Effect of gravity on helium II in aerogel

Abstract

We have observed for aerogel nearly filled with superfluid helium that the helium II liquid-vapour interface in the aerogel takes a shape that does not change with respect to gravity (the longest time studied was 3 hr.). the most likely reason we observe a fixed helium II liquid-vapour interface is due to the presence of capillary forces. We have concentrated our studies on the serogel nearly filled with superfluid helium since this is when capillary forces are weakest and least understood. The distribution of helium II in the aerogel is subsequently most susceptible to gravity. We conclude that the helium II liquid-vapour interface will not change shape due to gravitational forces of 1 g for any amount of helium in the aerogel. This makes aerogel a very promising candidate for gravitational missions such as STEP (Satellite Test of the Equivalence Principle) or GOCE (Gravity Field and Ocean Circulation Explorer) that are susceptible to helium II motion. In the case of STEP, gravity gradient forces of magnitude 10 −7 g act on the superfluid helium at twice orbital frequency. The motion of helium II produces a gravitational signal in the bandwidth of the STEP measurement. The use of aerogel would eliminate this signal. Thus, trying to isolate this signal by experimental procedure or data analysis is no longer necessary.