Studies on the Wettability and Durability Properties of Superhydrophobic Surfaces

Author:Zhi Jing Hui

Supervisor:zhang li zhi


Degree Year:2019





Due to the excellent anti-water property,superhydrophobic surfaces possess great potential applications in industrial and daily life.However,the durability make a severe obstacle to their widely used.In order to investigate the poor durability reasons of superhydrophobic surfaces,some research was carried out in this paper.As one of the very important conditions for preparing superhydrophobic surfaces,typical surface modification technologies were conducted to investigate the effects on durability of superhydrophobic surface.Based on the investigation,a new method namely hydrophilic-superhydrophobic bipolar film to prepare durable superhydrophobic surface is proposed.Through the hydrophilic connecting film,the superhydrophobic film and substrate were adhered tightly by convalent bond.As a result,the durability of the superhydrophobic surface are greatly improved.Meanwhile,the wettability of liquid on the surface is enhcanced.Through adjusting the preparation method,several durable superhydrophobic surfaces with more functions were obtained successfully,such as superhydrophobic and superoleophobic(SOS)surfaces and anti-reflection high transparency superoleophobic surfaces.For superhydrophilic surface,water droplets can spread out completely on the surface,so it can play an important role in enhancing heat and mass transfer.In this paper,durable superhydrophilic surfaces were also prepared and applied to dehumidifier to study its effect on enhancing heat and mass transfer process.The detail research projects are showed in follows.Firstly,eight different superhydrophobic surfaces were prepared by using different modification technologies to explore the influences of modification technologies on the durability of superhydrophobic surfaces.Sandpaper abrasion test was carried out to study the durability of the obtained surfaces quantitatively.In addition,surface morphologies and chemical compositions of the surfaces were detected to find out the key factor affecting the durability.The results show that the modification technologies can affect the durability of superhydrophobic surface through impacting on the surface roughness.The greater the surface roughness is,the better the durability of the superhydrophobic surface.Secondly,a new method for preparing durable superhydrophobic surfaces is proposed through using the key factor which affect the durability of superhydrophobic surface.Hydrophilic-superhydrophobic bipolar films were prepared to modify the substrate.The hydrophilic film acted as connecting layer to adhere the substrate and superhydrophobic film closely by covalent bond.Meanwhile,the connecting layer possessed cross-linked network structures which could enwrap micro-particles.The roughness of the superhydrophobic surface was increased.The combination of high roughness and convalent bond make the obtained surface displayed excellent durability.Thirdly,the formation mechanism of superhydrophobic and superoleophobic(SOS)surfaces were explored and prepared.During the preparing process,the particles were modified by fluoroalkylsilane twice to reduce the surface energy,and possessed two kinds of particle sizes.The surface energy of the obtained SOS surface was as low as 0.254 mJ/m2 and the roughness was as high as 15.61 um.The contact angles for water and liquid oil whose surface tension is greater than 33 mN/m are higher than 150°,and the rolling angles are less than 10°.Through blending the low surface energy modified particles into the connecting layer and the film showed concrete-like structures on substrate.The connecting layer was composed of organosilane crosslinking agent and organosilicon gel.Connecting layer could adhere the modified particles firmly together to form a stable and uniform film with concrete-like structures.The obtained film showed excellent durability properties.Fourthly,durable superhydrophobic surface with anti-reflection and high transparency properties was prepared by forming porous structures on the connecting layer.The porous structures were obtained by volatilization of pore-forming agent during calcination.The surface energy was reduced by adhering modified particles on the porous structures.The particles and porous structures formed duel-scale structures to enhcance the unwettability and durability of the obtained film.The refractive index of the film is only 1.3471.The average transmittance of the modified glass in the visible light range is up to 97.9%,which is nearly 9%higher than the glass substrate.Finally,durable superhydrophilic polytetrafluoroethylene(PTFE)plates were prepared and then used in liquid desiccant air conditioning system(LDACS).Firstly,uniformly arranged micro-rod structures were etched on PTFE plate,and then self-made superhydrophilic SiO2particles were adhered to the micro-rod structures to form durable superhydrophilic PTFE plate.The wettability test showed that the liquid desiccant could spread completely on the coated surface within 2 s,and the contact angle could be as low as 0°.Compared with the plate without coating film,the surface wetting area of the superhydrophilic PTFE plate increased by 1.5~5times,the average falling film thickness decreased by 1.4~2.4 times,the moisture removal rate increased by 1.3~1.8 times.The convective mass transfer process of the system is strengthened by using superhydrophilic PTFE plate.