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Geometry as a Catalyst: How Vapor Cavities Nucleate from Defects
journal contribution
posted on 2013-12-03, 00:00 authored by Alberto Giacomello, Mauro Chinappi, Simone Meloni, Carlo Massimo CasciolaThe 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.