DEPINNING OF THE SUPERFLUID HE-4-CS CONTACT LINE

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