Many applications of laser tweezers rely on the accurate measurement of the transverse or axial trapping force. We have concentrated on the transverse trapping force and the most common method used to measure it, applying a viscous drag force. A trapped sphere was subjected to a viscous drag force via a Stokesian flow. The flow was achieved by oscillating the sample stage at a constant speed of 750 microns/second. A Zeiss oil-immersion (N.A. equals 1.3) objective was used to focus a 1064 nm Nd:YVO4 laser beam in order to trap 6 microns diameter polystyrene spheres suspended in distilled water. The minimum power needed to hold the particle in the trap at a particular viscous drag force was then measured. The influence of trap depth, oscillation amplitude used and particle concentration have been investigated, in particular the effects caused by the characteristics of the function used to create the oscillation. The minimum laser power needed to trap a sphere was found to increase with a rise in oscillation amplitude. The velocity profile through the fluid, the rotation of the trapped particle and the effect of interactions with other particles is considered when explaining these effects.
The transverse force of an optical trap is usually measured by equating the trapping force to the viscous drag force applied to the trapped particle according to Stokes' Law. Under normal conditions, the viscous drag force on a trapped particle is proportional to the fluid velocity of the medium. In this paper we show that an increase of particle concentration within the medium affects force measurements. In order to trap the particle, 1064 nm light from a Nd:YVO4 laser was brought to a focus in a sample slide, of thickness around 380 microns, by using an inverted Zeiss microscope objective, with NA equals 1.3. The slide was filled with distilled water containing 6 micron diameter polystyrene spheres. Measurements were taken at a fluid velocity of 0.75 microns/sec, achieved by moving the sample stage with a piezo-electric transducer whilst a particle was held stationary in the trap. The laser power required to hold a sphere at different trap depths for various concentrations was measured. Significant weakening of the trap was found for concentrations >0.03% solids by weight, becoming weaker for higher trap depths. These results are explained in terms of aberrations, particle-particle interactions and distortion of the beam due to particle-light interactions.
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