Influence of substrate heterogeneities on the spreading of a drop

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Abstract
The dynamics of spreading of monolayers of completely wetting liquids on substrates with various degrees of heterogeneity is studied here experimentally and numerically at a microscopic level. The radius of spreading of the first layer of the drop always presents two different diffusive regimes as a function of time if the temperature is higher than the two-dimensional (2D) critical temperature of the liquid. The associated spreading rate increases when the substrate becomes heterogeneous at the molecular scale. If the temperature is lower than the 2D critical temperature of the liquid, the second diffusive regime occurs only on heterogeneous substrates. The spreading rate again increases with the substrate heterogeneity, at least for weak heterogeneities. The agreement between the experimental results and the simulation, using the Kawasaki double spin exchange dynamics [K. Kawasaki, in Phase Transitions and Critical Phenomena, edited by C. Domb and M. S. Green (Academic, London, 1972), Vol. 2] is very satisfactory.