Some insect bodies have evolved the ability to repel water and oil, adhere to different surfaces, and eliminate light reflections. Scientists have been studying the physical mechanisms underlying these properties found in nature and mimicking them to design materials for use in everyday life.
Several years ago, Brookhaven National Laboratory developed a nanoscale surface-texturing method for imparting complete water repellency to materials — a property inspired by insect exoskeletons that have tiny hairs designed to repel water by trapping air. Their method leverages the ability of materials called block copolymers (chains of two distinct molecules linked together) to self-assemble into ordered patterns with dimensions measuring only tens of nanometers in size. The scientists used these self-assembled patterns to create nanoscale textures in a variety of inorganic materials, including silicon, glass, and some plastics. Initially, they studied how changing the shape of the textures from cylindrical to conical impacted materials’ ability to repel water. Cone-shaped nanotextures proved much better at forcing water droplets to roll off, carrying dirt particles away and leaving surfaces completely dry.
Further work has demonstrated that the optimized nanotextures have excellent anti-fogging abilities, which could be applicable for condensing coils of steam turbine power generators, car and aircraft windshields, and other materials prone to fogging.
Fog forms when warm, moist air hits a cooler surface (such as a window or windshield) and forms water droplets. When water droplets are similar in size to the structural features of a textured hydro-phobic surface, they can get inside and grow within the texture, instead of remaining on top. Once the texture fills up, water landing on the material gets stuck, resulting in the appearance of fog.
Scientists previously observed that the wings of cicadas, which are covered by nanosized cone-shaped textures, have the ability to repel fog by causing water droplets to spontaneously jump off their surface — a phenomenon caused by the efficient conversion of surface energy to kinetic energy when two droplets combine. The team investigated how reducing texture size and changing texture shape impacts the anti-fogging ability of a model surface.
To simulate fogging conditions, the scientists heated water and measured the adhesion force as warm water droplets cooled upon contacting the nanotextured surfaces. These measurements revealed that droplet adhesion was significantly affected by the type of surface nanotexture, with warm drops strongly sticking to those with large textures, and hardly sticking to surfaces with the smallest ones. The droplets remain on top, essentially floating on the cushion of air trapped beneath.
The scientists next used an optical microscope connected to a high-resolution video camera to view droplet condensation on different textures during dew formation, when atmospheric moisture condenses faster than it evaporates. While all textures are initially covered by large numbers of microdroplets, over time, textures with a cylindrical shape become covered in water, while the ones with a conical shape spontaneously dry themselves. Conical-shaped textures resist dew formation because the water droplets are so lightly adhered to the surface that when two drops join together, they gain enough energy to spontaneously jump off the surface, similar to the mechanism observed in cicada wings.
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