Quartz Microbalance Study of Thick 4He Film Near the Superfluid Transition

We present the resugts of a quartz microbalance measurement of a ∼440 A thick 4He film adsorbed on Au. This measurement confirms the resugt of an earlier capacitance measurement that showed a dip in the film thickness just below Ta due to the critical Casimir force. The magnitude of the dip in the two measurements agrees within 15% . In addition to the dip, we see a signal due to the onset of superfluidity in the film. An apparent peak in the film thickness, not observed previously, is also found coinciding closely with the bulk Tλ. This extra feature may be due to a sound resonance in the cell associated with the bulk fluid.