EventsProf. Carlo Massimo Casciola

Superhydrophobic Surfaces and Heterogeneous Bubble Nucleation

Most applications of superhydrophobicity to date have concentrated on drops deposited on surfaces. On the other hand, there is a growing interest in the properties of submerged surfaces entrapping gas (Wenzel state) as a means to, e.g., reduce the liquid–solid contact, diminishing drag and preventing (bio)fouling. For submerged applications the central question is the resistance and durability of the gas pockets to pressure variations. Depending on the external conditions, however, superhydrophobicity may break down in the fully wet Wenzel state. In this context atomistic rare event simulations have unraveled the Salvinia paradox: a water fern leave with superior gas trapping capabilities. It is found that a re-entrant geometry with a hydrophobic interior, improves the stability of gas pockets against liquid intrusion and contaminants, while the hydrophilic top surface hinders bubble nucleation and coalescence.

Heterogeneous systems composed of hydrophobic nanoporous materials and water are capable, depending on their characteristics, to efficiently dissipate (dampers) or store energy (“molecular springs”). The results of advanced simulations based on the string method in collective variables elucidate the physical phenomena peculiar of nanoconfined water, paving the way for a better design of technological materials for energy applications. For instance, by decreasing the size of the nanopores alone it is possible to change the behaviour of materials from dampers to molecular springs.

In many cases, experimentally relevant, 3D nanotextures have a complex morphology which are often difficult to investigate directly via experiments, owing to the dimensions of the systems. We discuss how to tailor simulations in order to identify the collapse mechanism of a large sample of a complex surface.

Extending the analysis to even larger structure calls for a mesoscale approach. Recently we have been working on a diffuse interface formulation. The model we propose encompasses liquid-vapour phase transition, the presence of gas dissolved in the liquid, latent heat release, liquid compressibility and surface tension. Its potential is discussed by addressing the inertial collapse of micro/nano bubbles, showing transition to supercritical condition, shock wave radiation in the liquid and interaction with solid walls. If time will allow, the extension of the mesoscopic model to include thermally activated nucleation in the context of fluctuating hydrodynamics, approach pioneered by Landau and Lifshitz and becoming a hot topic in simulations today, will also be briefly addressed.

Acknowledgement: Research founded by ERC Programme (FP7/2007-2013)/ERC Advanced Grant agreement n. 339446, BIC – Following Bubbles from Inception to Collapse.

– M. Amabili, A. Giacomello, S. Meloni, and C.M. Casciola, “Collapse of superhydrophobicity on nanopillared surfaces”, Phys. Rev. Fluids 2017.

– M. Amabili, A. Giacomello, S. Meloni, C.M. Casciola, “Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity”, Advanced Material Interfaces 2015.

– F. Magaletti, L. Marino, C.M. Casciola, Shock formation in the collapse of a vapor nanobubble, Phys. Rev. Lett., 2015.

– F. Magaletti, M. Gallo, L. Marino, C.M. Casciola, Shock-induced collapse of a vapor nanobubble near solid boundaries, Journal of Multiphase Flows, 2016.

About The Speaker

Carlo-Massimo-Casciola-unam

Prof. Carlo Massimo Casciola leads a research group of about 20 among researchers, postdocs and PhD students working on the fluid dynamics of complex flows with a chiefly theoretical and numerical approach. His research is oriented to fundamental and numerical modelling, with substantial interactions with several neighbouring disciplines, such as engineering, physics, material science, chemistry, mathematics, biology, and medicine.

Most of the topics are characterised by the coupling of macroscopic flows with a micro-structure. After being awarded the ERC Advanced Grant 2013, BIC: Following Bubbles from Inception to Collapse, a significant part of the research work has been focused on different aspects of cavitation, e.g. heterogeneous nucleation, wettability, bubble collapse modelling, bubble-wall interactions. The tools employed span from Direct and Large Eddy Simulation, to Phase Field Models, Free Energy Methods, and Molecular Dynamics techniques specialised for Rare Events. Significant contributions concerns technics and models for Wettability and heterogeneous nucleation on complex surfaces; Phase field methods for (nano-)bubble dynamics and cavitation; Direct numerical simulations of polymer drag reducing flows; Scaling laws and energy fluxes in inhomogeneous turbulent flows; Particle transport in turbulent flows in the two-way coping regime. Recently, the research interests extended to experimental aspects of bubble dynamics, cavitation and micro fluidics for application in biology and medicine.