Prewetting transition on a weakly disordered substrate: Evidence for a creeping lm dynamics

We present the rst microscopic images of the prewetting transition of a liq uid film on a solid surface. Pictures of the local coverage map of a helium film on a cesium metal surface are taken while the temperature is raised t hrough the transition. The film edge is found to advance at constant temper ature by successive avalanches in a creep motion with a macroscopic correla tion length. The creep velocity varies strongly in a narrow temperature ran ge. The retreat motion is obtained only at much lower temperature, conformi ng to the strong hysteresis observed for prewetting transition on a disorde red surface. Prewetting transition on such disordered surfaces appears to g ive rise to dynamical phenomena similar to what is observed for domain wall motions in 2D magnets.