UC Irvine

Experiments on adsorption and wetting of quantum fluids (4He and 3He) on weakly binding alkali metal substrates are reviewed. Helium on weak substrates can undergo a variety of phase transitions including wetting, prewetting, layering, and liquid-vapor transitions. Another characteristic feature of weak substrates is the absence of an immobile quasi solid layer which is present on all conventional strong substrates. Both the absence of the immobile layer and the interaction with surface phase transitions can strongly affect superfluid onset. The Kosterlitz-Thouless superfluid transition can terminate in either a critical endpoint where it meets a first order structural transition or a tricritical point where it meets critical point such as the prewetting critical point or the 2D liquid-vapor critical point.