Improved condensation on hierachically structured surfaces

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Controlled generation of droplet condensation on microstructured surfaces enhances heat transfer. To achieve this we will combine strip-like surface structures with exceedingly low contact angle hysteresis with a set of integrated microstructured compartments exploited for individual droplet formation. The latter are cleared via the strip lines when the droplet dimension during the condensation process exceeds a threshold value. Through the integration of the condensation compartments besides the strip lines the generation of a continuous fluid film is prevented.The geometry and dimensions of the compartments need to be optimized to generate small droplets to improve heat transfer. The droplet size and velocity will be determined by an optical imaging set-up operated at slightly reduced temperatures in air. The same set-up is exploited to evaluate the dewetting process of droplets that are lifted up on the strip lines during condensation because their diameter exceeds the threshold value of the given surface structure. These droplet population data are necessary during the theoretical modelling of the heat transfer process to determine the thermal throughput of the given set-up. The dynamics of droplet transport is presumably supported by coalescence of already moving droplets with those still retained on the structure, leading to a sweeping effect along the strips. The overall efficiency of these heat exchange plates will be investigated with a second experimental set-up that is operated under real i.e. industrial conditions.All investigations will be first started using silicon as base material because the microstructuring processes are already available at the clean room facility of TUK. The optimized surface structures will then be transferred in a common action onto the surface of high-temperature polymers due to their industrial relevance. For this purpose, silicon masters of the negative surface structures are prepared and used to emboss the desired structure into the polymer surface. These processes need to be optimized with respect to the chemical nature of the involved surfaces to transfer nanostructures by embossing and facilitate the separation of the master and underlying polymer.The last project part is devoted to the question whether hierarchical surface structures improve the efficiency of the heat exchange plate by combining them with superhydrophobic and - or superhydrophilic nanostructures. The techniques used to generate the desired nanostructures need to be adopted to the material in use and may range from plasma etching of black silicon over direct laser writing of polymeric nanostructures to dry etching of the polymer using spincoated nanoscopic materials as a hard mask during plasma etching. Overall, the newly developed hybrid structures have a wide potential in other technical applications were non-wetting is desired (efflux from tubes - creams, ketchup, etc.).