Optical trapping of polystyrene beads in mixed solvents

Optical traps are devices used to micromanipulation microscopic objects using a focused laser beam in an optical microscope field of view. Trapping such objects is possible only when their refractive index is higher than the environment's. Typically, trapping experiments are carried out in aqueous solutions, exploiting the low refractive index of water. Experiments in different pure organic solvents were also reported, showing not very good dependence of the optical-trap force constant on the solvent refractive index. In this study, we carry out optical trapping experiments in mixed water:organic solvents where the organic components are dimethyl sulfoxide, ethylene glycol, and glycerol. Trends of corner frequencies measured in these mixtures follow the theoretical calculations well for all the studied systems in the whole molar-fraction range, indicating their dominant dependence on the refractive index. The conversion of the corner frequencies to the force constants of the trap is strongly influenced by the differences in viscosity throughout the molar-fraction range that emphasizes experimental errors in the regions where it is high. In addition, the force-constant and corner-frequency curves can serve as an indicator of ideality of the potential profile of the optical trap.