beam (e-beam). Both positive and negative e-beam tone polymers are used to produce variety of highly regular patterns over large area (∼mm2) in significantly lesser time as compared to e-beam lithography. Dewetting of selectively exposed thin films under a mixture of water and organic solvents enables the instability to grow much faster and in very confined domains. Patterns ranging from straight and cross channels, array of circular and square holes, aligned nanowires and square grid to the array of spherical droplets can be fabricated by selection of e-beam exposure patterns and the dewetting conditions. Fabrication of structures with sharp corners and edges becomes possible because of ultralow interfacial tension of polymer in the liquid mixture. Further, the length scale of pattern can be tuned over a wide range which in some case extends from about tenth of the natural wavelength of instability in dewetting (λ m ) to 2λ m . This is a significant improvement over the dewetting on physico-chemically patterned substrate where alignment of polymer structures is lost when substrate patterns are smaller than half of λ m . The dewetting mechanism of e-beam exposed films is proposed as the change in the effective viscosity of e-beam exposed region that leads to the faster growth of instabilities in the low viscous regions and results in the formation of regularly aligned structures. Nonlinear simulations are carried out which show very good agreement with the experimentally obtained patterns.