Magnetic compensation of gravity forces in (p-) hydrogen near its criticalpoint: Application to weightless conditions

We report a study concerning the compensation of gravity forces in two-phas e (p-) hydrogen. The sample is placed near one end of the Vertical z axis o f a superconducting coil, where there is a near-uniform magnetic field grad ient. A variable effective gravity level g can thus be applied to the two-p hase fluid system. The vanishing behavior of the capillary length l(c) at t he critical point is compensated by a decrease in g and cc is kept much sma ller than the cell dimension. For g ranging from 1 to 0.25 times Earth's gr avity (modulus g(0)) we compare the actual shape of the meniscus to the exp ected shape in a homogeneous gravity field. We determine Cc in a wide range of reduced temperature tau=(T-C-T)/T-C = [10(-4)-0.02] from a fit of the m eniscus shape. The data are in agreement with previous measurements further from Te performed in n-H-2 under Earth's gravity. The effective gravity is homogeneous within 10(-2)g(0) for a 3 mm diameter and 2 mm thickness sampl e and is in good agreement with the computed one, validating the use of the apparatus as a variable gravity facility. In the vicinity of the levitatio n point (where magnetic forces exactly compensate Earth's gravity), the com puted axial component of the acceleration is found to be quadratic in z, wh ereas its radial component is proportional to the distance to the axis, whi ch explains the gas-liquid patterns observed near the critical point.