(392a) Heterogeneous Cavitation of Metastable Nitrogen In Spherical Pores with Non-Wetting Defects

Cavitation of metastable fluids confined in nanoscale pores is displayed by sharp desorption branches of capillary condensation isotherms in several classes of mesoporous materials. Nucleation of a critical bubble necessarily precedes the process of spontaneous evaporation of fluid from the bottle-neck pores with sufficiently narrow necks. In previous works [1, 2], we showed that in the case of pores with wetting walls, the critical bubble is formed in the pore center and the nucleation proceeds in a homogeneous fashion. In this work, we investigate the onset of cavitation in the pores with non-wetting defects, and show that the increase of the defect size leads to a transition from the homogeneous to heterogeneous nucleation of critical bubbles. Using GCMC simulations, we studied the relative pressures at which the critical bubble forms in a spherical pore with a circular non-wetting defect.  As a practical example, established parameters for adsorption of nitrogen on silica were used. Calculation of a series of GCMC desorption isotherms enabled us to build a distribution of cavitation pressures for given pore and defect size. We find that the introduction of the non-wetting defect significantly shifts the cavitation pressure upward. By analyzing the simulation trajectories, we observed that the nucleation mode shifted from homogeneous in the defect-free pore to heterogeneous, with the critical bubble formed at the defect site. We conclude that inhomogeneities of pore walls may affect the cavitation pressure, and this effect may lead to the observation of desorption pressures exceeding the range predicted by the theories of homogeneous cavitation, which was recently found in some samples of porous silicon [3].

[1]  Neimark AV, Vishnyakov A. The birth of a bubble: A molecular simulation study. Journal of Chemical Physics. 2005;122(5):054707.

[2]  Rasmussen CJ, Vishnyakov A, Thommes M, Smarsly BM, Kleitz F, Neimark AV. Cavitation in Metastable Liquid Nitrogen Confined to Nanoscale Pores. Langmuir. 2010;26(12):10147-57.

[3]  Grosman A, Ortega C. Cavitation in Metastable Fluids Confined to Linear Mesopores. Langmuir. 2011;27(6):2364-74.