Compressibility of 4He Liquid Confined in Nuclepore

Adding and removing⁴He from an experimental cell which contains the porous material Nuclepore results in capillary condensation, hysteresis and avalanches. The volume of liquid in the nearly cylindrical pores is monitored with a capacitance technique. For the situation when all of the pores are full, the initial removal of⁴He from the cell results in a slight non-hysteretic decrease in the capacitance, which we attribute to the isothermal compressibility of⁴He. As⁴He atoms are removed, the surface tension at the meniscus applies a negative pressure to the liquid in the filled pores, which results in a reduction of the density and a decrease in the capacitance. We present results for⁴He capillary condensed in 2000 Å pore diameter Nuclepore.