Factors affecting the transverse force measurements of an optical trap: Part I.

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Abstract

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. An increase of particle concn. 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 mm, by using an inverted Zeiss microscope objective, with NA is 1.3. The slide was filled with distd. H2O contg. 6 mm diam. polystyrene spheres. Measurements were taken at a fluid velocity of 0.75 mm/s, achieved by moving the sample stage with a piezoelec. transducer while a particle was held stationary in the trap. The laser power required to hold a sphere at different trap depths for various concns. was measured. Significant weakening of the trap was found for concns. >0.03% solids by wt., 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. [on SciFinder (R)]

Keyword(s)

Piezoelectric transducers Viscosity (factors affecting transverse force measurements of optical trap) Trapping apparatus (optical factors affecting transverse force measurements of) Force (transverse factors affecting measurements of optical trap)

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