Anisotropic wettability of biomimetic micro/nano dual-scale inclined cones fabricated by ferrofluid-molding method

Learning from nature, a series of cone-shaped structures resembling trichomes of plants are fabricated by ferrofluid molding to understand the influence of geometry on wettability. Experimentally, ferrofluid microdroplets are generated under an external magnetic field, and their shape can be changed from right cones into oblique cones by tilting the external magnetic field. Followed by hard molds made with UV-curable tri(propylene glycol) diacrylate, polydimethylsiloxane microcones with different inclination angle (θ) are subsequently generated. Nickel thin film is deposited onto the microcones to form micro/nano dual-scale structures. The largest contact angle (CA) is obtained in nickel-deposited right cones (CA = 163.1° ± 2.5°). Anisotropic wettability is exhibited in oblique cones and the retention forces in the pin and release directions differ up to 12 μN (cones θ = 50°). As explained by a model as a function of the inclination angle of the cone structures, the contact and retention forces of droplet move in pin and release directions exhibit considerable differences. Results suggest the inclination of the trichomes assist the balance between repellency and retention of water in a direction-selective manner.