We manifest a significant influence of field direction and polarity on surface wetting, when the latter is tuned by application of an external electric field. Thermodynamics of field-induced filling of hydrocarbon-like nanopores with water is studied by open ensemble molecular simulation. Increased field strength consistently results in water-filling and electrostriction in hydrophobic nanopores. A threshold field commensurate with surface charge density of about one elementary charge per 10 nm2 suffices to render prototypical paraffin surfaces hydrophilic. When a field is applied in the direction perpendicular to the confining walls, the competition between orientational polarization and angle preferences of interfacial water molecules relative to the walls results in an asymmetric wettability of opposing surfaces (Janus interface). Reduction of surface free energy observed upon alignment of confinement walls with field direction suggests a novel mechanism whereby the applied electric field can operate selectively on water-filled nanotubes while empty ones remain unaffected.

Using molecular simulations of nanosized aqueous droplets on a model graphite surface, we demonstrate remarkable sensitivity of water contact angles to the applied electric field polarity and direction relative to the liquid/solid interface. The effect is explained by analyzing the influence of the field on interfacial hydrogen bonding in the nanodrop, which in turn affects the interfacial tensions. The observed anisotropy in droplet wetting is a new nanoscale phenomenon that has so far been elusive as, in current experimental setups, surface molecules represent a very low fraction of the total number affected by the field. Our findings may have important implications for the design of electrowetting techniques in fabrication and property tuning of nanomaterials.

We examine the effect of nanoscale roughness on spreading and surface mobility of water nanodroplets. Using molecular dynamics, we consider model surfaces with sub-nanoscale asperities at varied surface coverage and with different distribution patterns. We test materials that are hydrophobic, and those that are hydrophilic in the absence of surface corrugations. Interestingly, on both types of surfaces, the introduction of surface asperities gives rise to a sharp increase in the apparent contact angle. The Cassie-Baxter equation is obeyed approximately on hydrophobic substrates, however, the increase in the contact angle on a hydrophilic surface differs qualitatively from the behavior on macroscopically rough surfaces described by the Wenzel equation. On the hydrophobic substrate, the superhydrophobic state with the maximal contact angle of 180 degrees is reached when the asperity coverage falls below 25%, suggesting that superhydrophobicity can also be achieved by the nanoscale roughness of a macroscopically smooth material. We further examine the effect of surface roughness on droplet mobility on the substrate. The apparent diffusion constant shows a dramatic slow down of the nanodroplet translation even for asperity coverage in the range of 1% for a hydrophilic surface, while droplets on corrugated hydrophobic surfaces retain the ability to flow around the asperities. In contrast, for smooth surfaces we find that the drop mobility on the hydrophilic surface exceeds that on the hydrophobic one.

Interfacial polar molecules feature a strongly anisotropic response to applied electric field, favoring dipole orientations parallel to the interface. In water, in particular, this effect combines with generic orientational preferences induced by spatial asymmetry of water hydrogen bonding under confined geometry, which may give rise to a Janus interface. The two effects manifest themselves in considerable dependence of water polarization on both the field direction relative to the interface, and the polarity (sign) of the field. Using molecular simulations, we demonstrate strong field-induced orientational forces acting on apolar surfaces through water mediation. At a field strength comparable to electric fields around a DNA polyion, the torques we predict to act on an adjacent nanoparticle are sufficient to overcome thermal fluctuations. These torques can align a particle with surface as small as 1 nm 2 . The mechanism can support electrically controlled ordering of suspended nanoparticles as a means of tuning their properties, and can find application in electro-nanomechanical devices.

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The adsorption of CO2 gas on the MgO (100) crystal surface is investigated using grand canonical Monte Carlo simulations. This allows us to obtain adsorption isotherms that can be compared with experiment, as well as to explore the possible formation of monolayers of different densities. Our model calculations agree reasonably well with the available experimental results. We find a "low-density" adsorbed monolayer where each CO2 molecule is bound to two Mg2+ ions on the MgO substrate. We also observe the formation of monolayers of higher density, where some of the CO2 molecules have rotated and tilted to expose additional binding sites. Low-temperature simulations of both the low- and high-density monolayers reveal that these states are very close in energy, with binding energies of approximately 7 kcal/mol at T=5 K. The high-density monolayer given by our model has a density that is significantly less than the reported experimental value. We discuss this discrepancy and offer suggestions for resolving it.

We apply molecular dynamics (MD) simulations to study the final phase of electrospray ionization (ESI), where an ion loses all of its associated solvent molecules. By applying an electric field to a cluster of H(2)O molecules solvating an ion and including a surrounding gas of varying pressure, we demonstrate that collisions with the gas play a major role in removing this final layer of solvent. We make quantitative predictions of the critical velocity required for the cluster to start losing molecules via collisions with gas and propose that this should be important in real ESI experiments. Such collisions have heretofore not been explicitly considered in discussions of the ESI process.

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