Hierarchical surface coatings of polystyrene nanofibers and silica microparticles with rose petal wetting properties

• Hydrophobic surfaces with high water adhesion were prepared by simultaneously spray coating a polystyrene binder and silica particles.
• A hierarchical structure with decreasing size of silica particles was created by tuning the composition of the spray coating suspension.
• The resulting hierarchical structure also contained ∼100-nm diameter polystyrene nanofibers that interlaced throughout the coating.
• The hydrophobicity of these surfaces was tuned by incorporating a combination of microscale to nanoscale structures.
• The structured coatings were able to more efficiently condense water vapor under low humidity, yet, retained minimal water at high humidity.

Surfaces with rose petal like properties, simultaneously exhibiting a high degree of hydrophobicity and a high adhesion to water, were prepared by spray coating of progressively smaller hydrophilic silica particles along with hydrophobic nanofibers onto surfaces of interest. Various polymer structures were achieved by tuning the spray coating flow rates during deposition of the polymer nanofibers and silica particles. At a reduced flow rate, polystyrene fibers were formed with diameters of less than 100 nm. Water contact angles (WCAs) of coatings prepared from the hierarchical assemblies of silica particles blended with polystyrene nanofibers were greater than 110°. Coatings prepared from the hierarchical assemblies either with or without incorporation of the polymer nanofibers pinned water droplets to their surfaces even after inverting the substrates, similar to the properties of a rose petal. Hierarchical coatings of silica particles without the polystyrene nanofibers also exhibited a high adhesion to water, pinning at least 30% more water on its surfaces. Conversely, hierarchical coatings containing the polystyrene nanofibers exhibited an increased water mobility across their surfaces. Further water retention experiments were performed to determine the ability of the different coatings to efficiently condense water vapor, as well as their efficiency to remove this condensed liquid from their surfaces. Both types of hierarchical coatings exhibited an excellent ability to retain water at a low humidity, while establishing a self-limiting condition for retaining water at a high humidity. These coatings could be prepared on a relatively large-scale and with a relatively low cost on the surfaces of a variety of materials to enhance their water resistance, water retention and/or ability to condense water vapor.

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