Metastable Sessile Nanodroplets on Nanopatterned Surfaces

Abstract

Small aqueous droplets on homogeneous surfaces surrounded by a reservoir of vapor are inherently unstable. Depending on the humidity, they keep evaporating and ultimately disappear or grow until they fully wet the surface under supersaturation. We are considering a system departing from this common picture. For nanoscale droplets sitting above hydrophilic patches on a heterogeneous surface, there can exist a range of supersaturated pressures at which the droplets maintain a stable volume, determined by the pertinent contact angle and the size of the patches. The region under the droplet perimeter controls the drop's curvature. Vapor pressure rises along with increased curvature as soon as the drop extends into the hydrophobic area. The drop size may therefore remain stable when its base just covers the hydrophilic patch. The finite range of water−substrate interactions, however, blurs the boundaries between surface regions with different hydrophilicities; hence, the nanodrop contact angle varies with the patch size in a gradual manner. We use molecular simulations to examine this dependence on model surfaces with either chemical or topological heterogeneities. For both types of heterogeneities, our results show 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 for drop condensation and saturated vapor pressure above partially wetted nanopatterned hydrophilic/hydrophobic surfaces.

On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics

“…For a typical case as shown above: the case of a drop of roughly 26 nm in diameter and considering the planar limit of the fluid-vapor tension (72 mN·m . While this number seems to be different from those estimated from micrometer drop experiments [35] or from molecular simulation of atomistic water models [36,37]; it is appropriate to point out that values as low as ×10 −11 J·m −1 and as high as ×10 −5 J·m −1 have been reported [33], which place our results in the correct context. Obviously, there is scope for much more detailed research into this topic.…”

Interfacial Tension Does Not Drive Asymmetric Nanoscale Electrowetting on Graphene

“…We suggest that these asymmetries likely originate from molecular properties 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 23 while the contact area spans at least 11 nm in the same direction. In view of the work of Ritchie et al, 49 To provide a more direct evidence for the influence of the contact line on the observed asymmetry for pure water, the orientation ordering of the water molecules has been calculated. A similar orientation ordering is found for the 4 M NaCl system (not shown here), indicating that the structure of the water molecules next the co...…”

Surface nanobubbles and nanodroplets

“…They also lead to contact line hysteresis, i.e., the difference between an advancing and a receding contact angle (Neumann et al, 1974;Joanny and de Gennes, 1984;de Gennes, Brochard-Wyart, and Quere, 2004;Quere, 2008;Ramiasa et al, 2013). Indeed, Ramos, Charlaix, and Benyagoub (2003) showed that the amount of contact angle hysteresis linearly increases with the density of defects on nanostructured surfaces (provided it is not too high), due to the pinning-depinning process of the contact line at individual defects.…”