Research on novel and useful surface/thin-film phenomena found in nature
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Professor Robert E. Cohen, in collaboration with colleague Professor Michael F. Rubner of the Department of Materials Science and Engineering, has been directing his attention to novel and useful surface/thin-film phenomena that are found in nature. In one example, their discovery of a successful synthetic route to robust superhydrophobic surfaces led to a patent application, and a communication in NanoLetters that was flagged by the press in a June 7, 2004, Chemical and Engineering News feature "Lessons from Lotus Leaves." Quoting C&E News: |
Prof. Cohen's work with novel surface/thin-film phenomena is also discussed in the
May 8th New Scientist
(A British equivalent to Scientific America).
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"A surface of micrometer-sized hills and valleys dotted with waxy nanoparticles gives the lotus leaf its superhydrophobic self cleaning properties. Water droplets bead up and roll off the rough surface, taking dirt and debris with them. Using a simple, water-based process, researchers from MIT have created a polyelectrolyte multilayer coating that mimics the leaf's tidy topography [Nano Lett, published online May 18, http://dx.doi.org/10.1021/nl049463j]. The group, led by Robert E. Cohen and Michael F. Rubner, first creates micrometer-sized pores in a polyelectrolyte surface (shown) via multiple low-pH treatments.They then add nanoscale texture by depositing silica nanoparticles onto the material, followed by a semifluorinated silane coating. The material retains its superhydrophobic character even after being immersed in water for a week. By eliminating the semi-fluorinated silane coating step, the team can make the material superhydrophilic."
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Figure 1
Cohen, Rubner and their postdoctoral associate Dr. Lei Zhai, have been similarly motivated by optical properties of hummingbird wings which demonstrate magnificent changes in color as the angle of observation varies. In this case, spatially controlled variations in porosity in ultra thin films are responsible for the photonic mirror effects. Using the same aqueous-based layering techniques mentioned above, the team has produced lamellar structures that mimic the wing's alternating nanoporous/fully dense ultrastructure (Figure 2.) The tunable Bragg reflectors, based on the step changes in refractive index that result from the porous to dense excursions, were first disclosed in a patent application and then published in the ACS journal, Macromolecules, 37, 6113 (2004).
Figure 2
In a further improvement aimed at eliminating unwanted side bands in the reflection spectra of the photonic films, Cohen used his well-known in-situ "nanoreactor chemistry" techniques, Supramolecular Science, 1, 117 (1994), to generate, with nanometer length-scale precision, arrays of high index silver nanoparticles inside the ultrastructure. The challenge, described in a publication in Langmuir, 20, 3304 (2004), was to generate a sinusoidally varying refractive index gradient in the film (known as a Rugate structure) to suppress the side bands. As shown in Figure 3 below, this strategy produced remarkable nanocomposite structures comprised of as many as 1500 polylectrolyte layers with appropriately embedded silver nanoparticles. The locations and magnitudes of the narrow, nearly side-band-free, reflectance peaks depend on the predetermined density of the silver nanoparticles in the high index regions of the designed heterostructure.
Figure 3
Attention is now focused on two other natural phenomena: Water gathering/channeling properties of the surface of the Namid desert beetle and antireflectivity of moth eyes. The former is based on juxtaposed patchy regions of superhydrophilic and superhydrophobic character on the back of the beetle, a structure that Cohen and Rubner have already reproduced successfully in their laboratories. Tiny droplets of water accumulate on the superhydrophilic regions, grow in size and eventually roll freely along superhydrophobic zones to a pre-determined location; in the beetle's case, the water is directed to its mouth thereby directly linking this novel surface physics to its survivability in arid circumstances. The moth eye provides inspiration for synthetic antireflection coatings that are again based on combinations of polyelectrolyte multilayers and inorganic nanoparticles. Working with a design team from Rockwell International under DARPA funding, the MIT group is currently addressing the challenge of producing antireflection coatings on flexible, fluid-filled lenses.
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