Wetting of Superfluid 4He by Liquid 3He

We have investigated optically the spreading of 3He on top of the 4He-rich solution in phase-separated helium-mixture films, 20-50 μm thick. In equilibrium, the 3He layer wets the 4He-rich phase completely, but nearly circular or stripelike pools of 3He-rich phase are stabilized instead when 4He atoms are condensed to the liquid sample at the rate 1015-5 × 1015 atoms/cm2s. For the contact angle we obtain about 10 mrad, which suggests a fractional change of the 3He surface tension by a factor of 10−5 from the equilibrium value.