Walk This Way
Scientists at Eindhoven University of Technology in the Netherlands and Kent State University in Ohio have developed a new material that can undulate and therefore propel itself forward under the influence of light. To achieve this, the scientists clamp a strip of this polymer material in a rectangular frame. When illuminated, it goes for a walk all on its own. This small device, the size of a paperclip, is the world’s first machine to convert light directly into walking, simply using one fixed light source. The researchers, including Professor Robin Selinger of Kent State’s Liquid Crystal Institute® in the College of Arts and Sciences, published their findings on June 29 in the scientific journal Nature.
The maximum speed of the device is equivalent to that of a caterpillar, about half a centimeter per second. The researchers think it can be used to transport small items in hard-to-reach places or to keep the surface of solar cells clean. The researchers placed grains of sand on the strip, and these were removed by the undulating movement. The mechanism is so powerful that the strip can even transport an object that is much bigger and heavier than the device itself uphill.
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The motion of the new material is due to the fact that one side contracts in reaction to light while the other side expands, causing it to bulge when illuminated. That deformation disappears instantaneously once the light is gone. Although the material looks transparent to the human eye, it fully absorbs the violet light the researchers used, thus creating a shadow behind it.
The scientific team, led by Professor Dick Broer of Eindhoven University of Technology, was able to create a continual undulating movement, using this “self-shadowing” effect. The team members attached a strip of the material in a frame shorter than the strip itself, causing it to bulge. Then the researchers shone a concentrated LED light on it from in front. The part of the strip that is in the light starts to bulge downward, creating a “dent” in the strip. As a consequence, the next part of the strip comes in the light and starts to deform. This way, the “dent” moves backward, creating a continual undulating movement. This sets the device in motion, walking away from the light. When the device is placed upside down, the wave travels in the opposite direction, causing it to walk toward the light.
The research team managed to reach this specific behavior of the material using liquid crystals, familiar in liquid crystal displays or LCDs. The principle relies on the incorporation of a fast-responding, light-sensitive variant in a liquid crystalline polymer network. The researchers engineered a material in such a way that this response is translated to an instantaneous deformation of the strip when illuminated and relaxation when the light is gone. Computer simulations demonstrated the self-shadowing mechanism and explained the directionality of light-driven motion.
The research team’s paper in Nature is titled “Making Waves in a Photoactive Polymer Film.” Joining Selinger and Broer as authors are Anne Hélène Gélébart, Dirk Jan Mulder, Ghislaine Vantomme and Bert Meijer of Eindhoven University of Technology and Michael Varga and Andrew Konya of Kent State. The empirical study took place at Eindhoven University of Technology with the corresponding theoretical model being developed at Kent State. The research was made possible by subsidies from the Dutch research funding agency NOW, the European Research Council and grants from the U.S. National Science Foundation.