Rupture of radially spreading liquid films

The rupture of thin water films (thickness of ∼ 100 μ m ) spreading radially outward on a solid surface was studied experimentally. Circular water films were produced by directing a 1 mm diameter water jet onto a flat horizontal plate for 10 ms. Jet impact velocity was varied from 1.6 to 6.1 m/s, giving Reynolds numbers from 1600 to 6100. Three different plates were used (glass, Plexiglas, and wax) to vary substrate wettability. Both rough and smooth wax surfaces were tested. Water films did not rupture on glass and Plexiglas plates, but holes formed and expanded in films spreading on wax, which had the highest liquid-solid contact angle. Tests in which the spreading film was not supported on a surface, but spread after impact on a pin to simulate a perfectly nonwetting surface, did not show formation of holes. A thermodynamic stability analysis predicted film rupture behavior by showing that films would be stable at very small or very large contact angle but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness.