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Interaction dynamics of two colloids in a single optical potential.

The interaction of two diffusing particles is strongly influenced by their hydrodynamic coupling. At a tracking rate of 10 kHz we are able to measure the 3D trajectories of two colloidal spheres in a single harmonic potential, which was generated by scanning line optical tweezers. This common potential enables tilting, rotational, and translational dynamics of the spheres, which we analyzed via the spheres position cross-correlations C(τ) over a time range of 10(-4)-2 s. We found that the dynamic interaction of the colloids is controlled by short-range surface forces F(s), which are attractive in one direction and repulsive in the other two directions. This unexpected behavior is supported by a theoretical model using two Langevin equations, which decouple for linear F(s), allowing a description with autocorrelation functions for collective and relative motions. We further demonstrate that variations in salt concentration and reaction volumes significantly influence C(τ) and the mean contact times between the particles, which may offer new insights into biological particle interaction.