2 μm Tm-doped fiber laser trapping of colloidal particles by temperature gradient

A temperature gradient induced by a focused 2 μm Tm-doped fiber laser is used for opto-thermal trapping of colloidal particles in an aqueous solution. The water has a large absorption peak around 2 μm in wavelength due to its vibrational modes, and some local temperature gradient is generated around the focus where the colloidal particles are migrated when salt is slightly added to the solution. In this study, the experimental results under different salt (electrolyte) concentrations are compared in order to clarify the role of an electrostatic force generated due to ions redistribution in the temperature gradient. As a result, the particles are trapped when the salt concentration is higher than 10 μg/ml, whereas they are not trapped under this concentration. Although a prediction of the electrostatic field near the heat source is difficult, our findings suggest that the mechanism of trapping in our system may ascribe to thermoelectric effect.