July 11, 1995
Using light to manipulate small particles
We manipulate individual micron-size colloidal particles with light beams in order to measure the tiny repulsive forces between them that create colloidal crystals.
Schematic diagram of the dual optical tweezer system used to measure pairwise interactions between colloidal microspheres. The microscope objective lens focuses two laser beams into the sample volume. Each focus forms an optical trap for a submicron sphere. Once a pair of spheres is brought to a fixed separation they are released by interrupting the beams and their motions analyzed by digital video microscopy to extract the interaction potential. The vertical tubes in the drawing contain reservoirs of ion exchange resin which maintain chemical purity in the sample volume.
Measurement of pair-wise colloidal interaction potentials from digital video data using the apparatus sketched above. Interaction potential U(r) in units of the thermal energy kT as a function of center-to-center separation r measured in units of sphere radii. The solid line is a fit to the DLVO theory with corrections for charge renormalization Inset: modified semi-logarithmic plot to emphasize the screened-Coulomb nature of the measured interaction.
- "A Microscopic Measurement of the Pair Interaction Potential of Charge-Stabilized Colloid," J. C. Crocker and D. G. Grier, Phys. Rev. Lett., 73, 352 (1994).