Stability of the submerged superhydrophobic state via rare event molecular dynamics simulations

In the present thesis the stability of the superhydrophobic state on submerged nanotextured surfaces with complex chemistry and geometry is investigated via rare event molecular dynamics simulations. Superhydrophobicity stems from the presence of vapor or gas pockets trapped within the surface texture sustaining the liquid atop of the textures. This superhydrophobic --or Cassie-- state gives rise to remarkable macroscopic properties, such as enhancing liquid slip at the surface or resulting into anti-fouling capabilities. These and many other properties make superhydrophobic surfaces promising for a wide range of technological applications. Superhydrophobicity can be lost via two mechanisms: i) liquid intrusion, i.e. the filling of the liquid inside the textures, leading to the fully wet Wenzel state, or ii) vapor cavitation, i.e. the formation, growth, detachment of a (supercritical) vapor or gas bubble, and the ensuing replacement of the vapor pocket by the liquid. Understanding the mechanism of the Cassie-Wenzel and cavitation transitions is crucial in order to define quantitatively the stability of the superhydrophobic state. These transitions are typically characterized by large free-energy barriers. The presence of these barriers make the study of the transitions particularly challenging due to the diverse timescales present in the problem. Indeed, these transitions are rare events, i.e., they happen on timescale which cannot be simulated by standard molecular dynamics methods. Thus, in order to tackle this issue, rare event methods are used which allow one to compute the free-energy barriers and the mechanism of the transition. This information is essential for developing new design criteria which can pave the way to a new generation of surfaces with more stable superhydrophobic properties in submerged conditions. In the first part of the thesis, re-entrant surface textures are investigated. The focus on this geometry is due to the increasing interest on textures which show omniphobic properties, i.e., which allows the formation of the Cassie state also for low surface tension liquids. Our atomistic results are compared with a macroscopic sharp-interface continuum model. While the two models are generally in fair agreement, quantitative differences were found in the cavitation free-energy barriers, with the macroscopic model overestimating them. The major qualitative difference concerns the behaviour of the system in the Wenzel state, where only the atomistic model can capture the presence of the confined liquid spinodal. These results also allowed us to develop design criteria for textured submerged superhydrophobic surface. The role of the chemistry of the surface was also studied. Pure hydrophobic, pure hydrophilic, and mixed, i.e. internally hydrophobic and externally hydrophilic surfaces, were considered. It was found that the free energy of the mixed chemistry surface closely resemble the superimposition of the one of pure hydrophilic and hydrophobic chemistries. The mixed chemistry shows improved stability of gas pockets against both liquid intrusion and vapor cavitation. Finally, the combined effect of complex chemistry and geometry, such as re-entrant pore morphologies, was also investigated. This latter part of the work was primarily inspired by the natural case of Salvinia molesta, which is a floating fern capable of remaining dry even after a long underwater immersion. Salvinia leaves show similar features with respect to the proposed textures; it is characterized by hairs with a peculiar re-entrant structure with a heterogeneous chemistry: a hydrophobic interior and an hydrophilic patch on the hairs top. In the second part of the thesis, the Cassie-Wenzel transition was investigated on a submerged 3D nano pillared surface. Here, a state-of-the-art technique, the string method, was employed in order to compute the most probable transition mechanism and the corresponding free-energy barrier. The coarse-grained fluid density field was used as a collective variable to characterize the transition. Results are both of applicative and of methodological interst. For the former, the string method revealed the actual transition mechanism, which proceeds by breaking the 2D translational symmetry of the surface textures. These results are interpreted in terms of a sharp-interface continuum model suggesting that nanoscale effects, e.g., line tension, play a minor role in the considered conditions. Concerning to the former, the effect of the choice of the collective variables, i.e. different level of coarse-graining of the fluid density field, was studied. Results show the correct level of coarse-graining suited to correctly capture the transition mechanism and the free-energy barrier.