What experiments on pinned nanobubbles can tell about the critical nucleus for bubble nucleation

Abstract

Abstract. The process of homogeneous bubble nucleation is almost impossible to probe experimentally, except near the critical point or for liquids under large negative tension. Elsewhere in the phase diagram, the bubble nucleation barrier is so high as to be effectively insurmountable. Consequently, there is a severe lack of experimental studies of homogenous bubble nucleation under conditions of practical importance (e.g., cavitation). Here we use a simple geometric relation to show that we can obtain information about the homogeneous nucleation process from Molecular Dynamics studies of bubble formation in solvophobic nanopores on a solid surface. The free energy of pinned nanobubbles has two extrema as a function of volume: one state corresponds to a free-energy maximum ("the critical nucleus"), the other corresponds to a free-energy minimum (the metastable, pinned nanobubble). Provided that the surface tension does not depend on nanobubble curvature, the radius of the curvature of the metastable surface nanobubble is independent of the radius of the pore and is equal to the radius of the critical nucleus in homogenous bubble nucleation. This observation opens the way to probe the parameters that determine homogeneous bubble nucleation under experimentally accessible conditions, e.g. with AFM studies of metastable nanobubbles. Our theoretical analysis also indicates that a surface with pores of different sizes can be used to determine the curvature corrections to the surface tension. Our conclusions are not limited to bubble nucleation but suggest that a similar approach could be used to probe the structure of critical nuclei in crystal nucleation.Bubble nucleation is a very common process but the observation of truly homogenous bubble nucleation is very rare [1]. The reason is that the free-energy barrier for homogenous bubble nucleation tends to be very high (order 10 3 -10 4 eV), except near the critical point (typical T > 0.9T c ) where the surface tension is very low, or under conditions of extreme under-pressure [2]. For this reason, bubble nucleation under less extreme circumstances (e.g., bubble formation in boiling water) is always dominated by heterogeneous nucleation. The rate of heterogeneous nucleation is normally extremely sensitive to the properties of the surface, in particular to the value of the solid-liquid and solid-vapor surface free energies. Hence, normally, heterogeneous bubble nucleation does not provide direct information about the homogenous nucleation process.Supplementary material in the form of a .pdf file available from the Journal web page at http://dx.doi.org/10.1140/epje/i2017-11604-7 a e-mail: df246@cam.ac.uk b e-mail: jd489@cam.ac.uk c e-mail: zhangxr@mail.buct.edu.cnThe situation is very different in the case of welldefined hydrophobic (or, more generally, "solvophobic") nanopores in a hydrophilic ("solvophilic") surface. Inside these pores, vapor may nucleate easily, sometimes even at a pressure where the bulk liquid is still thermodynamically stable. As the pressure of t...

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