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.

Water droplets tend to bead up on rough or textured hydrophobic surfaces by trapping air on the crevices underneath resulting in “Cassie” state of wetting. When a textured hydrophobic surface is immersed in water, the resulting underwater “Cassie” state can lead to significant drag reduction. The entrapped air pockets act as shear free regions and the composite interface consisting of alternate no slip and no shear regions thus formed can deliver substantial drag reduction during flow. The magnitude of drag reduction depends not only on the fractional coverage of air on the surface, but also on the size of the air pockets, with larger sized air pockets facilitating larger drag reduction. It is a common observance that Lotus leaf when kept immersed in water for a few minutes loses its water repellency due to the loss of entrapped air on the surface. Underwater Cassie state on textured hydrophobic surfaces is also not sustainable because of the depletion of air pockets caused by the diffusion of trapped air into water. This causes the drag reduction to diminish with time. Rate of diffusion of air across the water–air interface depends on the concentration gradient of air across the interface. Under flow conditions, removal of entrapped air is further enhanced by convection, leading to more rapid shrinkage of the air pockets. In order to sustain the Cassie state, it is thus necessary to continuously supply air to these air pockets. In this work, we explore the possibility of supplying air to the cavities on the textured surface inside a microchannel by controlling the solubility of air in water close to the surface. The solubility is varied by i) Controlling the absolute pressure inside the channel and ii) Localized heating of the surface To trap uniform air pockets, a textured surface containing a regular array of blind holes is used. The textured surface is generated by photo etching of brass and is rendered hydrophobic through a self-assembled monolayer. The sustainability of the underwater Cassie state of wetting on the surface is studied at various flow conditions. The air trapped on the textured surface is visualized using total internal reflection based technique, with the pressure drop (or drag) being simultaneously measured. Water which is initially saturated with air at atmospheric conditions, when subjected to sub-atmospheric pressures within the channel becomes supersaturated causing the air bubbles to grow in size. Further growth causes the bubbles to merge and eventually detach from the surface. The growth and subsequent merging of the air bubbles leads to a substantial increase in the pressure drop because as the air pockets grow in size, they project into the flow and start obstructing the flow. On the other hand, a pressure above the atmospheric pressure within the channel makes the water undersaturated with air, leading to gradual shrinkage and eventual disappearance of air bubbles. In this case, the air bubbles do cause reduction in the pressure drop with the minimum pressure drop (or maximum drag reduction) occurring when the bubbles are flush with the surface. The rate of growth or decay of air bubbles is found to be significantly dependent on the absolute pressure in the channel. Hence by carefully controlling the absolute pressure, the Cassie state of wetting can be sustained for extended periods of time. A drag reduction of up to 15% was achieved and sustained for a period of over 5 hours. Temperature of water also influences the solubility of air in water with higher temperatures resulting in reduced solubility. Thus locally heating the textured hydrophobic surface causes the air bubbles to grow, with the rate of growth being dependent on the heat input. The effect of trapped air bubbles on thermal transport is also determined by measuring the heat transfer rate through the surface in the presence and absence of trapped air bubbles. Even though the trapped air bubbles do cause a reduction in the heat transfer coefficient by about 10%, a large pressure drop reduction of up to 15% obtained during the experiments helps in circumventing this disadvantage. Hence for the same pressure drop across the channel, the textured hydrophobic surface helps to augment the heat transfer rate. The experiments show that, by varying the solubility of air in water either by controlling the pressure or by local heating, underwater Cassie state of wetting can be sustained on textured hydrophobic surfaces, thus delivering up to 15% drag reduction in both cases for extended periods of time. The results obtained hold important implications towards achieving sustained drag reduction in microfluidic applications.