Dynamics of wetting of tiny drops: Ellipsometric study of the late stages of spreading

Abstract

Via ellipsometry, we study the profile evolution of very small drops (about ${10}^{\mathrm{\ensuremath{-}}4}$ \ensuremath{\mu}l) of nonvolatile liquids, spreading completely on silicon wafers. The evolution of the profiles are studied at molecular thicknesses, and for the first time it is shown that (i) the final stage of spreading in the cases studied is not a pancake, as predicted by the theory of de Gennes and Joanny (for S/\ensuremath{\gamma}\ensuremath{\ll}1), but rather is a two-dimensional gas and (ii) short-range forces and molecular dymanics may drastically affect the profile of the droplet.