J. BHATT ET AL.
298
Figure 4. The experimental data showing variation of trap-
ping force on the particles with distance from the trap centre.
The size of the trap is 0.96 µm.
significant directional flow to take place since the trap is
switched on.
We observe that the theoretical approach which takes
spot size w as the relevant length scale of interaction and
valid for all the particle sizes [13], should have given the
correct range of forces to us. However, it is not very
successful in the present case, although it provides us a
qualitative explanation regarding range of forces for
bigger and smaller trap sizes. We can say that the gradi-
ent forces alone cannot describe this kind of directional
flow, one need to consider hydro-dynamical processes
involved in the system.
We know that the spherical aberrations [16-18] which
come into play because of refractive index mismatch in
the glass-water boundary decrease the stability of the
optical traps due to distorted focus. However, for a com-
parative study like ours, this does not affect the conclu-
sions, the aberrations being same throughout the experi-
ment.
5. Conclusion
In conclusion, we show an obvious but unexplored me-
thod to control the flow of particles that does not use
micro-channels and may allow the possibility of an en-
tirely fluidic process. Earlier studies used the light inten-
sity of the trap to control the flow of particles and used
the conventional method of overfilling the objective lens.
In contrast, we show that it’s not always required to
overfill the objective; instead one can use the original
beam and its different magnifications to control the flow
of microscopic particles.
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