Complete Electrolytic Plastron Recovery in a Low Drag Superhydrophobic Surface.

We present a superhydrophobic surface capable of recovering the lubricious gas layer known as the "plastron" from a fully wetted state underwater. It is shown that full plastron recovery is possible without a second layer of structural hierarchy, which is prone to irreversible wetting transitions. This allows us to use a cheap, fast, and potentially scalable method to fabricate the surface from silicone and carbon black in a molding process. We demonstrate plastron recovery from the fully wetted state and immediate plastron recovery after pressure-induced wetting transitions. The wetting state can be measured remotely and quickly by measuring the capacitance. The slip length is measured as ∼135 µm, agreeing well with the theory given the geometry of the surface. The ability of the surface to conform to small radii of curvature and withstand damage from loading is also demonstrated. The work presented here could allow superhydrophobic surfaces to reduce drag on ships and in pipes where the plastron would otherwise rapidly dissolve.

Active gas replenishment and sensing of the wetting state in a submerged superhydrophobic surface.

Previously superhydrophobic surfaces have demonstrated effective drag reduction by trapping a lubricious gas layer on the surface with micron-sized hydrophobic features. However, prolonged reduction of drag is hindered by the dissolution of the gas into the surrounding water. This paper demonstrates a novel combination of superhydrophobic surface design and electrochemical control methods which allow quick determination of the wetted area and a gas replenishment mechanism to maintain the desirable gas filled state. Electrochemical impedance spectroscopy is used to measure the capacitance of the surface which is shown to be proportional to the solid/liquid interface area. To maintain a full gas coverage for prolonged periods the surface is held at an electrical potential which leads to hydrogen evolution. In the desired gas filled state the water does not touch the metallic area of the surface, however after gas has dissolved the water touches the metal which closes the electrochemical circuit causing hydrogen to be produced replenishing the gas in the surface and returning to the gas filled state; in this way the system is self-actuating. This type of surface and electrochemical control shows promise for applications where the gas filled state of superhydrophobic surfaces must be maintained when submerged for long periods of time.

Wetting of Surfaces Made of Hydrophobic Cavities.

Templated electrodeposition through a close packed, monolayer array of 3 µm polystyrene spheres followed by removal of the template by dissolution in an organic solvent was used to fabricate sphere segment void (SSV) surfaces in gold with heights up to 1.5 µm. These surfaces were made hydrophobic by treating with 1-dodecanethiol. Contact angle measurements show that the wetting behavior of these surfaces change significantly with film thickness. The apparent advancing contact angle increases from 110° for the flat 1-dodecanethiol-coated gold surface to 150° for the film with a close-packed array of hemispherical cavities, in good agreement with the behavior predicted by the simple Cassie-Baxter equation. In contrast, the apparent receding angles have significantly smaller values in all cases, and water droplets are strongly pinned at the surface. Thus, these surfaces demonstrate "rose petal" behavior, in which a large apparent advancing contact angle, typical of a superhydrophobic surface, is accompanied by significant contact angle hysteresis. Observation of the shapes of drops on the surface during evaporation-driven recession shows that the drops adopt a dodecagonal shape, in which the drop perimeter is selectively pinned along the ⟨10⟩ and ⟨11⟩ directions on the hexagonally close-packed surface.