Conical micro-structures as a route for achieving super-repellency in surfaces with intrinsic hydrophobic properties

Abstract

Hydrophobic surfaces like Lotus leaves show amazing self-cleaning properties with the apparent water contact angle above 150° and contact angle hysteresis below 10°. Thus, at low inclination angles, millimeter drops can roll-off easily. This effect can be a consequence of the air trapped below the drop, which allows the droplet to reach a superhydrophobic Cassie-Baxter state. However, the superhydrophobic state can be accompanied by very different adhesive properties due to the pinning of the droplet to the microstructures, implying that even in a hydrophobic or superhydrophobic state, the droplet might not roll-off easily. A superhydrophobic state with minimum adhesion to the surface has been the pursuit in many applications where a super-repellent state is highly desired. Many microstructures have been shown to be able to reach a superhydrophobic state, but only a few have been shown to be capable of achieving a super-repellent state without the help of more complex hierarchical structures. Here, we show that conical structures provide a template for designing super-repellent surfaces where the wetting characteristics look to be invariant in the microscale range. The conical structures can maintain a super-repellent state for all intrinsic contact angles larger than 90°, and the transition from the Cassie-Baxter to the Wenzel state is controlled by the apex angle of the conical structures. This finding advances the understanding of why conical structures can show a superhydrophobic state, which will be beneficial for the design of super-repellent surfaces with a wider intrinsic contact angle range.