Fabrication Of Functional Cellulose Fabric Surfaces With Special Wetting Behaviors

As is ubiquitous in nature, the surface wettability is one of the most important propertiesof natural and artificial surfaces, exerting profound influences on our daily life by providingimportant industrial and scientific applications. The surface wettability is characterized by thecontacting liquid contact angle. A surface is superhydrophobic if the water contact angle isgreater than150o, while it is superhydrophilic if the contact angle is less than5o. Similarly, asurface is superoleophobic if the oil contact angle is greater than150o, while it issuperoleophilic if the contact angle is less than5o. Surfaces with special wetting behaviors arealways displaying extreme affinity or repellency for a contacting liquid. Generally, the basicsurface extreme wetting performances include superhydrophilicity, superhydrophobicity,superoleophilicity, superoleophobicity, their combinations and also transitions between eithertwo wetting sates.Cellulose, one of the most sustainable natural resources, is one of the most common andessential materials in our daily life. However, cellulose fabrics tend to get contaminatedresulting from the affinity to liquids as cellulose is presenting a lot of hydroxyl groups at thefabric surface. Therefore, the functionalization of cellulose fabric surface is on demand foranti-fouling clothing and related apparels. In addition to the abundant reactive hydrophilichydroxyl groups at the surface, cellulose fabric is also possessing porous rough textures, thefunctionalization of which will render a great opportunity in eliciting the relationship betweenthe surface texture and surface performances. Moreover, the study on stimuli-responsivenessof the fabric wetting property will also bring about great scientific interests in the fieldsincluding fluid control, surface self-cleaning, smart materials and other related interfacialmaterials with special wetting properties.In this thesis, work has been carried out on the topic of surface wetting phenomena. Therelations between a surface wettability and its surface chemical structures were studied byfabricating azobenzene fluoride derivatives, investigating the surface wetting property ofsmooth and rough coatings, and analyzing wetting mechanisms. The following aspects aremainly included in this dissertation:(1) Studies on the relationship between the surface chemistry and surface wettability offlat substrates. Silicon wafers were first coated with fluoro-azobenzene derivatives orfluoro-silanes and the surface free energy was calculated based on the liquid contact angles onthe surface using Owens-Wendt approach. It was first revealed quantitatively that azobenzenederivatives switch between high energy Cis state and low energy Trans state and display switchable wettability. Surface affinity to water and resistance to oil may be tailored byfluorosilanization of superhydrophilic surfaces with controllable reactive fluoro-silanes.(2) Studies on superhydrophobic fabric surfaces and photo-responsive wettability.Superhydrophobic surfaces with ultra-low contact angle hysteresis was fabricated byanchoring fluoro-aniline onto the cellulose fabrics, on which water bounces freely as there isno hysteresis between water and substrate. While, fluoro-azobenzene functionalized fabricdisplays controlled extreme wetting behaviors resulting from the photoisomerization ofazobenzene molecules. Under visible light irradiation, fluoro-azobenzene molecules are inlower energetic Trans state, which provides superhydrophobicity for the fabric, while underUV light, azobenzene switches to higher energetic Cis state rendering superhydrophilicity forthe fabric. Moreover, superhydrophobicity will be restored under visible light illumination.(3) Design of surfaces with counterintuitive wettabilities. Hydrophilic surfaces maydisplay resistance to oil in air by introducing chemical heterogeneity to the surface. Theabundant reactive and hydrophilic hydroxyl groups of cellulose fabrics may chemically reactwith fluoro-silanes with moderate reactivity but low surface energy using chemical vapordeposition method. As the relatively low reactivity of oxyethyl groups in fluoro-silane,oil-resistance may be introduced to the water-loving cellulose fabric without sacrificing itsintrinsic hydrophilicity. Water-loving and oil-hating surfaces may render great potentials inself-cleaning clothing, chemical shielding and oil-water separations.