Total Pageviews

Monday, January 9, 2012

The world’s smallest Stirling engine is powered by laser!

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?

Figure 1. The realization of the microscopic Stirling engine. Courtesy of V. Blickle and C. Bechinger in Nature Physics doi:10.1038/nphys2163 (2011).
Not only the realization of microscopic machines is presented, but also its power and efficiency are characterized.  They have shown that if the entire cycle is working at a rate of 7.2 s, the power is at its maximum. So a micro-machine has a working ethic of macroscopic time scale. They also calculate the average work output, which is in the range of 10^-21 J! Since the machine is tiny, it is prone to the stochastic fluctuation. As a result, some of the cycles are exerting negative works. But fortunately, when average over many cycles, the machine works reliably.

Optical tweezers technique has been used for various applications. Beside the one we just present, Block’s group in Stanford is the true master in applying it to biophysical study. They have used this technique to investigating the transcription of the DNA and the motions of kinesin motors inside the cell, just to name a few. The basic principle is to attach the molecule of interest to the bead or beads, exert the force to the bead(s) through the laser, just like figure 2. By carefully controlling the laser intensity, the force can be finely tuned in a delicate way. So the step motion or the transcription of the DNA can undergo in a subtle and controllable way. In addition, by monitoring the location of the bead, we can extract the size of the step motion of the molecules (the molecules themselves are too small to see). This sophisticated setup has been perfected by Block’s group, and they are able to extract the step motion of kinesin and DNA transcription with a resolution of a few nanometers. This is another master piece of scientific work, because we are talking about a motion in a microscopic world through macroscopic technique. I strongly encourage people who are interested in this topic to take a look of their research website, since it contains lots of information, even some insightful literature!

Figure 2. Using the optical tweezers to probe microscopic world. The force, when tuned properly, can switch the reaction on and off since the reaction involves the change of the molecular length. Also by monitoring the bead location, we can know the step size of the motion. Courtesy of Block's research group website.
I keep wondering when the Nobel Prize was granted on the optical tweezers, did they envision its cool applications we described here already? Maybe...

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.