Taking the plunge into forced wetting with unprecedented precision

When a flat solid substrate is swiftly thrust across the interface between two fluids (e.g., water and air), it entrains a thin film of the trailing fluid attached to its surface. The onset of this ‘forced wetting’ phenomenon has been extensively studied in terms of a two-dimensional geometry in which no flows occur transverse to the direction of motion of the solid surface. Contrary to this assumption, we have discovered that in steady state the fluid interface develops a characteristic three-dimensional structure: there are multiple thin and thick regions of the fluid film alternating in the transverse direction. This structure occurs robustly both in wetting (solid plunged into the liquid) and dewetting (solid removed from liquid). This shows that the flow behind the contact line is not invariant in the transverse direction suggesting the existence of a new instability. Using a new interference technique, we measure the thickness of the thick and thin regions as a function of substrate velocity and fluid viscosity. We compare these results with the Landau-Levich-Derjaguin model generalized to incorporate viscous effects from both fluids.