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.
Si Nanostructured Surfaces Krupenkin et al had recently fabricated Si nanostructures that were coated with a hydrophobic coating. The result was what the authors termed "superhydrophobicity", where water that was placed on the surface displayed very high contact angles and very small contact line hysteresis.