Abstract
The wetting properties of superhydrophobic surfaces have generated worldwide research interest. A water drop on these surfaces forms a nearly perfect spherical pearl. Superhydrophobic materials hold considerable promise for potential applications ranging from self cleaning surfaces, completely water impermeable textiles to low cost energy displacement of liquids in lab-on-chip devices. However, the dynamic modification of the liquid droplets behavior and in particular of their wetting properties on these surfaces is still a challenging issue. In this review, after a brief overview on superhydrophobic states definition, the techniques leading to the modification of wettability behavior on superhydrophobic surfaces under specific conditions: optical, magnetic, mechanical, chemical, thermal are discussed. Finally, a focus on electrowetting is made from historical phenomenon pointed out some decades ago on classical planar hydrophobic surfaces to recent breakthrough obtained on superhydrophobic surfaces.
Highlights
Biological surfaces, like lotus leaves, exhibit the amazing property for not being wetted by water leading to a self cleaning effect
In order to mimic these properties, artificial superhydrophobic surfaces have been prepared by several means, including the generation of rough surfaces coated with low surface energy molecules [1,2,3,4,5,6], roughening the surface of hydrophobic materials [7,8,9], and creating well-ordered structures using micromachining and etching methods [10, 11]
We have shown for the first time that reversible electrowetting is possible on superhydrophobic surfaces that display specific geometrical criteria as predicted by Bico [24]
Summary
Biological surfaces, like lotus leaves, exhibit the amazing property for not being wetted by water leading to a self cleaning effect. This leads to a coexistence of the two modes, as described in Fig. 10: when a drop is deposited on a rough surface, a Cassie–Baxter regime occurs even when h \ hc (for water, h \ 120°) [27,28,29] This state is metastable, i.e. by applying a pressure to the drop, for example, it is possible to reach the Wenzel regime: stable and displaying an important hysteresis [30]. For pH values of 2 and 11, the surfaces are superhydrophobic and superhydrophilic, respectively, whatever the temperature (Fig. 15) Another point is that, as compared to previously related reports on thermally responsive materials, the film can be hydrophobic at low temperature and hydrophilic at high temperature. Similar microsystems have been developed and patented within the framework of contract BIOCHIPLAB [83]
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