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.

Keywords: MD simulations; adhesion; cavitation; contact angle; solvation.

Fig. 1.

(A) Snapshot of the periodic simulation system. Two parallel surfaces consisting of hydroxyl-terminated alkane chains with different polarity parameters ${\alpha }_1$ and ${\alpha }_2$ interact across a water layer at fixed chemical potential. (B) Density profiles of water and head-group oxygens for the asymmetric scenario ${\alpha }_1=0$ and ${\alpha }_2=1$.

Fig. 2.

Single surface results. Wetting coefficient $k_{\text{w}}$ is shown as a function of the surface polarity parameter α (black solid circles); the corresponding contact angle θ is shown on the right axis. The wetting transition, defined by $\theta =0°$, occurs at $\alpha \simeq 0.9$. Red open circles denote the vacuum wetting coefficient $k_{\text{w}}^{\text{vac}}$ obtained in the absence of a water film at the surface–vapor interface. Inset shows the areal water density of the adsorbed water film, $n_{\text{A}}$. Lines are guides to the eye.

Fig. 3.

Simulation results for the surface free energy in the hydrated state, $f\left(D\right)$ (dark blue lines), and in the vacuum state, $f_{\text{vac}}\left(D\right)$ (red dashed lines), for three surface polarity combinations that illustrate the scenarios of (A) hydration repulsion, (B) dry adhesion without cavitation, and (C) cavitation-induced attraction. The number of water molecules per surface group $N_{\text{w}}/N_{\text{s}}$ is shown in turquoise (right scale). The green horizontal dashed lines denote the cavitation free energy at infinite separation $f_{\text{cav}}^{\infty }$, which includes the effects of formation of water-wetting films on the surfaces.

Fig. 4.

Adhesion free energy in vacuum $f_{\text{vac}}^{\text{adh}}$ as a function of the vacuum wetting coefficient $k_{\text{w}}^{\text{vac}}$ for the symmetric scenario, ${\alpha }_1={\alpha }_2\equiv \alpha$.

Fig. 5.

Adhesion diagram for two surfaces represented in terms of (A) the two surface polarity parameters ${\alpha }_1$ and ${\alpha }_2$ and (B) the two surface contact angles ${\theta }_1$ and ${\theta }_2$, featuring the regimes of hydration repulsion (white), dry adhesion without cavitation (blue), and cavitation-induced attraction (orange). The asymptotic law for the adhesion transition, Eq. 7 (blue dashed line), agrees well with the simulation results (blue solid line) for not too asymmetric surfaces. The cavitation transition that separates the dry adhesion and the cavitation-induced attraction regimes is exactly described by the cavitation law, Eq. 2 (red solid line). The diamonds indicate the three scenarios considered in Fig. 3.