Modeling Cassie droplets on superhydrophobic coatings with orthogonal fibrous structures

Superhydrophobic coatings comprised of orthogonally layered fibers are studied in this paper in terms of their ability to accommodate water droplets in the non-wetting Cassie state. The effects of microstructural properties of these coatings on droplet contact angles and Cassie state stability are investigated via numerical simulation. More specifically, mathematical expressions are derived to predict whether or not such fibrous coatings can provide sufficient capillary forces for the droplet to remain in the Cassie state. For comparison, similar coatings comprised of parallel fibers are also studied, as a droplet may only interact with the first layer of fibers (parallel fibers) when the fiber spacing is smaller than some critical spacing value. Considerable differences were observed between droplet contact angles on coatings made of orthogonally layered fibers and those having multiple layers of parallel fibers. Our numerical simulations conducted using the Surface Evolver finite element code indicated that apparent contact angle of a droplet can be different in longitudinal and transverse directions, and they both increase by decreasing the diameter of the fibers or by increasing their spacing. It was also found that contact angle in the longitudinal direction is more sensitive to the spacing or the diameter of the fibers. It was also found that a droplet may achieve higher contact angles on.a coating with orthogonally layered fibers than on its parallel-fiber counterpart. (C) 2016 Elsevier B.V. All rights reserved.