research: fluctuations

Optical Trapping of a Colloidal Particle

By studying a submicron colloidal particle trapped by optical tweezers, we have demonstrated that even a weak periodic force can give rise to a significant particle flux, or directional diffusion. A net force is produced by modulating a spatially symmetric double-well potential.

A rendering of the 0.6 µm glass sphere in a trap formed by two highly-focused laser beams is shown in A. The laser traps a sphere near its beam waist. The potential U(r) created by the beam-particle interaction is obtained by measuring the probability density of the particle in the 3-dimensional trap.

A 2-dimensional cross-section of the trapping potential is shown, with each color representing energy contours differing by 1 kBT. The trap is tilted to the left and to the right by modulating the laser intensity, as shown by the half-cycle sequence B ®C ®D.

If the trap is initially symmetric, how does the particle pick out a particular direction to diffuse?

By modulating the intensity with a biharmonic waveform, the preferred direction and rate of interwell transitions are controlled by adjusting the phase angle between the two harmonics.

A large response is observed even for rapid nonadiabatic modulation of the potential. Nonadiabatic means that the modulation rate is greater than the intrawell relaxation time which is about 10 ms due to viscous damping of the 0.6 µm sphere in water.

We believe that the principles demonstrated here should be useful in designing highly selective devices for separating biological macromolecules and other polydisperse suspensions.

To learn more...

Activated Escape of Periodically Driven Systems,

M.I. Dykman, B. Golding, L.I. McCann, V.N. Smelyanskiy, D.G. Luchinsky, R. Manella, and P.V.E. McClintock, Chaos 11, 587-594 (2001).

Thermally Activated Transitions in a Bistable Three-Dimensional Optical Trap, Lowell McCann, Mark I. Dykman, and Brage Golding, Nature 402, 785-7 (1999).

Time Oscillations of Escape Rates in Periodically Driven Systems, V.N. Smelyanskiy, M.I. Dykman, and B. Golding, Physical Review Letters 82, 3183-7(1999).

Support…

Thanks to the NSF Division of Materials Research for funding. And to collaborator Mark Dykman.