Figure 4.

(a) A snapshot of suspended polystyrene beads (500 nm in diameter) driven laterally to two opposite sides by the TOW tweezers. The locations of the two main intensity spots from the ‘diverging’ TOW beams are marked by two dashed circles, from where the beads are pushed away (Supplementary Movie 3). (b) Illustration of time-averaged direction and magnitude of the flowing beads when the two beams constituting the TOW tweezers are 5 μm apart at the focal plane. The colour represents the magnitude of the normalized average particle velocity, and arrows indicate the direction of particle flow. (c) Trapping force resulting from only one arm of the TOW tweezers while the other is absent (that is, that part of the beam is blocked). Measured results are plotted in solid curves with shaded area representing the error. The theoretically calculated force in x-direction is shown on the same graph (dashed curve) for comparison. (d) The force profile of a 1-μm ‘bacterium-like’ particle with a refractive index 1.38 calculated via the generalized Lorenz–Mie theory. Red and blue colours represent the magnitude of the force in positive and negative directions, respectively. The overlay shows the forces (in normalized units) along the dotted line. The hair-like wisps represent paths taken by simulated particles in the absence of Brownian motion. The dotted box indicates the region similar to that shown in c.