Thermally activated motion of the contact line of a liquid 4He meniscus on a cesium substrate

We have measured the contact angle of a liquid helium-4 meniscus on a cesium substrate as a function of the velocity of the three phase contact line, at various temperatures. The velocity is found to vary exponentially with the applied force. We show that the contact line advances through thermally activated jumps. Their area of about 125 nm(2) is probably related to the roughness of the cesium substrate which is evaporated at low temperature.