title: From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces.
abstract: Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall charge-neutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can-if strong enough-give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface-surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles.
authors.affiliation: Department of Physics, Free University Berlin, D-14195 Berlin, Germany; Department of Theoretical Physics, J. Stefan Institute, SI-1000 Ljubljana, Slovenia matej.kanduc@ijs.si.
authors.forename: Matej
authors.initials: M
authors.lastname: Kanduč
authors.affiliation: Department of Physics, Free University Berlin, D-14195 Berlin, Germany;
authors.forename: Roland R
authors.initials: RR
authors.lastname: Netz
keywords: MD simulations; adhesion; cavitation; contact angle; solvation