Title:
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Factors affecting the characterisation of an optical trap using the viscous drag force method
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A computer controlled optical trapping system has been designed and built, with the
aim of being as flexible, compact and user friendly as possible. The optical trap was
formed with a 1064 nm TEMoo mode laser beam and a microscope objective with a
numerical aperture of 1.3. The system is based on a Leica DM IRB inverted
microscope and programmed using the Lab VIEW programming language from
National Instruments.
The optical trapping system was used to investigate factors that affect the transverse
trapping force measurements of an optical trap, in particular when using the viscous
drag force method to characterise the trap. Measurements were made of the
minimum power required to trap, trapping efficiency, trap stiffuess, potential well
and transverse trapping force. The particle or fluid displacement functions, position
of the particle within the sample and particle concentration, were all examined.
When a particle was trapped off-axis by 33 J.lm in the x direction, the efficiency of
the trap was found to reduce by roughly 32 %, compared to a particle trapped onaXIS.
The minimum power required to retain a particle in an optical trap was
independent of particle concentration, for concentrations below 0.1 % solids by
weight. For a 6 J.lm diameter polystyrene sphere, an increase in power of
approximately 50 % was required to trap a particle when the amplitude of distance
moved by the sample stage was increased from 20 J.lm to 60 J.lm. This effect was
only present below an amplitude of 53 ± 6 J.lm, above this value the power required
to trap a 6 J.lm diameter sphere was independent of amplitude of distance moved.
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