Understanding and optimizing laser-induced thermoelectric forces for enhanced trapping and manipulation of colloidal particles

Several studies have been proposed to control particle trajectory in liquid solutions using optically induced thermal gradient. Upon introducing different solutes such as salts and surfactants along with microparticles in these solutions, an additional optically induced thermoelectric trapping force is generated due to the differential motion of ions in the solution under thermal field. As the complexity of the solution increases, it becomes increasing difficult to understand particle response towards laser irradiance. More importantly, the existing models to study the thermoelectric behavior of the particle assumes a constant temperature gradient across the particles, which becomes obsolete in the micro-regime due to discontinuity of thermal conductivity at the particle-solution interface. For a better understanding of trapping and manipulation behavior of particles under light induced thermoelectric field, the temperature gradient distortion must be considered. In this work, full-scale finiteelement solver model has been proposed to determine the temperature variation around a microparticle under laser heating. The resultant temperature distribution is utilized to numerically evaluate the thermoelectric field and the trapping potential of the laser induced opto-thermoelectric trap. To experimentally validate this methodology, polystyrene micro-particles are trapped opto-thermoelectric-ally in CTAC solution and compared the experimental trapping stiffness to theoretical estimates obtained from the model. It is observed that trapping stiffness saturates as surfactant concentration increases which can be optimized by choosing the lowest CTAC concentration at the onset of saturation. The model implemented here can be easily extended to arbitrarily shaped particles, particles with non-uniform surface morphology, different combinations of core-shell particles and electrolyte solutions, which can be implemented to study different phenomenon such as optical pulling, rotation and translation.