Shape of a droplet atop a surface heterogeneity at a nanoscale. Small aqueous droplets on homogeneous surfaces, surrounded by a reservoir of vapor are inherently unstable. In contact with supersaturated vapor, the drops will keep growing until they coalesce and form a contiguous aqueous phase. Alternatively, if vapor pressure is below that of the droplets, the droplets gradually evaporate. Departing from this common picture, when nanoscale droplets sit above hydrophilic patches on a heterogeneous surface, at certain conditions they can maintain a stable volume, determined by the pertinent contact angle and the size of the patches. Only the region under the droplet perimeter controls the contact angle, which in turn determines the drops curvature for given volume and the vapor pressure of the liquid in the drop. The drop size may therefore stop changing when its base just covers the hydrophilic patch. The finite range of water-substrate interactions, however, blurs the patch boundaries hence the nanodrop geometry varies with the patch size in a gradual manner. We use molecular simulations to examine this dependence on graphene-like surfaces with topological heterogeneity as complementing studies of chemical heterogeneity (John Ritchie, Master Thesis, VCU, 2010). We measure the microscopic analogue of the contact angle of aqueous nanodrops above circular hydrophilic or hydrophobic patches of varied size. For both the chemically and topographically heterogeneous surfaces, the results confirm the contact angle of a nanodroplet can be predicted by the local Cassie-Baxter mixing relation applied to the area within the interaction range from the drop’s perimeter, which, in turn, enables predictions of condensation and saturated vapor pressure above nanopatterned hydrophilic/hydrophobic surfaces. Switchable nanowetting dynamics. Understanding the dynamic response of contact angle on switchable hydrophobic-hydrophilic surfaces is key to the design of nanofluidic and optical devices. We use molecular dynamics simulation for water droplets with different number of molecules on a molecularly smooth and corrugated substrate. We monitored the relaxation of the droplet geometry in response to a change in surface hydrophobicity. From the time correlation function for the height of the drop’s center of mass we estimate the rates of relaxation for wetting/dewetting processes following the change between hydrophobic and hydrophilic character of the surface. On molecularly smooth surfaces, we find similar forward/backward rates revealing insignificant hysteresis. Calculations on corrugated surfaces, however, reveal quite different relaxation times for forward (Cassie state to Wenzel state) and reverse processes. The observed hysteresis is associated with different friction forces between the droplet and the surface during advancing and receding processes. We calculate the friction coefficient of the corrugated surface for the forward process following the increase in surface hydrophilicity. We compare continuum hydrodynamic (HD) and molecular kinetic theories (MKT) for calculation of the friction coefficient. Although the small size of our system suggests the use of molecular description of the surface, incorporated in MKT, we obtain essentially equal friction coefficients from both theories. This information indicates an overlap between continuum hydrodynamics and molecular dynamics regimes, with both the HD and MKT theories being applicable at the nanoscopic lengthscales we consider. Water dynamics inside nanospheres. Chemical nature of a spherical confinement has significant effect on dynamics of water molecules outside the cage. In a separate study we examined the effect of chemical nature of the cage on the dynamics of water molecules inside the cage. Calculations have been made for variety of time correlation functions of water in four different sizes of spherical hydrophobic/hydrophilic confinements, Cx x=320, 500, 720, 1500 based “hollow buckyballs”, with different spherical pore diameters. Calculated water hydrogen bond lifetimes, diffusion coefficients and rotational relaxation times in these systems reveal a distinctly different water dynamics compared to interfacial water dynamics outside the cage: interestingly we find insignificant changes in time scales for water dynamics in hydrophilic and hydrophobic carbon cages. Even adding partial charges to hydrophilic confinement did not make a big effect on results compared to hydrophobic case. These findings are suggesting that in highly symmetric confinement water molecules do not care about the type of interaction with the wall because of cancellation of forces in different directions.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-1297 |
| Date | 12 September 2011 |
| Creators | Seyed, Yazdi Jamileh |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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