Abstract
Surfaces patterned with alternating (binary) superhydrophobic-superhydrophilic regions can be found naturally, offering a bio-inspired template for efficient fluid collection and management technologies. We describe a simple wet-processing, thermal treatment method to produce such patterns, starting with inherently superhydrophobic polysilsesquioxane-silica composite coatings prepared by spray casting nanoparticle dispersions. Such coatings become superhydrophilic after localized thermal treatment by means of laser irradiation or open-air flame exposure. When laser processed, the films are patternable down to ∼100 μm scales. The dispersions consist of hydrophobic fumed silica (HFS) and methylsilsesquioxane resin, which are dispersed in isopropanol and deposited onto various substrates (glass, quartz, aluminum, copper, and stainless steel). The coatings are characterized by advancing, receding, and sessile contact angle measurements before and after thermal treatment to delineate the effects of HFS filler concentration and thermal treatment on coating wettability. SEM, XPS and TGA measurements reveal the effects of thermal treatment on surface chemistry and texture. The thermally induced wettability shift from superhydrophobic to superhydrophilic is interpreted with the Cassie-Baxter wetting theory. Several micropatterned wettability surfaces demonstrate potential in pool boiling heat transfer enhancement, capillarity-driven liquid transport in open surface-tension-confined channels (e.g., lab-on-a-chip), and select surface coating applications relying on wettability gradients. Advantages of the present approach include the inherent stability and inertness of the organosilane-based coatings, which can be applied on many types of surfaces (glass, metals, etc.) with ease. The present method is also scalable to large areas, thus being attractive for industrial coating applications.
Highlights
Polyhedral Oligomeric Silsesquioxanes or, in short polysilsesquioxanes (POSS), are organic– inorganic hybrid materials having unique molecular structures.[1]
We describe a simple wet-processing, thermal treatment method to produce such patterns, starting with inherently superhydrophobic polysilsesquioxane-silica composite coatings prepared by spray casting nanoparticle dispersions
With proper combination of hydrophobic nanoparticles, it seems possible to achieve superhydrophobic coatings based upon POSS materials, thereby circumventing problems associated with organic polymers, and in turn, offering improved coating thermal stability and scratch resistance
Summary
Polyhedral Oligomeric Silsesquioxanes or, in short polysilsesquioxanes (POSS), are organic– inorganic hybrid materials having unique molecular structures.[1]. Similar logic applies for silane or siloxane modified silica (e.g., HFS) as under oxidizing conditions silicon will readily abandon linkages with hydrogen or carbon in preference for oxygen, inducing hydrophilicity.[11] If it is possible to induce a wettability transition to a POSS nanocomposite coating by a fast thermally assisted process—such as with flame exposure or laser irradiation— one may be able to fabricate chemically stable surfaces patterned with superhydrophobic and superhydrophilic area regions by large-area processing. Applications are envisioned in microfluidics and other small liquid volume handling technologies Due to their ability to withstand temperatures well above the boiling point of water, one possible application of such patterned binary-wettability coatings is in the area of enhanced heat transfer (e.g., pool boiling). This feature, along with the thermal stability of the present patterns, opens new horizons in technologies involving high temperature fluids
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