Water–substrate physico-chemistry in wetting dynamics

Abstract

We consider the wetting of water droplets on substrates with different chemical composition and molecular spacing, but with an identical equilibrium contact angle. A combined approach of large-scale molecular dynamics simulations and a continuum phase field model allows us to identify and quantify the influence of the microscopic physics at the contact line on the macroscopic droplet dynamics. We show that the substrate physico-chemistry, in particular hydrogen bonding, can significantly alter the flow. Since the material parameters are systematically derived from the atomistic simulations, our continuum model has only one adjustable parameter, which appears as a friction factor at the contact line. The continuum model approaches the atomistic wetting rate only when we adjust this contact line friction factor. However, the flow appears to be qualitatively different when comparing the atomistic and continuum models, highlighting that non-trivial continuum effects can come into play near the interface of the wetting front.

Molecular origin of contact line friction in dynamic wetting

“…These simple atoms are non-charged and do not form hydrogen bonds with water molecules. See [12] for further details. The initial radius R of the water droplets are 50 nm and their width w = 4.67 nm.…”

“…A recent study has shown that the no-slip condition in dynamic wetting is a consequence of liquid molecules forming hydrogen bonds with substrate molecules [12]. Hydrogen bonds are a particular class of transient and non-covalent electrostatic bonds between polar molecules, such as water.…”

Steady moving contact line of water over a no-slip substrate

“…From the applied mathematical point of view, Navier slip regularisation leads to an approximation in which the curvature still diverges logarithmically at the contact line and to a contradiction if the velocity field is continuous [20,21]. Moreover certain fluid and surface combinations have very small slip lengths, below the nanometer scale [22,23]. In such systems, if one considers the problem at smaller and smaller scales, other molecular effects will become relevant before the slippage effects.…”

Theoretical Calculation of Thermodynamic Properties and Diffusion Coefficients for Pure Ethanol, Pure Water and Binary Mixture of (Ethanol + Water) as Function of Temperature by Molecular Dynamic Simulation

“…Temperature (v-rescale) and pressure (Parrinello-Rahman) were controlled by using weak coupling thermostats [15,16]. Electrostatic interactions were treated by means of Particle Mesh Ewald (PME) approach [17,18]. Initial atomic velocities were created on the basis of Maxwellian distribution at the absolute temperature [19,20].…”