Geometry as a Catalyst: How Vapor Cavities Nucleate from Defects

Abstract

The onset of cavitation is strongly enhanced by the presence of rough surfaces or impurities in the liquid. Despite decades of research, the way the geometry of these defects promote the nucleation of bubbles and its effect on the kinetics of the process remains largely unclear. We present here a comprehensive explanation of the catalytic action that roughness elements exert on the nucleation process for both pure vapor cavities and gas ones. This approach highlights that nucleation may follow nontrivial paths connected with a sharp decrease of the free energy barriers as compared to flat surfaces. Furthermore, we demonstrate the existence of intermediate metastable states that break the nucleation process in multiple steps; these states correspond to what is commonly known as cavitation nuclei. A single dimensionless parameter, the nucleation number, is found to control this rich phenomenology. The devised theory allows one to quantify the effect of the geometry and hydrophobicity of surface asperities on nucleation. Within the same framework, it is possible to treat both vapor cavitation, which is relevant, e.g., for organic liquids, and gas-promoted cavitation, which is commonly encountered in water. The theory is shown to be valid from the nano- to the macroscale.

Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity

“…[ 1 ] Moving toward submerged applications requires a new inspiration: a promising candidate is the Salvinia molesta (Figure 1 a), because of its superior gas trapping capabilities. [ 6,[18][19][20] The gas entrapped within surface asperities can be either air or the vapor phase coexisting with the liquid: albeit the partial pressure of the other gases stabilizes the Cassie state, their presence is not a requirement for (meta)stable superhydrophobicity [ 21 ] (see the Supporting Information for additional details on the role of dissolved gases). The entrapped gas may be lost through different mechanisms, analyzed in detail below, determining the failure of superhydrophobicity:…”

“…This is our main result, which at the same time clarifi es in quantitative terms the function of a complex biological structure, fi rst described by Barthlott and co-workers, [ 2,6 ] and suggests how to exploit it in the design of simpler bioinspired surfaces. Figure 2 a addresses the effect of the pressure on the free energy profi les, which amounts to adding to Ω(Φ) a term ∼ΦΔ P ; [ 21,28 ] this linear shift changes the location of the minima and determines the stability of the Cassie state: for instance, increasing the pressure always favors the Wenzel state. At suffi ciently large pressures, the Cassie minimum disappears and 0 CW † ΔΩ → : this is the spinodal pressure Δ P max for the Cassie-Wenzel transition, i.e., the maximum pressure before the mechanical destabilization of superhydrophobicity (mechanism 1).…”

Pressure control in interfacial systems: Atomistic simulations of vapor nucleation

“…What sets these systems apart is that different subdomains can have different pressures. A broad range of phenomena falls into this class, including homogeneous and heterogeneous vapor nucleation, [6][7][8][9][10][11][12] nucleation of polymorphic crystals, [13][14][15][16] dissolution of bubbles and droplets, and condensation or evaporation. In this work we show that in such cases, in which the relative amount of the two phases changes along the process, the pressure of the preexisting bulk metastable phase might change during the process when one uses global barostats, which is different from the condition at which experiments are carried out.…”

Shock Wave Formation in the Collapse of a Vapor Nanobubble

“…In these conditions, away from solid walls (see e.g. [12] for the role of asperities on solid surfaces as a catalyst of bubble nucleation), local density fluctuations can generate the critical vapor nucleus from which the eventual bubble is formed.Intermingled phenomenologies, [13,14], such as interface dynamics [15,16], thermodynamics of phase change [17], and dissolved gas diffusion [18], are a challenge to theoretical modeling of cavitation. The available descriptions combine two distinct adjoining regions, liquid and vapor phase, respectively, with vapor pressure taken to be the saturation pressure [19] and phase transition accounted for through suitable kinetic equations and latent heat release [18].…”

“…In these conditions, away from solid walls (see e.g. [12] for the role of asperities on solid surfaces as a catalyst of bubble nucleation), local density fluctuations can generate the critical vapor nucleus from which the eventual bubble is formed.…”