INFLUENCE OF SUBSTRATE HETEROGENEITIES ON THE SPREADING OF A DROP

The dynamics of spreading of monolayers of completely wetting liquids on substrates with various degrees of heterogeneity is studied here ex perimentally and numerically at a microscopic level. The radius of spr eading of the first layer of the drop always presents two different di ffusive regimes as a function of time if the temperature is higher tha n the two-dimensional (2D) critical temperature of the liquid. The ass ociated spreading rate increases when the substrate becomes heterogene ous at the molecular scale. If the temperature is lower than the 2D cr itical temperature of the liquid, the second diffusive regime occurs o nly on heterogeneous substrates. The spreading rate again increases wi th 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 Tran sitions and Critical Phenomena, edited by C. Domb and M. S. Green (Aca demic, London, 1972), Vol. 2] is very satisfactory.