Cavitation in heterogeneous nanopores: The chemical ink-bottle

Abstract

The emptying of nanoporous media may either occur by meniscus recession from the outside or proceed via bubble formation in the core of the fluid (cavitation). In the latter case, the system has to overcome the high energy barrier associated with liquid rupture and can be observed only if the porous network impedes meniscus recession. It is generally admitted that the ink-bottle geometry fulfills this condition: large cavities should be connected to the outside only through thin throats. We show that chemical heterogeneities can play a similar role (the so-called chemical ink-bottles) where cavities correspond to the pores with the lowest fluid–wall affinity, while throats are replaced by channels with the highest fluid–wall affinity. In addition, the chemical ink-bottle has been revealed as powerful in monitoring the out-of-equilibrium fluid cavitation. We consider a simple molecular model of nitrogen adsorption in a slit pore with different fluid–wall interactions to mimic the possible variations in the surface chemistry of realistic adsorbents. The highest-to-lowest affinity ratio is shown to be a key parameter that controls the metastability of the confined fluid. The direct measurement of the nucleation rate (lifetime method) compares well with the classical nucleation theory, with better agreement if one takes into account the reduction in the surface tension of small bubbles.