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
Don's comments: What silane was used to make the surface hydrophobic? How does the contact angle of water on a flat surface treated with this silane compare to water on the superhydrophobic array?
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
Sung Hoon's comments: It's very interesting study that the authors could switch the wetting behavior of the surface using humidity. Is there any other study which showed switching of wetting by stimuli other than humidity?
Don's comments: When a drop of water was placed on the surface with the hydrogel in the dry state, did the hydrogel eventually absorb the drop and switch to the wet state? Could this be used as a humidity sensor of some sort?
Scott's comments: Yes, there are a number of other studies on the switching of wetting behavior. One very popular method is switching by EWOD (electrowetting-on-dielectric). Refer to "Equilibrium behavior of sessile drops under surface tension, applied external fields, and material variations" by Shapiro and Kim. Another method is called optoelectrowetting, which does the same thing (lowers surface tension) using a laser. Refer to "PICO LITER DROPLET MANIPULATION BASED ON A NOVEL CONTINOUS OPTO- ELECTROWETTING MECHANISM" by Chiou and Wu. In the "reverse response" case, I think it may be possible for the drop to contact the surface, wet the surface, and then cause the hydrogel to swell and the surface to become hydrophobic. The "direct response" case probably would not do this because the drop would just sit on top of the pillars and not wet the surface to swell the hydrogel. This could definitely be a humidity sensor. The authors also talked about the "reverse response" being able to act like "smart clothes", which are able to attract moisture when it is dry and repel water when it is exposed to a humid environment.
Alex's comments: when the surface is in the wetting state, do the authors say whether it is completely wetting or partially wetting? My guess is completely, if hydrogel chains are hydrophilic.
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