Unidirectional Wetting in the Hydrophobic Wenzel Regime

Abstract

wetting property on desired surfaces. [1][2][3] In addition, it was also found that unidirectional wetting can also be achieved on a surface through the deposition of a coating with a surface energy gradient, [ 10 ] or by decorating it with an orderly array of Janus nanostructures, which possess a hydrophilic face on one side and a hydrophobic face on the other. [ 5,11,12 ] When a water droplet is deposited on these nanostructures and if the substrate material is suffi ciently hydrophilic ( α < 65°, where α refers to the intrinsic water contact angle of the substrate material), the liquid can infi ltrate into the space between the nanostructures, wetting their sides and the base. [ 1 ] The droplet then exhibits a net contact angle that is less than 90°. We shall refer to this case as the hydrophilic Wenzel state. However, if the substrate material is not as hydrophilic (65° < α < 90°) and the water droplet still wets the nanostructures fully, the droplet will most likely exhibit a net contact angle that is greater than 90° due to strong contact line pinning at the edges of the nanostructures. [ 13 ] This shall be referred to as the hydrophobic Wenzel state. Finally, if the substrate material is intrinsically hydrophobic ( α > 90°), then the liquid droplets would adopt the Cassie-Baxter state instead, wetting only the tips of the nanostructures, effectively sitting on a fl at, composite surface made up of air and nanostructure tips. [ 2,14 ] In all of the cases studied thus far on structurally asymmetric nanostructures, droplets exhibiting unidirectional wetting characteristics are found to adopt either the hydrophilic Wenzel state [ 1,3,5,15 ] or Cassie-Baxter state. [ 2 ] To achieve unidirectional wetting in the hydrophobic Wenzel regime, current theoretical models predict that heavily bent nanostructures are required, [ 1,15,16 ] and experimental data for borderline cases ( α = 65°) appear to support this hypothesis. [ 1,15 ] In this report, however, we show that, in contrast to these expectations, unidirectional wetting in the hydrophobic Wenzel regime can be induced by slightly bent nanostructures. The hydrophobic Wenzel state was achieved by allowing water droplets to spread on an orderly array of bent hydrophilic Si nanowires protruding from a hydrophobic base of Au coating, an arrangement that is made possible by the use of oblique angle deposition and metal assisted chemical etching of Si. Furthermore, through the novel use of simple NaCl precipitation, we were able to observe the nanoscale features of the droplet near the triple phase contact line, obtaining important experimental insights into the mechanism of unidirectional wetting on asymmetric nanostructures.Unidirectional wetting surfaces can cause liquid droplets to fl ow/move in one direction while pinning them in the other directions, a feature that is useful for biosensing, adhesives, thermal management, and microfl uidics. Such surfaces can be fabricated by employing structurally or chemically asymmetric nanostructures. While unidirectional wet...

Wetting against the nap – how asperity inclination determines unidirectional spreading

“…Interestingly, we find that the allowed line motion is not with the cones, as intuition suggests, but against the cones. This result also contrasts with most previous studies with tilted surface features [14][15][16][17]20 where easy motion is found along the feature direction. Motion against the features has only been observed on sharp tilted cones, a system very similar to ours 19 and asymmetric high aspect ratio polymeric posts 24 .…”

“…Therefore, this data point has not been included in the following analysis. We first compare these experimental results to a simple line average model 16,19,26 . Following the rule of mixture, we average wetting conditions along the triple line 21,22 .…”

“…In practice, one possibility is to resort to biomorphological features 12,13 or naturally occurring asymmetries during material elaboration 14 . A more controlled way is offered by more or less complex lithographic processes 15,16 , usually involving some oblique incidence modification step [17][18][19] .…”

“…Precise numerical investigations however demonstrate that the result holds only for relatively hydrophilic surfaces 20 . Experimentally, it was also found to fail for denser structures 16 .…”

Directional imbibition on a chemically patterned silicon micropillar array

“…19 Surface energy patterns combined with pillar topographies can also be utilized for creating anisotropic wetting, as demonstrated by uni-bi-, and -tridirectional wetting based on oblique glancing angle deposition. 20,21 Anisotropic wetting can also be achieved based strictly on surface energy patterns without a topographic contribution, as demonstrated by bidirectional wetting on top of hydrophilic grooves. 22 Temperature tunable unidirectional wetting has been achieved through a poly-(N-isopropylacrylamide) coating on top of the prism structure 23 or a half shell nanocone structure.…”