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On-demand liquid release from a sponge-like coating

(Nanowerk Spotlight) Liquid-impregnated coating technologies such as LiquiGlide, developed in 2009 by MITís Kripa Varanasi and David Smith, or SLIPS (Slippery Liquid-Infused Porous Surfaces), developed in 2011 by Joanna Aizenberg's lab at Wyss Institute, involve nanoscale texturing of a surface, which is then coated with a – usually lubricating – liquid.
Many of these coatings are stimuli responsive, meaning that the coating can release a liquid upon being triggered by an external stimulus such as temperature or light. These type of coatings could be used for self-lubricating devices; drug delivery; self-cleaning of contaminated surfaces; or adhesion/release on command.
In new work, reported in Advanced Functional Materials ("Photoresponsive Sponge-Like Coating for On-Demand Liquid Release"), researchers present a novel approach to deliver liquids from a solid coating, both location- and time- controlled. This sponge-like coating can repeatedly absorb liquids and release them again. The type of release, either droplets or film, can be initiated by a short exposure with UV light. The uptake is accelerated by exposure with blue light.
"The general objective of our work is to make a solid coating that reversibly releases and uptakes a liquid material thereby changing surface properties like adhesion or friction," Dirk J. Broer, a Professor at Eindhoven University of Technology, and head of the Laboratory for Functional Organic Materials & Devices, tells Nanowerk.
The principle of the team's coating is based on a photoresponsive liquid-filled smectic crystal network (LCN) coating as illustrated in the figure below.
Principle of a photosponge coating
Principle of a photosponge coating. During the formation of the coating the mobile liquid is phase separated from the polymer network (left). The white area resembles here the continuous phase of the polymer network and the green area the phase separated 8CB molecules. The insert shows that both the network, consisting of the blue polymerized reactive mesogens, and the green unbound 8CB molecules have adapted the smectic order. Upon reduction of the molecular order by light the liquid is expelled at the surface in the form of initially small droplets (green) which may coagulate in a later phase to form a continuous wet film (right). (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Broer's group is focused on developing responsive coatings. For instance, they just published a report in Advanced Materials on a coating that wipes off dust and sand from a surface by an electrical trigger, e.g. to clean solar panels in desert like conditions ("Oscillating Chiral-Nematic Fingerprints Wipe Away Dust").
Going forward, the researchers will attempt to make the sponge-like coating accessible for a variety of different liquids, varying from water to oil, and from chemicals for a chemical reaction to medicines and drugs.
In addition, they are studying alternative triggers for release and uptake – other wavelengths of light, electrical fields and contacts with gases or other liquids.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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