Controlled pattern formation using Optical Tweezers

This is probably the most practically useful application that we have developed with our OT apparatus till date. This was discovered almost accidentally (as have most great discoveries in Physics!!) by Basudev when he was working with the SOMs, found these large bubbles being created, tried to move the laser spot away so the bubbles died down, but found the bubble following instead! The bubbles were actually getting trapped by the laser, and could be manipulated with forces MUCH greater than that for particles trapped conventionally using OT (nano-Newtons against pico-Newtons). It took us a long time to understand what was happening - essentially the bubble gets created in a SOM dispersion with water where some of the SOMs get adsorbed on the top slide of our sample chamber. These SOMs are absorbing at 1064 nm, so a hot-spot is created on the top slide, which forms a bubble. Now the bubble causes convection currents about it due to the difference in surface tension at its top and bottom surface (since the temperature is different - the top part of the bubble in contact with the top slide has higher temperature than the bottom). This so-called Gibbs-Marangoni convection draws in particles from the dispersion which accumulate around the bubble base (see figure). Now, when the laser source is moved (we typically achieve this by moving the microscope sample-holder stage), the hot-spot follows, since ther eis now this ring of absorbing material formed around the bubble! It can be shown by calculations that the convection currents would actually displace the bubble in the direction of the hot spot, but one may intuitively try to understand this by realizing that it is energetically less costly for the older bubble to move towards the hot spot than for a new bubble to be formed at the hot spot. The bubble thus becomes 'trapped' by thermo-phoretic forces generated optically, and keeps on accumulating material about it as it follow s the laser when the latter is moved. Now the material accumulated undergoes a phase transition from soft metal to a crystalline phase (we don't know why still, but TEM studies do prove the crystallinity - check out our publication), and we can thus generate permanent patterns (of dimensions in the microns, since the bubbles themselves are of that order in size) on the glass substrate of the top slide! We have made a large variety of patterns (see figure), including writing IISER, and have not only patterned SOMs, but also other materials such as dyes, carbon nano-tubes mixed with SOMs. This technique should essentially work for any material, provided one uses the right wavelength where the material absorbs! We haven't checked this out as yet, but as should be very clear by now, a lot of work needs to be done on this! Presently, we have had some success in making electronic circuits on glass by making patterns of a conductive material along with the SOMs (details will be coming soon). Imagine how cool it would be if one could make micro-circuits (or even nano - the size of the patterns depend on the laser wavelength - presently the best we have achieved is around 1 um) without lithography-based methods, in about ten minutes, and at the cost of just about a few thousand dollars (the entire optical setup we use now costs less than 2.5k USD)!

You may like to watch this animation to understand the schematic of the pattern formation process better.

This work was started by Basudev and is now being continued by Subhrokoli. Our collaborators are Dr. Roy, of course, and Dr. Partha Mitra of DPS, IISER Kolkata.