Interfacial Water at Hydrophobic and Hydrophilic Surfaces: Slip, Viscosity, and Diffusion

Abstract

The dynamics and structure of water at hydrophobic and hydrophilic diamond surfaces is examined via non-equilibrium Molecular Dynamics simulations. For hydrophobic surfaces under shearing conditions, the general hydrodynamic boundary condition involves a finite surface slip. The value of the slip length depends sensitively on the surface water interaction strength and the surface roughness; heuristic scaling relations between slip length, contact angle, and depletion layer thickness are proposed. Inert gas in the aqueous phase exhibits pronounced surface activity but only mildly increases the slip length. On polar hydrophilic surfaces, in contrast, slip is absent, but the water viscosity is found to be increased within a thin surface layer. The viscosity and the thickness of this surface layer depend on the density of polar surface groups. The dynamics of single water molecules in the surface layer exhibits a similar distinction: on hydrophobic surfaces the dynamics is purely diffusive, while close to a hydrophilic surface transient binding or trapping of water molecules over times of the order of hundreds of picoseconds occurs. We also discuss in detail the effect of the Lennard-Jones cutoff length on the interfacial properties.

The interplay between apparent viscosity and wettability in nanoconfined water

“…4a. First, these values are in good agreement with existing experimental or theoretical values found in literature 18,[20][21][22][23] (see Supplementary Table S3). Second, the good superposition of the fitting curves with the experimental data points confirms that equation (4) is able to capture the physics behind the different AFM energy dissipation measurements.…”

“…leaving b as a free-fitting parameter, to be compared with data measured or calculated in literature with different methods [18][19][20][21] . The fitting process is illustrated in Fig.…”

Cutting Ice: Nanowire Regelation

“…The observed behavior is somewhat counterintuitive: A hydrophobic surface in contact with water should move more easily than a hydrophilic one, since surface trapping and short slip lengths, leading to increased friction [36], are expected on a hydrophilic surface, but not on a hydrophobic one [37]. Here, however, hydrophobicity makes the wire resist water molecules from covering its surface and the remaining solid obstructs the liquid from moving around and past the wire.…”

Direct observation of stick-slip movements of water nanodroplets induced by an electron beam

“…These theories imply that perturbations, either from external physical forces or chemical nonuniformities are coupled to dynamics of a nanodroplet through changes in shape and thus causing translocation of nanometer size droplets. Interestingly, very recent simulations also predict that nanometer-diameter water droplets will slip on hydrophilic surfaces (15) similar to those on hydrophobic surfaces (16,17). However, studying the structural dynamics of nanodroplets is experimentally challenging because one needs to be able to externally induce the movement and be able to image the subsequent dynamic process of nanoscale droplets.…”