At the transition to the nanoscale, finite-size effects become significant which are often neglected in phenomenological models. The equilibrium properties and the dynamics of droplets in contact with nanostructured surfaces are hard to capture experimentally due to the high resolution which is required and due to the short time scale at which significant fluctuations may occur.
In the present work, molecular dynamics simulation is applied to wetting phenomena for systems where the fluid-fluid, fluid-solid, and solid-solid interactions are modelled by the Lennard-Jones truncated and shifted pair potential. Scenarios with different surface morphologies are considered: For sessile droplets on a perfectly planar solid substrate, the dependence of the contact angle on the strength of the fluid-solid interaction, the temperature, and the density of the solid is characterized [1]. Furthermore, droplets are simulated in the epitaxial Cassie configuration, where the contact line of an advancing droplet reaches the edge of a surface structure. For sessile droplets on patterned surfaces in the impregnation wetting regime, equilibrium contact angles and non-equilibrium contact line dynamics are studied. In this way, the influence of the surface morphology on the contact angle and the contact line is captured with molecular resolution.
In the epitaxial Cassie state, the range of contact angles which occur in mechanical equilibrium at an edge is confirmed to agree with phenomenological predictions from the Gibbs inequality. However, the exact position at which the contact line is pinned deviates from the position of the edge slightly. For the equilibrium contact angle on impregnated surfaces, both homogeneously and heterogeneously structured surfaces are considered, and significant deviations from predictions by the Cassie model are found.
The dynamics of advancing contact lines, during spreading processes of droplets over a lyophilic surface which is structured at the nanometre length scale, are characterized by activated transitions as discussed by de Gennes [2]. Thereby, first, a liquid protrusion is formed in radial direction; subsequently, the protrusion advances in azimuthal direction. Depending on the initial configuration and the boundary conditions, this may be a single-step or a multi-step process, and the spreading process may terminate in a symmetrical or an asymmetrical state. Accordingly, it is crucial to account for deviations of the instantaneous shape of the droplet from spherical symmetry. On the basis of the present results, the perspective of a multiscale simulation approach is discussed where phase field modelling and molecular dynamics simulation results are combined [3].
[1] S. Becker, H. M. Urbassek, M. Horsch, H. Hasse, Contact angle of sessile drops in Lennard-Jones systems, Langmuir 30(45), 13606-13614, 2014.
[2] P. G. de Gennes, Wetting: Statics and dynamics, Reviews of Modern Physics 57, 827-863, 1985.
[3] F. Diewald, C. Kuhn, R. Blauwhoff, M. Heier, S. Becker, S. Werth, M. Horsch, H. Hasse, R. Müller, Simulation of surface wetting by droplets using a phase field model, Proceedings in Applied Mathematics and Mechanics 16, 519-520, 2016.
