A new mechanism for the surface instability and dewetting of thin films on chemically heterogeneous substrates is identified and simulated. The time scale for instability varies inversely with the potential difference due to the heterogeneity. Heterogeneities can even destabilize spinodally stable films, reduce the time of rupture substantially for thicker films, and produce complex and locally ordered morphological features (e.g., ripples and castle-moat structures, lack of undulations before hole formation) that are not predicted by the spinodal mechanism.

Electrostatic field induced instability, morphology, and patterning of a thin liquid film confined between two electrodes with an air gap are studied on the basis of nonlinear 3D simulations, both for spatially homogeneous and heterogeneous fields. In addition to the spinodal flow resulting from the variation of field because of local thickness changes, a heterogeneous imposed field also moves the liquid from the regions of low field to high field, thus allowing a more precise control of pattern. Hexagonal packing of liquid columns is observed for a spatially homogeneous electric field, which is in accord with the e-field experiments on thin polymer films (Schaffer et al. Nature 2000, 403, 874). For a large liquid volume fraction in the gap, varphi > or = 0.75, the coalescence of columns causes a phase inversion, leading to the formation of air columns or cylindrical holes trapped in the liquid matrix (air-in-liquid dispersion). Locally ordered aligned patterns are formed by imposing a spatial variation of the electrostatic field by using a topographically patterned electrode. For example, multiple rows/lines of liquid columns are formed near the edge of a step-like heterogeneity of the electrode and annular rings of ordered columns or concentric ripples are formed around a heterogeneous circular patch. Simulations predict that the electrode pattern is replicated in the film only when the pattern periodicity, L(p), exceeds the instability length scale on the basis of the minimum interelectrode separation distance, L(p) > or = lambda(m)-d(min). Thus, the formation of secondary structures can be suppressed by employing an electrode with deep grooves and stronger field gradients, which produces almost ideal templating. The number density of the electric field induced patterns can be altered by tuning the mean film thickness (or the volume fraction of liquid in the gap), periodicity and depth (amplitude) of the grooves on the top electrode, and the applied voltage. The implications are in electrostatic lithography, pattern replication in soft materials, and the design and interpretation of thin film experiments involving electric fields.

The instability, dynamics and morphological transitions of patterns in thin liquid films on periodic striped surfaces (consisting of alternating less and more wettable stripes) are investigated based on 3-D nonlinear simulations that account for the inter-site hydrodynamic and surface-energetic interactions. The film breakup is suppressed on some potentially destabilizing nonwettable sites when their spacing is below a characteristic lengthscale of the instability (λ h ), the upper bound for which is close to the spinodal lengthscale. The thin film pattern replicates the substrate surface energy pattern closely only when, (a) the periodicity of substrate pattern matches closely with the λ h , and (b) the stripe-width is within a range bounded by a lower critical length, below which no heterogeneous rupture occurs, and an upper transition length above which complex morphological features bearing little resemblance to the substrate pattern are formed. PACS numbers: 68.15.+e, 47.20.Ma, 47.54.+r, 68.08.De, 68.08.Bc Self-organization during dewetting of thin films on deliberately tailored chemically heterogeneous substrates is of increasing promise for engineering of desired nano-and micro-patterns in thin films by templeting [1,2,3,4,5,6,7,8,9,10,11]. On a chemically heterogeneous substrate, dewetting is driven by the spatial gradient of micro-scale wettability [12], rather than by the non-wettability of the substrate itself. The latter occurs in the so called spinodal dewetting on homogeneous surfaces [13,14]. While the rupture of a thin film on a single heterogeneous patch is now well understood, patterned substrates pack a large density of surface features that are closely spaced. How does hydrodynamic interactions between the neighboring heterogeneities affect the pattern evolution dynamics and morphology in thin films? This question, which is addressed here, is central to our understanding of how faithfully the substrate patterns are reproduced in a thin film spontaneously, i.e., how effective is the templeting of soft materials by dewetting route and what are the conditions for ideal templeting? An associated question for both the patterned and naturally occurring heterogeneous surfaces is whether all the potentially dewetting sites remain active or "live" in producing rupture when they are in close proximity. These questions are resolved based on 3-D nonlinear simulations of the stability, dynamics and morphology of thin films on periodic chemically heterogeneous surfaces.The substrate considered consists of alternating less wettable and more wettable (or completely wettable) stripes that differ in their interactions with the overlying film. The key parameters of the substrate pattern are its periodicity interval (center-to-center distance between two consecutive stripes, L p ) and the length-scale of the less wettable stripe (stripe-width, W ). The following nondimensional thin film equation governs the stability and spatio-temporal evolution of a thin film system subjected to the excess intermolecular in...

The instability, dynamics, and morphological transitions of patterns in thin liquid films on physically and chemically heterogeneous patterned surfaces are investigated on the basis of 3D nonlinear simulations. On a chemically striped surface (consisting of alternating less and more wettable stripes) the film breakup is suppressed on some potentially destabilizing nonwettable stripes when their spacing is below a characteristic length scale of instability (λh), which is of the same order as the spinodal length scale (λl) of instability on the less wettable stripes. The thin film pattern replicates the substrate surface energy pattern closely only when (a) the periodicity of the substrate pattern lies between λh and 2λh and (b) the less wettable stripe width is within a range bounded by a lower critical length, below which no heterogeneous rupture occurs, and an upper transition length, above which complex morphological features bearing little resemblance to the substrate pattern are formed. The thin film pattern on a periodic physically heterogeneous surface shows the loss of ideal templating when the periodicity of the surface is smaller than about 0.8 times the spinodal wavelength evaluated at the minimum film thickness. On a physicochemically patterned periodic surface, the chemical heterogeneity largely controls the thin film pattern and the effect of small to moderate physical heterogeneity is minimal.