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ABSTRACT: Surface tension confined (STC) open tracks for pumpless transport of low-surface tension liquids (e.g., acetone, ethanol, hexadecane) on microfluidic chips are fabricated using a large-area, wet-processing technique. Wettable, paraffin-wax, submillimeter-wide tracks are applied by a fountain-pen procedure on superoleophobic, fluoroacrylic-carbon nanofiber (CNF) composite coatings. The fabricated anisotropic wetting patterns confine the low-surface-tension liquids onto the flow tracks, driving them with meniscus velocities up to 3.1 cm s(-1). Scaling arguments and Washburn's equation provide estimates of the liquid velocities measured in the STC tracks. These tracks are also shown to act as rails for directional sliding control of mm-sized water droplets. The present facile top-down patterned wettability approach can be extended to deposit micrometer-wide tracks, which bear promise for pumpless handling of low-surface tension liquids (e.g., aqueous solutions containing alcohols or surfactants) in lab-on-a-chip type applications or in low power, high-throughput bio-microfluidics for health care applications.
Lab on a Chip 11/2012; · 5.67 Impact Factor
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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.
Nanoscale 07/2012; 4(17):5378-85. · 5.91 Impact Factor
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ABSTRACT: We report a highly efficient technique to form novel fluoropolymer blend dispersions containing poly(vinylidene fluoride) (PVDF) and a fluorinated acrylic copolymer using a cosolvent system comprising N-methyl-2-pyrrolidone (NMP), acetone, and water under pH control. We also show that certain surface-functionalized, high-aspect-ratio nanostructured materials, such as organoclay and carbon nanowhiskers (CNWs), can be easily dispersed in these fluoropolymer blends to fabricate durable and functional superhydrophobic composite coatings upon spray casting. Both clay and CNW superhydrophobic coatings also repel lower surface tension liquids, such as water–alcohol mixtures (40 mN/m). Repellency is characterized using droplet sessile contact angle and contact angle hysteresis. Both clay and CNW-based composite coatings display self-cleaning properties (low contact angle hysteresis) for both water and water–alcohol mixtures. Additionally, electrical conductivity measurement of CNW composite coatings demonstrates the ability to fabricate multifunctional superhydrophobic composites using these fluoropolymer dispersions. The nanoparticle concentration required in these composite coatings for water and water–alcohol repellency is compared with a previously reported PVDF-based coating system. Wettability is interpreted within the framework of the Wenzel and Cassie–Baxter wetting theories.
09/2011;
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ABSTRACT: Super-repellent nanostructured composite coatings applied over large areas by spray and subsequent thermal treatment are reported. Solution blending of poly(vinylidene fluoride) and poly(ethyl 2-cyanoacrylate) is implemented to formulate filler particle dispersions used to apply these coatings. The wettability of these coatings is manipulated using hydrophobic poly(tetrafluoroethylene) and hydrophilic zinc oxide particle fillers or their combination. The resulting coatings feature contact angles up to 164 degrees for water and 154 degrees for a water and isopropyl alcohol mixture (9:1 weight ratio; surface tension approximately 40 mN/m). A self-cleaning ability is revealed by droplet roll-off angles below 10 degrees . The results show that the fillers affect the coating surface energy and surface roughness, in turn influencing the wettability of the coatings.
ACS Applied Materials & Interfaces 04/2010; 2(4):1114-9. · 4.53 Impact Factor
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Macromolecular Materials and Engineering 10/2009; 294(11):775 - 780. · 1.99 Impact Factor
