Identification of the seeding mechanism in the spinodal instability of dewetting liquids

Abstract

Hypothesis: Spinodal dewetting is one of the basic processes inducing a spontaneous withdrawal of a liquid from a substrate surface. In the accepted theory, thickness fluctuations generated by thermally activated capillary waves are amplified by the competing actions of surface tension and disjoining pressure. Ubiquitous sub-nanometric substrate roughness also produces thickness fluctuations and may play a role analogous but even more efficient in seeding the process.

Modelling: Analytic calculations valid at the early linear stage of the process and simulations extending the study to its whole non-linear development have been performed to compare features and the relative relevance of the two seeding mechanisms.

Findings: Calculations and simulations have shown that substrate roughness can replace capillary waves in seeding spinodal dewetting. A typically larger amplitude and a steady nature compared to the transitory one of capillary waves allow us to conclude that, contrary to the common view, substrate roughness is the prevailing seed of the spinodal instability. The consequence of our statement is that spinodal dewetting loses most of its stochastic nature and becomes, in principle, a process that can be tuned by engineering substrate roughness.

Keywords: Capillary waves; Dewetting; Substrate roughness; Thin fluid films.