Use of Superhydrophobic Surfaces for Performance Enhancement of Aerial–Aquatic Vehicles

Abstract

experience tradeoffs in propulsion, structural design, and vehicle configuration. [1][2][3] Superhydrophobic surfaces show great potential in providing performance benefits for aerial-aquatic robots as a passive drag reduction technique. Such surfaces do not add complexity or significant additional mass to the vehicle nor do they need actuators to display the desired properties on the vehicle they are applied to. In addition, superhydrophobic surfaces can exhibit self-cleaning properties, preventing dirt or contaminants dissolved in water to adhere to the robot's surface, providing corrosion and fouling resistance. [4] This paper examines the application of superhydrophobic surfaces in drag reduction, which small unmanned aerial-aquatic vehicles (UAAVs) can utilize to minimize their power consumption and improve their mission capabilities when transitioning between air and water.Superhydrophobic surfaces are defined as surfaces with which, when laid flat, a drop of water forms an angle larger than 150 . [5] Superhydrophobicity is achieved by producing a surface topology, which enables gas to be trapped in surface voids preventing the liquid from penetrating. This is referred to as the Cassie-Baxter state. [6] The required surface topology must have roughness in the micrometer range or smaller, with surfaces consisting of nanometer scale structures typically achieving higher contact angles. These can also be combined, forming surfaces that have nanometer scale roughness on top of micrometer scale structures, with the purpose of minimizing the contact area of the water with the surface, known as the Lotus state. [7] The use of superhydrophobic surfaces for drag reduction is a developing field that is recently gaining traction. [8] So far, the focus of the research in the field has been primarily on micro-channel flow and less research has been carried out on external hydrodynamics over 3D geometries. As such, questions remain about the fundamentals of drag reduction over these surfaces, especially in the turbulent regime. [8] For the laminar flow through a micro-channel up to 40% reduction in pressure drop was found when the walls were made superhydrophobic by etching a series of square wave patterns onto them. [8] Another channel flow study found no drag reduction for laminar flow but up to 50% drag reduction in the turbulent flow regime. [9] These experiments also showed a large reduction in drag with