Effects of media viscosity and particle size on optical trapping of microspheres

In this study, we investigated the effects of size and surrounding media viscosity on trapping of microspheres. A continuous wave ytterbium fiber laser with a 1064 nm wavelength was used to create an optical tweezers system for optical manipulation experiments. Briefly, the system consisted of an inverted microscope, and a 100X 1.4 NA oil immersion objective through which the laser beam converged to form the optical trap. The laser beam was collimated, steered, and coupled to the microscope through the epifluorescence microscope port. The laser power at the trap focal spot was determined by measuring the input power at the back aperture of the objective multiplied by the objective transmission factor at 1064 nm measured by a modified dual objective method. Polystyrene microspheres varying in diameter from 5 to 15 microns were suspended in liquid media in glass bottom petri dishes prior to trapping experiments. The microspheres were trapped at different trapping powers, and fluidic viscous drag forces where applied to the optically trapped microspheres by driving a computer controlled 2D motorized microscope stage at known velocities. The drag forces were calculated at the point that the microspheres fell out of the trap, based on the Stokes equation for flow around spheres. The data show a linear relationship between trapping force and trap power within the range of the microsphere diameters and media viscosity values used. The work includes calculation of the dimensionless trap efficiency coefficient (Q) at 1064 nm wavelength and the corresponding effects of media viscosity and microsphere size on (Q).