Repelling Liquids via Polymeric Silicone Surfaces


Student thesis: Doctoral Thesis

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Award date29 Mar 2019


Creating surfaces with stable liquid repellency are of great interest in various applications, including self-cleaning, anti-smudge, antifouling, oil-water separation, particle fabrication, fluid manipulation, biomedical and analytical technologies. One challenge in designing liquid-repellent coatings for their applications lies in developing a stable repellent layer that allows effective repulsion of low surface tension liquids. However, the current strategies for creating the stable repellent layers are engaged in preventing the liquid impalement by exploiting sophisticated geometries and fluorinated chemistries, which have significantly limited the general application due to the complexity of processing or environment concerns. An alternative approach for creating stable liquid repellent surfaces is sought by the formation of a flat, non-textured repellent layer, which could avoid the impalement of liquids. Polydimethylsiloxane (PDMS) is the promising material that has been extensively used in the fabrication of liquid-repellent surfaces due to its liquid-like features. Although these studies demonstrated excellent liquid repellency with features such as good thermostability, transparency, tunable surface reactivity, durability or easy fabrication, a general strategy for polymeric coatings that integrates all above merits is yet to be reported.

In the first part of this thesis, we introduce a flat polymeric repellent layer made of highly crosslinked polydimethylsiloxane networks that demonstrated stable liquid repellency to various liquids. The coatings show stable repellency to ultralow surface tension liquid. The surfaces can also bear chemical activities without losing their liquid repellency by simply adding functional motifs.

Second, liquid-repellent surfaces were obtained via coating liquid-like copolymers brush. The stable and transparent nanocoatings showed liquid repellency to a broad range of organic liquids even in the presence of reactive sites. Functional molecules could be covalently immobilized onto the liquid-repellent surfaces. Moreover, the liquid repellency can be maintained or finely tailored after post-chemical modification via synergically tailoring the polymer brush thickness, selection of capping molecules, and labeling degree of the capping molecules.

Third, based on reactivity of the liquid repellent surface, we reveal the reactive liquid-repellent surface as a new platform for surface micro-patterning and functionalization via the condensation-enrichment approach. As a demonstration, fluorescent molecules were immobilized on the reactive liquid-repellent surface by the high-density and high-resolution patterning of microdroplet arrays which was rarely achieved on other surfaces. Our results indicate a synergetic effect of the oil-repellent surface and condensation-enrichment on both high-density micropatterning of organic droplets and surface functionalization. This method demonstrated a simple approach to spatially control the surface chemistry and wettability which could be employed in many analytical applications.

In summary, we present a general strategy to create liquid repellent surfaces. Liquid repellency on a flat surface is realized by the synergy of two criteria: the incompatibility and mobility of repellent layer. Liquid repellency is independent of liquid surface tension due to elimination the effect of liquid impalement of surface texture. Therefore, the surfaces show stable repellency to ultralow surface tension liquid, even at an environment of ultralow temperature. Also, the surfaces can bear chemical activities without losing their liquid repellency under the premise that the mobility of the repellent layer is maintained. We believe that these unique properties of the solid-based liquid-repellent coating make it highly practical not only for surface coating but also broaden the application of molding and replication of micro/nanoscale structures, fabrication of microfluidics.