Blog RSS FeedThis post originally appeared on CLEO BLOG by Frank Kuo and is reproduced with permission from its author.
Figure 1. The realization of the microscopic Stirling engine. Courtesy of V. Blickle and C. Bechinger in Nature Physics doi:10.1038/nphys2163 (2011).
When people mention the word “laser” to you, what is the first thing coming to your mind? Most of us associate lasers to their scary and destructive power, just like how we are educated in the Star Wars movie series. In reality, lasers can be quite gentle and perform very accurate and precise assignments, like micro-machining (Jim has a nice article about it). In fact, laser can be so gentle that researchers have used it to power the world’s smallest Stirling engine, which is composed of single tiny melamine bead (~ 3 um in diameter) in the water bath.
To realize how this ingenious microscopic engine works, we have to step into the phenomenon of optical trapping/tweezers first. Thanks to the detailed illustration on wiki, I can just summarize it in a few sentences — When the laser is tightly focused, or when it has the Gaussian beam intensity distribution, the tiny particle will be trapped in the focus or the center of the Gaussian beam, just like being trapped in a potential well. This is a result of momentum conservation. When the refracted light rays exit the particle, they exert momentum kicks to the particle, and the net result of these kicks is a force that traps the particle at the center of the focus. If the particle is in the focus, this force is zero. If the particle drifts away from the center, the kicks will be imbalanced and a net force will pull it back to the center. This particle behaves exactly like it is in a potential well. The steepness of the well depends on the laser intensity as you might guess it already. And our talented researchers use this technique to power the microscopic engine.
Here is how it goes. Figure 1 shows the comparison of a microscopic Stirling engine with a macroscopic one. As shown in step (1), the bead is trapped in a potential well by a focused laser beam. From step (1) to (2), the laser intensity is increased such that the bead would be confined in a smaller volume due to the steeper potential well. This is similar to moving a piston to squeeze the volume in the chamber. From (2) to (3), the water bath is heated by another NIR laser, and this step is similar to heating a macroscopic chamber. From step (3) to (4), the potential well is relaxed and the work is exerted from the bead to the surrounding, just like in macroscopic world, the gas is pushing the piston to exert work for useful application. From (4) to (1), the NIR laser is turned off, and the bead is cooled down, just like in the traditional Stirling engine, the gas is cooled back to the ambient temperature. Smart and elegant design, isn’t it?
