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Different Ways to Switch Water Droplet Adhesion
2011-05-20 ArticleSource:General Office & State Key Laboratory for Solid Lubrication
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In recent years, many studies have been devoted to hydrophobic and oleophobic materials. However, as an important physical property of material surface, water droplet adhesion, especially tunable water drop adhesion, have not received much attention.

The research group of material surface and interface behavior of the Lanzhou Institute of Chemical Physics, CAS, has developed several ways to achieve tunable and switchable water droplet adhesion.

The group achieved tunable and switchable water droplet adhesion on a TiO2 nanotube film by UV mask shining and annealing in a micro scale level. They then reported a general approach to switch water droplet adhesion by using sparsely grafted responsive polymer brushes on rough surfaces.

As for photoirradiation induced switchable adhesion, the group developed a new strategy which can reversibly control water droplet transformation between superhydrophobic high adhesion (high sliding angle, SA>90℃) state and superhydrophobic low adhesion state (low sliding angle, SA < 90℃) under alternate UV and vis irradiation by virtually controlling the CA hysteresis of spherical water droplets based on the dynamically varied surface chemistry. The substrate always possesses superhyrophobicity regardless of UV or visible light irradiation, which is a prerequisite for switchable droplet mobility.

Based on the above work, recently the group has demonstrated that wetting transitions and hysteresis can be closely related to the surface composition at the molecular level. Sparsely distributed grafted responsive polymers on the surface don’t significantly change the surface wetting properties, but lead considerably to surfaces with responsive wetting transitions and hysteresis characteristics. When the probing droplet interacts with polymers and they become hydrated, the wetting can be easily realized from the Cassie mode to the Wenzel mode, with the characteristics of high hysteresis and a decrease in contact angle. On the other hand, if the probing droplet doesn’t interact with polymers, meaning that polymer will remain in a collapsed state, the droplet will remain in the stable Cassie wetting mode. The responsive surface composition regulated wetting will be very useful in understanding wetting theory, and will be helpful experimentally in designing smart surfaces in, for example, microfluidic devices.

The group’s work is significant for designing surfaces for microdroplets in microfluidics, smart coatings, and self-cleaning surfaces.

Their work has been supported by the ‘‘Hundred Talents Program’’ of CAS, National Program on Key Basic Research Project of China, National Natural Science Foundation of China and State key research project.

The findings have been published in J. Phys. Chem. C (2010, 114, 9938–9944) , Chem. Comm., (2009, 7018–7020) Soft Matter (2009, 5, 3097–3105; 2011, 7, 515–523; 2011, 7, 3331–3336) and Langmuir(2010, 26(14), 12377–12382).

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