Controlled switching of the wetting behavior of biomimetic surfaces with hydrogel-supported nanostructures
Original entry by Sagar Bhandari, APPHY 225 Fall 2010
Controlled switching of the wetting behavior of biomimetic surfaces with hydrogel-supported nanostructures,Alexander Sidorenko, Tom Krupenkinb and Joanna Aizenberg, J. Mater. Chem., 2008, 18, 3841-3846
The authors demonstrate the application of a variation of biomimetic surfaces capable of dynamic actuation in controlled reversible switching of the surface wetting behavior. The authors described a method of fabricating Si nanostructured surfaces with high-aspect-ratio features as shown in Figure 1a. When a hydrophobic coating is applied, the nanostructured surfaces demonstrate remarkable superhydrophobicity. As shown in Figure 1b, the water droplet remains in almost spherical shape when it's on the superhydrophobic surface.
In this paper, the authors report on the synthesis of new designs that combine hydrogels with an array of such high-aspect ratio nanostructures thereby demonstrating the application of these structures in controlling surface wetting behavior. Two designs are reported in the current paper which offer the ability of controlled, dynamic switching of the surface wetting properties - one is 'direct' response mode which undergoes reversible transitions from a superhydrophobic state to a hydrophilic state before/after exposure to water where as the other is 'reverse' which undergoes reversible transitions from a hydrophilic state to a superhydrophobic state before/after exposure to water.
Fig. 2 shows the scheme of integration of microscopically thick hydrogel film with array of isolated rigid setae (AIRS) to fabricate the hybrid device. The drying cycle of the hydrogel film in such a hybrid structure induces bending of highaspect-ratio nanocolumns. The extent of bending can be controlled by regulating the aspect ratio of the AIRS. The architecture as shown in Fig 2 results in switching from the superhydrophobic to the hydrophilic state upon exposure to water and therefore, it is "direct mode" surface.
Design of surfaces with 'reverse response' consists of the transfer of AIRS setae on the confining surface. The basic technique in making such surface is by depositing a droplet of the polymerization solution between AIRS and the solid confining surface. The confining surface is modified with an anchoring layer. After cleaving the assembly by tangential stress, they are transfered onto the confining surface. As a result, the hydrogel film is covalently grafted to the flat, confining surface and the setae are partially embedded into the film.
In this case, when the polymer film are dried, they contract and nanocolumns redirect the tensile forces from the gel into a lateral actuation that results in the tilt of the partially exposed nanostructures. The tilt angle can be controlled by the volume change of the gel and can be therefore regulated by the appropriate choice of the polymer.