Wetting behavior of liquid He on rough Cs films: Pinning, memory effect, and micropuddles

The liquid He-cesium system is a nearly ideal one for studying wetting phenomena. However, it can show nonideal behavior such as an extreme wetting hysteresis and a memory of being in contact with liquid He. We believe that this is caused by the roughness of the Cs surface. We review the wetting characteristics of Cs surfaces produced by various methods, and we qualitatively classify Cs surfaces according to the strength of pinning of the contact line. New data are presented on quench-condensed Cs surfaces that show that the pinning can be weak and the contact line can move freely to dewet this Cs. We discuss how micropuddles can form on strong-pinning surfaces, and we discuss how this leads to the memory effect and changes the effective pinning. These phenomena should be generally relevant to the wetting behavior of rough surfaces.