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108 3 Optical Tweezers
20 d = 10 mm
Minimum trapping power (mW) 10 5 n = 1.33
h = 1.0 mPa•s
15

H = 150 mm
T = 55 mm

0
0 50 100 150 200 250
Stage velocity (mm/s)

Fig. 3.31. Dependence of minimum transverse trapping power on the velocity for
d =10 µm polystyrene particle
100
Minimum trapping power (mW) 80 Polystyrene

60
40
20
0
20
30
0
10
Particle diameter (mm) 40 50
Fig. 3.32. Dependence of minimum transverse trapping power on diameter of
polystyrene microspheres

Figure 3.31 shows the dependence of P trans on sphere velocity for d =
min
10 µm polystyrene particles (small gravity). P trans increases as bead velocity
min
increases and the experimental values are fairly in good agreement with the
theoretical ones.
Figure 3.32 shows the dependence of P trans on sphere size (diameter) for
min
polystyrene particles with a downward-directed beam. P trans substantially
min
increases as d increases due to the Stokes dragforce increase. The discrepancies
between the theory and experiment for the trappingforces at d> 30 µm is due
to the fact that the trappingposition moves to upward from the transverse
center line in the sphere, and that Q becomes smaller, since the gravitational
force increases for such large spheres [3.18] (Q3.7).

3.3.4 Optical Fiber Trapping

To realize a simple and inexpensive implementation of an optical trap, 3-D
trappingusingthe counterpropagatinglight beams from two optical fibers
and 2-D trappingusinga solitary fiber were demonstrated. Figure 3.33 shows