Electric Control of Wetting by Salty Nanodrops: Molecular Dynamics Simulations

Abstract

Aqueous nanoscale systems feature a distinct electrowetting behavior, sensitive to not only the strength but also the direction and polarity of the applied electric field. These effects have so far not been tested in solutions of ions, although electrolytes are commonly present in electrowetting experiments. We describe atomistic simulations of sessile salty nanodrops on an apolar substrate under electric fields in a miniature mimic of the experimental setup of a drop inside a capacitor. In the absence of field, the contact angle rises with salt concentration. The effect is weaker when the surface is more hydrophobic. The electric field lowers the contact angle, but the polarity dependence, observed in neat water and related to directional hydrogen bond effects, is diminished in the presence of screening ions. This is important for nanofluidics, as the addition of salt can improve electrowetting inside a nanopore where uneven wettabilities of opposing walls are found in pure water. The different hydration properties of cation and anion are reflected in their density profiles. The droplet curvature enhances ion withdrawal from the liquid/vapor interface, the depletion being more pronounced for cations (Na+, Mg2+) than anions (Cl–). The dependence of the contact angle on the field strength varies nonmonotonically with salt concentration, suggesting that moderate concentrations represent the optimal regime for the electric control of wetting properties at the nanoscale.