The extended surface forces apparatus. IV. Precision static pressure control

We report on design and performance of an extended surface forces apparatus (eSFA) built into a pressurized system. The aim of this instrument is to provide control over static pressure and temperature to facilitate direct surface force experiments in equilibrium with fluids at different loci of their phase diagram. We built an autoclave that can bear a miniature eSFA. To avoid mechanical or electrical feedtroughs the miniature apparatus uses an external surface coarse approach stage under ambient conditions. The surface separation is thus pre-adjusted to approximately ~3 μm before sliding the apparatus into the autoclave. Inside the autoclave, the surface separation can be further controlled with a magnetic drive at sub-Ångstrom precision over a 14 μm range. The autoclave pressure can then be set and maintained between 20 mbar and 170 bars with few mbar precision. The autoclave is connected to a specially designed pressurization system to precondition the fluids. The temperature can be controlled between -20 and 60 °C with few mK precision. We demonstrate the operation of the instrument in the case of gaseous or liquid carbon dioxide. Thanks to a consequent decoupling of the eSFA mechanical loop from the autoclave structure, the obtained measurement stability and reproducibility, at elevated pressures, is comparable to the one established for the conventional eSFA, operated under ambient conditions.