Figure 17.3 — Electrostatic interactions between trapped colloidal particles Figure 17.3 — Electrostatic interactions between trapped colloidal particles. (a) Dual blinking optical tweezers experimental set-up for measuring the electrostatic interaction between two colloidal particles. (b) Measured interaction potential for a pair of spheres with radius a = 0.326 μm as a function of their separation x . For larger separations, the interaction potential is well fitted by the potential predicted by the DVLO theory with fitted parametersZ* = 1991 e– and lD = 161 nm. The inset is a semilogarithmic plot whose gradient is used to determine the screening length. (c) Measurement of pairwise electrostatic interaction potentials showing the importance of correcting the particles’ positions obtained from digital video microscopy at small particle separations. The upper trace shows that the interaction potential (vertically shifted by 4kBT ) for large separations of the particles is fitted well by a screened repulsive Coulomb interaction. When the particles are allowed to sample small separations (less than approximately twice their diameter), an apparently attractive potential appears, as shown in the middle curve (vertically shifted by 2kBT ). After correction for artefacts arising from imaging particles at small separations, the interaction potential is found to be repulsive at all separations and in agreement with the DLVO theory, as shown by the lower trace.Figures (a) and (b) are reprinted from Crocker and Grier, Phys. Rev. Lett. 73, 352–5. Copyright (1994) by the American Physical Society.Figure (c) is reproduced by permission from Baumgartl et al., Soft Matter 2, 631–5. Copyright (2006) The Royal Society of Chemistry.