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Geometry as Catalyst : How Vapor Cavities Nucleate from Defects

Carlo Massimo Casciola

Mercredi 19 février 2014

à 11h en salle C. Brot

The onset of cavitation is strongly enhanced by the presence of rough surfaces or impurities in the liquid. Despite decades of research, the way these defects promote the nucleation of bubbles and their effect on the kinetics of the process remain largely unclear. We present here a comprehensive explanation of the catalytic action that roughness elements exerts on the nucleation process [1]. We reconstruct the free-energy profiles of the process via molecular dynamics (MD) simulations, by exploiting rare events methods. These techniques allow to determine the stable and metastable states along the nucleation process, to isolate the relevant mechanism, and to evaluate the associated free-energy barriers. On this nanoscale system, we assess the effects of geometry, pressure, and temperature on the nucleation process. Along the lines of [2], we also devise a thermodynamic continuum theory of the cavitation process based on a constrained approach inspired by restrained MD. This second step allows to extend the description to the macroscopic scale. Overall, this approach highlights a sharp decrease of the free energy barriers connected with cavitation and demonstrates the existence of intermediate metastable states that break the nucleation process in multiple steps. A single dimensionless parameter, the nucleation number, is found to control this rich phenomenology. The devised theory allows to quantify the effect of geometry and hydrophobicity of surface asperities on cavitation from the nano- to the macroscale. These results pave the way for new methods for cavitation control that resort on micro- or nanostructured surfaces engineered to promote or delay cavitation. In particular, this strategy for cavitation control may have a signifcant impact in the sonochemical synthesis of bio- and nanostructured materials.

[1] A. Giacomello, M. Chinappi, S. Meloni, C.M. Casciola, Langmuir 2013.

[2]A. Giacomello, M. Chinappi, S. Meloni, C.M. Casciola, Phys. Rev. Lett. 2012.

C.M. Casciola (Mechanical and Aerospace Dept., Sapienza University of Rome)


Fluides & Matériaux Complexes