M.J. Cook, J.H. Johnston, J. Leveneur
Victoria University of Wellington,
Keywords: wetting, polymer, surfaces, micro-structured, water harvesting
Summary:We present an attractive new technology for the design and fabrication of micro-structured water harvesting surfaces through a unique combination of 3D printing and hydrophilic/hydrophobic surface modification. For many years, researchers have examined the wetting behaviours of natural surfaces and applying similar principles to newly created surfaces. For example, the high degree of superhydrophobicity exhibited by the famed “Lotus Leaf” has been replicated several times using novel methods developed in the laboratory. The same is true for anisotropic wetting behaviours, such as that which occurs on the back of the Stenocara gracilipes beetle. Utilising areas of both hydrophilicity and hydrophobicity allows the creature to collect liquid water from the foggy morning air via condensation and directional flow. The beetle’s back has hydrophilic bumps to facilitate condensation of water droplets. These droplets then grow and coalesce until they are sufficiently large that they fall into surrounding hydrophobic troughs that channel the water to the beetle's mouth, allowing it to survive in the arid Namib desert. In this work we aimed to simplify the water-harvesting process demonstrated by the beetle, using facile manufacturing and processing methods to produce an innovative water-harvesting device. Anisotropic wetting behaviour can be induced in a surface by creating 1-dimensional roughness on the surface. This can be achieved by forming a series of suitably sized lines/channels in the surface that extend in only one direction. This promotes the spreading and movement of water droplets in the direction parallel to these lines/channels and hinders the spreading and movement in the perpendicular direction. In our work presented here, 3D printing was used as a method of rapid prototyping to produce ABS polymer surfaces with micro-channels of varying size and shape. The wettability of the surfaces was assessed and the optimum structure was determined, to facilitate the movement of droplets in only the direction parallel to the channels. These surfaces were then treated with a fluoroalkyl silane (FAS) to further increase their hydrophobicity. They were then patterned with a novel physical method. The physical treatment caused the surface to be rendered hydrophilic in the desired patterned areas. This was demonstrated by a contact angle of less than 65° and droplets observed as sitting in a Wenzel state. The combination of 1-dimensional anisotropic surface roughness and patterned areas of hydrophilicity and hydrophobicity, has led to the creation of a surface with hydrophilic lines surrounded by hydrophobic channels that allow water flow in only the direction parallel to the lines/channels. This surface proved to be an effective water-harvesting device. The manufacturing of this device was indeed facile and commercially viable. The 3D printed surface can be easily reshaped and resized to suit different end-use water harvesting requirements and devices, and the post treatments can easily be performed on a smaller or larger scale. As the surface chemistry of the polymer can be altered using FAS surface functionalisation and patterned physical modification, this method can also be extended to metals and other substrates, depending on the particular application.