Surface tension in liquid 3He and 4He near the liquid-gas critical point

The surface tension of liquid 3He and 4He was measured near the gas-liquid critical points in the reduced temperature range 3 x 10-4 < t < 2 x 10-1, where t is identical to (T /sub c/ - T)/T /sub c/. The critical exponents were found to be ≤3 = 1.289 + or - 0.015 for 3He and ≤4 = 1.306 + or - 0.017 for 4He. These values are very close to those for classical liquids, and are consistent with the value of 1.28 predicted by Widom, but are apparently different from the exponents previously obtained for liquid helium isotopes, which are near unity. The critical coefficients show good agreement with the quantum-corrected corresponding states theory for the Lennard-Jones 6-12 potential discussed by Young. The interface thickness is deduced from Widom's theory to be d = d0t /sup -v/ with d30 = 0.14 + or - 0.03 nm and v /sub '3/ = 0.57 + or - 0.04 for 3He, and d40 = 0.37 + or - 0.07 nm and v /sub '4/ = 0.58 + or 0.01 for 4He