The Factors Influencing The Transition Of Solid Surface Properties Rfom Superhydrophobic To Adhesive

The wettability is one of the most important properties of a solid surface. When a liquid rests on a solid surface surrounded by gas, it may spread or form a droplet, which depends on the system. A surface on which a water drop assumes a nearly spherical shape with a contact angle larger than150°and a sliding angle less than10°is usually considered a superhydrophobic surface. The phenomenon of superhydro-phobicity, the most well known as "lotus effect", is originated from nature. The lotus leaf from which water drops can readily roll away together with any dust and dirt exhibits an impressive self-cleaning property, resulting from micro/nano-binary structures with epidermal wax crystals. Inspired by the lotus leaf, biomimetic superhydrophobic surfaces have been fabricated via various smart methods. In recent years, studies on superhydrophobic surfaces have experienced transitions from natural to biomimetic to functional. The useful functions of superhydrophobic surfaces and factors influencing these functions have received more attention.Although it has been confirmed that a superhydrophobic surface may be adhesive due to water vapor condensation, the environmental factors, such as air temperature, relative humidity (RH) and so on, could not be effectively controlled during condensation. The experimental results obtained under a specific condensation condition lacked the widespread guiding significance and were hard to repeat. In this thesis, we first designed a convenient and reliable system for synchronous measurement under controllable temperature and relative humidity. After that, the lotus leaf and biomimetic superhydrophobic surfaces with different architectural features or low-surface-energy coatings were tested to systematically analyze the structural, environmental and chemical factors influencing the transition of solid surface properties from superhydrophobic to adhesive. At last, the feasibility of superhydrophobic surfaces as potential icephobic materials for power line was also evaluated. The main achievements and innovations of this thesis are:(1) On basis of CAM200optical angle meter, we independently designed and assembled a sealed chamber, a rotatable platform, an environmental control system and a high-speed data acquisition system to synchronously measure liquid temperature, contact angle and sliding angle from-80℃to300℃under RH of0-100%. The instrumental modification could overcome existing restrictions to observe static and dynamic behaviors of liquid drops on a solid surface in a wide range of temperatures and RHs. Two patents about the design of the experiment device which was original at home and abroad have been authorized in2011.(2) Water contact and sliding angles were first systematically evaluated on fresh lotus leaves from-10℃to80℃under either extremely low or high RH (RH<10%or>90%). A scanning electron microscopy (SEM) was utilized to find the correlation between morphology change and water repellency. The experiment was of great importance because it has specified the effect of environmental factors on superhydrophobicity of the lotus leaf, and provided a new way to investigate natural and biomimetic superhydrophobic surfaces.(3) We have fabricated biomimetic superhydrophobic surfaces on paint films and aluminum substrates by a dip-coating process and a chemical etching method, respectively. The contact angles on these surfaces were all above160■, and the sliding angles were less than5°at room temperature. To comparatively investigate the influence of environmental factors on superhydrophobic behaviors of surfaces with different topographies or low-surface-energy coatings under controlled condensation conditions, contact and sliding angles were measured from-10℃to30℃under a series of RHs ranging from10to90%, respectively. The structural, environmental and chemical factors influencing the transition of solid surface properties from superhydrophobic to adhesive have been discussed in detail, and the results were useful to assess the stability of superhydrophobicity under condensation.(4) We have prepared five surfaces with representative wettabilities:i.e. superhydrophilic, hydrophilic, critical, hydrophobic and superhydrophobic. The freezing processes of individual water droplets on both clean and artificially contaminated surfaces were in situ investigated by a high-speed data acquisition system and a high-speed camera. Ice accretion was also tested by spraying supercooled water microdroplets to the as-prepared surfaces below0℃. The correlation between surface topography and ice formation has been revealed. Additionally, how contamination affected the freezing was first reported. In summary, the feasibility of superhydrophobic surfaces as potential icephobic materials for power line was demonstrated in theory and practice.