Abstract
Underwater superoleophobic surfaces have different applications in fields from oil/water separation to underwater lossless manipulation. This kind of surfaces can be easily transformed from superhydrophilic surfaces in air, which means the stability of superhydrophilicity in air determines the stability of underwater superoleophobicity. However, superhydrophilic surfaces fabricated by some existing methods easily become hydrophobic or superhydrophobic in air with time. Here, a facile method combined with electrochemical etching and boiling water immersion is developed to fabricate long-term underwater superoleophobic surfaces. The surface morphologies and chemical compositions are investigated. The results show that the electrochemically etched and boiling-water immersed Al surfaces have excellent long-term superhydrophilicity in air for over 1 year and boehmite plays an important role in maintaining long-term stability of wettability. Based on the fabricated underwater superoleophobic surfaces, a special method and device were developed to realize the underwater lossless manipulation of immiscible organic liquid droplets with a large volume. The capture and release of liquid droplets were realized by controlling the resultant force of the applied driving pressure, gravity and buoyancy. The research has potential application in research-fields such as the transfer of valuable reagents, accurate control of miniature chemical reactions, droplet-based reactors, and eliminates contamination of manipulator components.
Highlights
Superhydrophilicity on Al substrates obtained by HCl etching or electrochemical etching can keep no more than 2 days; the superhydrophilicity on Cu substrates obtained by chemical oxidation only keep for 8 days; and the superhydrophilicity on Al substrates obtained by laser etching can keep no more than 8 days
We show that the superhydrophilic surfaces are transformed into underwater superoleophobic surfaces when immersed in water
When the etched and boiling-water immersed (EEBWI) Al surfaces were immersed in water, the polar water became entrapped in between the micro/ nanometer-scale structures because of superhydrophilicity, forming a repellent conformal barrier to non-polar organic liquids, resulting in underwater superoleophobicity with small adhesion force
Summary
Superhydrophilicity on Al substrates obtained by HCl etching or electrochemical etching can keep no more than 2 days; the superhydrophilicity on Cu substrates obtained by chemical oxidation only keep for 8 days; and the superhydrophilicity on Al substrates obtained by laser etching can keep no more than 8 days. Yong et al realized the in-situ transfer oil droplets in water based on underwater superoleophobic surfaces by adding sugar in the water and switching the density of the water solution[28]. This method need to change the characteristic of the environment water, resulting in a complex manipulation process. Since the underwater lossless manipulation of non-polar organic liquids has potential application in research-fields such as the transfer of valuable reagents, accurate control of miniature chemical reactions, droplet-based reactors, and eliminates contamination of manipulator components, a simple route for the in-situ lossless controllable manipulation of non-polar organic liquid droplet with a big volume need to be studied. The operable limiting volume of the droplet is shown to be determined by the contact angle, interface tension, and density of organic liquid, which even reaches to 1406 μL for peanut oil
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