The Self-Cleaning Property Of Superhydrophobic Surfaces Based On The Phenomenon Of Droplet Rebound

The self-cleaning property of superhydrophobic surfaces is very important in real life. Several self-cleaning phenomena are studied in this paper, including the self-cleaning for micro-drops, the self-cleaning for dust particles and the the self-cleaning for condensate droplets. These self-cleaning properties are closely related to the rebound behavior of droplets. The droplets rebound from surfaces can not only prevent the liquid from wetting the solid surfaces, but also pick up dust particles by the spreading and recoiling processes of drops. In addition, the self-propelled jumping hebavior of coalescent condensate droplets on a superhydrophobic surface enables the cooling surface to be self-cleaning for condensate drops.The effect of the rebound behavior of drops on the self-cleaning is studied systematically in this thesis.Firstly, superhydrophobic epoxy resin particulate films are prepared. This film is prepared by coating method, which is simple. It can satisfy the requirement of large-scale preparation. The curing time of epoxy suspension is an important condition to obtain films with good comprehensive performances. The coated surface is highly hydrophobic and the impacting water droplets are bounced off from the surface, resulting in the surface to be self-cleaning under impinging conditions.Secondly, the effect of the geometrical shape of micro-pillars on the rebound behavior are investigated by CFD simulation method. Six types of pillars are prepared, namely, triangular prism, square pillar, pentagonal prism, cylindrical pillar, and crisscross pillar. The CFD model is verified with experimental data and an analytical mode(AM). The AM is established to predict the penetrating depth and the maximum spreading diameter of an impinging droplet. The bouncing ability of an impinging droplet on textured surfaces can be illustrated from three aspects, namely, the contact time, the ranges of velocities for rebound and the penetrating depth of liquid in the maximum spreading stage. The results show that the crisscross pillar surface exhibits the best ability to rebound drops, resulting in an excellent self-cleaning property. When droplets impinge on surfaces with the same geometrical shapes and capillary pressures, the smaller the sizes of the pillars are, the better the anti-wetting ability is.Thirdly, the self-cleaning experiments for dust particles are conducted on the prepared epoxy resin particulate surfaces. Under the condition of drop impinging, the effects of surface wettability and dust properties on the self-cleaning are studied. Four types of surfaces and three types of dust particles are used here. The self-cleaning is realized by two steps:(1) the pickup of particles by the water-air interface of an impinging droplet;(2) the release of the impinging droplets from the surface. It can be observed that only the trailing edges of the droplets can pick up particles when the droplets recoil from the inclined surfaces. The hydrophilic surface can also achieve self-cleaning, but under some conditions. This interesting finding may be helpful for the successful implementation of self-cleaning with common surfaces.Fourthly, a transparent superhydrophobic surface(TSHSs) is of vital importance for the self-cleaning of solar cell surface. A sol-gel method is used to fabricate the TSHSs with transmittance closing to the smooth glass surface. Five different experiments are designed to invesitage the anti-soiling effect of these coatings. The results show that these TSHSs can keep the anti-soiling ability under different conditions. The surface energy and roughness of coatings both have influences on the amount of dust adhesion. The lower of the surface energy and the roughness are, the less the dust adhesion will be. When the coating surface is superhydrophobic, the surface roughness has little influences on the adhesion amount.Lastly, the self-cleaning phenomenon for condensate droplets on cooling surpehydrophobic surfaces are investigated. The results show that nanostructures on copper surface are superior to lattice-like microstructures on aluminum surface for preventing large droplets condensate from forming and for delaying icing. The self-propelled jumping(SPJ) behavior of condendate drops are the key to realize the anti-dewing and anti-icing properties. An easy surface modification method by silica sol coating is utilized to allow the SPJ under high humidity and low temperature conditions. The results show that the optimized surface modification method reduces the demands for structures of SHSs that only some “properly designed” structures can induce the SPJ motion. The anti-dewing and anti-frosting properties of superhydrophobic surface with silica sol modification are improved significantly.