Single beam thermo-optical tweezers has been used for rotating particles continuously by 360o in pitch sense. The particle is rotated by placing it in a sample chamber with a thin layer of gold coated coverslip in an optical tweezers setup. The combined effects of convection current generated from heating of water and the optical trap rotate the particle in pitch sense near the hot spot present at the gold layer. This method works effectively until a water vapor bubble is formed due to continuous heating of the gold layer. We have tried to delay the onset of bubble formation by using viscoelastic medium instead of water. Viscoelastic medium is prepared by mixing polyacrylamide in water. Pitch rotation of hexagonal shaped particle has been observed for media having different concentrations of polyacrylamide. The frequency of rotation is measured at various concentrations. A suitable mathematical function is determined empirically which can describe the change in frequency while varying the concentration.
It is well known that thermo-optical tweezers leads to deposition of continuous patterns on substrates mediated by Marangoni convection currents around micro-bubbles. While performing the deposition, we also find that there is an accompanying emission. On first look, one would expect this to be thermal broadband emission. However, the spectrum of emission seems to start from green and extend all the way to near infra red. Such peak wavelengths would correspond to 3000 K or even higher. Generation of such high temperatures at the local hot spot would melt the glass substrate. However, such melting facets have never been seen. Thus we speculate that the emission is actually two photon fluorescence from the incident light on the deposited pattern. Soft Oxometallate material is known to exhibit photoluminescence in the green-red region of spectrum. This kind of emission is also observed in carbon nanotubes when incident with a focused 1064 nm light, the origin of which appears to be similar multiphoton fluorescence processes.