Comparison of interparticle force measurement techniques using optical trapping

Timothy P. Koehler, Christopher M. Brotherton and Anne M. Grillet

Optical trapping has become a powerful and common tool for sensitive determination of electrostatic interactions between colloidal particles. Two optical trapping based techniques, blinking laser tweezers and direct force measurements, have become increasingly prevalent in investigations of interparticle potentials. The blinking laser tweezers method repeatedly catches and releases a pair of particles to gather physical statistics of particle trajectories. Statistical analysis is used to determine drift velocities, diffusion coefficients, and ultimately colloidal forces as a function of the center–center separation of the particles. Direct force measurements monitor the position of a particle relative to the center of an optical trap as the separation distance between two continuously trapped particles is gradually decreased. As the particles near each other, the displacement from the trap center for each particle increases proportional to the interparticle force. Although these techniques are commonly employed in the investigation of interactions of colloidal particles, there exists no direct comparison of these experimental methods in the literature. In this study, we compare measurements of interparticle forces applying both methods to a model system of polystyrene particles in an aerosol-OT (AOT) hexadecane solution where the screening lengths are very large. We found that the interaction forces measured using the two techniques compare quantitatively with each other and Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Additionally, our studies show that direct force measurements can be far more sensitive than previous studies have reported and nearly as sensitive as the blinking method.

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