Molecular Dynamics simulations of polymer liquids on substrates of different topography
von Nikita Tretyakov
Datum der mündl. Prüfung:2012-12-17
Erschienen:2013-02-04
Betreuer:Prof. Dr. Marcus Müller
Gutachter:Prof. Dr. Marcus Müller
Gutachter:Prof. Dr. Reiner Kree
Zum Verlinken/Zitieren:
http://dx.doi.org/10.53846/goediss-3703
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Zusammenfassung
Englisch
The properties of polymer films and droplets at substrates of different topography are studied employing particle-based simulation techniques (Molecular Dynamics). The liquid is modeled by short coarse-grained polymer chains of 10 monomers, while the temperature of the system is controlled by the Dissipative Particle Dynamics (DPD) thermostat that conserves momentum locally and provides correct hydrodynamics. Throughout this dissertation we show that macroscopic concepts cannot be straightforwardly extrapolated down to microscopic systems. At first the flat topography of the substrate is studied. A parameter-passing technique is explored that bridges particle-based MD simulations with continuum descriptions (CD) of the liquid. In particular, the liquid-vapor, solid-liquid and solid-vapor interfacial tensions, and the interface potential are determined by MD simulations. This information is then introduced into continuum models accounting for (i) the full curvature and (ii) a long-wavelength approximation of the curvature (thin film model). A comparison of the dependence of the contact angle on droplet size indicates that the theories agree well if the contact angles are defined in a compatible manner. Then, we proceed with substrates that are structured symmetrically. Their surface represents a regular array of grooves. The crucial feature of this system is that the typical dimensions of corrugations are of the order of ten diameters of fluid particles. We investigate the influence of corrugation, wettability and pressure on slippage and friction at the solid-liquid interface. For symmetrically structured substrates we observe a gradual crossover between the Wenzel state, where the liquid fills the grooves, and the Cassie state, where the corrugation supports the liquid and the grooves are filled with vapor. Using two independent flow set-ups, we characterize the near-surface flow by the slip length, $\delta$, and the position, $z_h$, at which viscous and frictional stresses are balanced according to Navier’s partial slip boundary condition. This hydrodynamic boundary position depends on the pressure inside the channel and may be located above the corrugated surface. In the Cassie state, we observe that the edges of the corrugation contribute to the friction. Finally, we consider asymmetrically structured substrates. This type of topography implies an asymmetric response of the droplet’s shape onto periodic vibrations. Hence, directed motion of droplets can be achieved. By an analytical phenomenological model we explain the direction of motion and verify it by several computations. Then, the mechanism of the driving is investigated: along with the commonly described motion due to contact lines, we find that the contact area itself may additionally drive the droplet. We show that modifying the roughness of the substrate, one controls the dissipations of the input power due to substrate vibrations and different regimes of droplet motion may be established.
Keywords: Molecular Dynamics; Microfluidics; Slippage; Corrugated substrate; Directed Transport; Wetting; Superhydrophobicity