Bian, Yifei; (2020) Droplet Behaviour on Superhydrophobic surfaces. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Superhydrophobic surfaces, implied by its name, refer to ‘ultra-water-repellent surfaces’. They were observed in nature and researched by people for their intrinsic mechanism behind low water adhesion. Massive researches have been conducted recently for novel fabrications and further applications of superhydrophobic surfaces. This thesis will start from the synthesis, then present studies on several behaviours of water droplets on surfaces, including sliding, evaporation and collision. Firstly, a new synthesis approach of superhydrophobic thin films by modification of a Co3O4 film was illustrated. The surface morphology could be controlled by the reaction time, and the investigation into the relationship between the water contact angle, sliding angle, droplet size and surface micro-structures showed that the watersurface adhesion can be effectively manipulated through tailoring the morphology or the size of the micro-structures of the surfaces. After that, evaporation of water droplets was studied as a dynamic behaviour, and the effect of temperature was taken into account. An experimental and theoretical understanding of the evaporation of sessile water droplets was reported at different static water contact angles on hydrophobic and superhydrophobic substrates. With the assumption that two evaporation modes, constant contact radius (CCR) mode and constant contact angle (CCA) mode dominate successively during the process the lifetime of the droplets was analyzed with substrate temperature. The result shows that generally the lifetime increases as surface contact angle rises whereas it decreases when the temperature rises. Evaporative cooling effect was taken into consideration and shown to have a minor influence on the process. Finally, the thesis reports another water droplet behaviour. The coalescence and rebound process of binary droplets with different temperatures on superhydrophobic surface were studied. During the experiments, a ‘stationary’ droplet was kept on a superhydrophobic surface at room temperature and an ‘impacting’ droplet with temperature ranging from room temperature to 70 ℃ was released from above. The findings reveal that an increase in the temperature of the impacting droplet facilitated the coalescence process between the binary droplets. When both the temperatures of the impacting droplet and of the stationery were at room-temperature, a large deformation was observed before merging, while the deformation reduced as the temperature of the ‘impacting’ droplet increased. Energy dissipation was also considered in the study

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