Depinning of the cuperfluid 4He-Cs contact line

Abstract

We have studied the spreading of helium-4 on cesium substrates with random disorder. When the characteristic length scale of the disorder is of the order of 20 micrometers, we have been able to measure the roughness exponents of the contact line, and found that they are in agreement with theoretical predictions. The temperature dependence of the roughness is also consistent with the theory. However, the same model predicts an hysteresis much smaller than the measured one. Better cesium substrates with optical quality yield a very straight contact line but the same large hysteresis. This hysteresis is presumably due to the pinning of the line on microscopic defects of the evaporated cesium film. The velocity of the contact line increases exponentially with the applied force, suggesting that the depinning is thermally activated.