Cavitation in metastable liquid nitrogen confined to nanoscale pores

We studied cavitation in metastable fluids drawing on the example of liquid nitrogen confined to spheroidal pores of specially prepared well-characterized mesoporous silica materials with mean pore diameters ranging from ∼6 to ∼35 nm. Cavitation was monitored in the process of evaporation/desorption from fully saturated samples with gradually decreasing vapor pressure at the isothermal conditions. The onset of cavitation was displayed by a sharp step on the desorption isotherm. Wefound that the vapor pressure at the onset of cavitation depended on the pore size for the samples with pores smaller than ∼11 nm and remained practically unchanged for the samples with larger pores. We suggest that the observed independence of the cavitation pressure on the size of confinement indicates that the conditions of bubble nucleation in pores larger than ∼11 nm approach the nucleation conditions in the bulk metastable liquid. To test this hypothesis and to evaluate the nucleation barriers, we performed grand canonical and gauge cell Monte Carlo simulations of nitrogen adsorption and desorption in spherical silica pores ranging from 5.5 to 10 nm in diameter. Simulated and experimental adsorption isotherms were in good agreement. Exploiting the correlation between the experimental cavitation pressure and the simulated nucleation barrier, we found that the nucleation barrier increased almost linearly from ∼40 to ∼70 k_BT in the range of pores from ∼6 to ∼11 nm, and varied in diapason of 70-75 k _BT in larger pores, up to 35 nm. We constructed the dependence of the nucleation barrier on the vapor pressure, which asymptotically approaches the predictions of the classical nucleation theory for the metastable bulk liquid at larger relative pressures (>0.6). Our findings suggest that there is a limit to the influence of the confinement on the onset of cavitation, and thus, cavitation of nanoconfined fluids may be employed to explore cavitation in macroscopic systems.