Thin liquid films on chemically heterogeneous substrates: self-organization, dynamics and patterns in systems displaying a secondary minimum

Abstract

Surface instability in a thin liquid film engendered by a micro-scale wettability contrast, resulting from the spatial gradients in the intermolecular interactions, is investigated based on 3D simulations for a system displaying both the primary and secondary minima in its force vs. distance curve. Characteristic dynamical and morphological features of the evolution on a chemically heterogeneous substrate are identified for two different cases: (a) a single patch or step of heterogeneity (b) multiple periodically arranged stripes of heterogeneity. The presence of heterogeneity can cause true rupture at the primary minimum by surmounting the energy barrier between the two minima. The breakup time on heterogeneous surfaces varies inversely with the gradient in the potential induced by the heterogeneity and can be several orders of magnitude smaller than the spinodal dewetting time on homogeneous surfaces. Heterogeneity can also cause rupture in spinodally stable films and produce complex and locally ordered morphological features (e.g., ripples) absent in the spinodal dewetting. On a chemically patterned surface composed of alternating more and less wettable stripes, film breakup is suppressed on some potentially destabilizing nonwettable sites when their spacing is below a characteristic lengthscale of instability, λ_h which is close to the spinodal lengthscale. Thin film pattern ideally replicates the surface energy pattern only when, (a) the periodicity of substrate pattern is greater than λ_h, (b) width of the less wettable stripe is lower than a transition length above which complex morphological features are formed.