Controlled Switching of the Wetting Behavior of Biomimetic Surfaces with Hydrogel-Supported Nanostructures
By Lidiya Mishchenko
In this paper, Aizenberg et. al. utilized nanopost arrays to allow structures to switch between wetting and non-wetting states (superhydrophobic and not). These arrays (as synthesized) are silicon nanostructures with high aspect ratio features. When coated with a hydrophobic silane, they demonstrate superhydrophobicity (through coupling of high contact angle and structure). The behavior of a device that switches from nonwetting to wetting in a humid environment was called "direct response" and one that switches from wetting to non-wetting in a humid environment was called "reverse response" (See captions).
A large portion of the paper focused on creating a hydrogel layer that was anchored properly to the substrate and could reversibly switch for many cycles.
Some interesting ideas in the paper:
- Hydrogels were defined as "responsive materials composed of cross-linked flexible polymeric hydrophillic chains whore elastic networks can swell in water to the desired degree of hydration"
-They noted that hydrogels are "shape-memory" polymers and this allows for the repeatability of the process. Also noted was that hydrogels respond to a variety of stimuli: humidity, pH, etc.
-In their intro, they gave a nice definition of superhydrophobicity: "a very high water droplet contact angle and a very small advancing-receding hysteresis... a water droplet deposited on a superhydrophobic surface maintains its almost spherical shape and easily slides over the surface"
-The bio-inspiration for responsive superhydrophobic structures comes from: legs of water spiders and beetles. They use nanostructured hydrophobicity for "water repellence, movement, and water capture"
-Finally, they mentioned another paper that demonstrates (for block copolymer coatings) "water-induced increase of superhydrophobicity of the polymer coatings...caused by the dynamic rearrangement of the flourinated polymer segments" [] Makal, U. and, Kenneth J. Wynne. Water Induced Hydrophobic Surface. Langmuir 2005 21 (9), 3742-3745
By Scott Tsai
In this paper, Sidorenko et al demonstrated a hybrid system of rigid nanostructures with responsive hydrogel films to create a superhydrophobic-hydrophilic switch. They demonstrate this for two cases. The first case is for a "direct response", where the unactuated state of the system is superhydrophobic, and the second case is for a "reverse response", where the unactuated state of the system is hydrophilic.
Hybrid System of Si Nanostructured Surface and Hydrogel Layer
Here, the authors have attemped to combine the Si nanostructed surface with a layer of hydrogel. They were inspired to do so by a number of biological materials, such as a gecko's toe . To integrate the hydrogel layer to the Si nanostructured surface, a bottom-up layering approach was applied. The process involved layering PGMA on the Si substrate, immersing the surface acrylic acid, and initiating the polymerization with either UV light or thermoinitiator.
Direct Response and Indirect Response Systems The direct response system involves the integration of a thick hydrogel layer into a nanostructured surface. As shown in Fig.2, when the hydrogel is in a "dry state", all of the features of the nanostructured layer stick up, and cause a pendent drop of water to show almost spherical shape, displaying superhydrophobicity. But when the hydrogel is in its "wet state", it swells, causing the drop of water to wet the surface.
1. T. Krupenkin, J. A. Taylor, T. M. Schneider and S. Yang, Langmuir, 2004, 20, 3824-3827
2. K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili and R. J. Full, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 12252-12256