NOISE FROM RAINDROPS - FUNDAMENTAL-STUDIES OF BUBBLE ENTRAINMENT IN PURE HE-4

The impact of a liquid drop into a liquid surface has been extensively studied using acoustical techniques over the past decade. Experimenta l results have been difficult to reproduce because of impurities at th e surface of water, the liquid usually used by past experiments. Using liquid helium, above and below the lambda point, as an impurity free simple liquid, we have been able to observe small drops impacting onto the liquid surface. One finds a phase diagram showing a region (deter mined by the size of the drop, impact velocity, etc.) for which bubble entrainment is possible. Bubble entrainment has been observed to occu r under conditions which agree with theoretical studies.