Thinning and Rupture of Liquid Films by Moving Slot Jets

We
present systematic experiments of the rupture and dewetting
of thin films of a nonvolatile polar liquid on partially wetting substrates
due to a moving slot jet, which impinges at normal incidence. The
relative motion was provided by a custom-built spin coater with a
bidirectionally accessible axis of rotation that enabled us to measure
film thickness profiles in situ as a function of substrate velocity
using dual-wavelength interference microscopy. On partially wetting
polymeric substrates, dry spots form in liquid films with a residual
thickness well below 1 μm. We measured the density of dry spots
as well as the density and size distribution of the residual droplets
as a function of film thickness. In a certain parameter range, the
droplet distributions exhibit pronounced anisotropy due to the effect
of long-range shear stresses on the dewetting rim instability. We
find robust power-law scaling relations over a large range of film
thicknesses and a striking similarity to literature data obtained
with ultrathin polymer melt layers on silicon substrates.