  Colloidal dispersions composed of polymethylmetacrylate particles dispersed
in a mixture of cyclohexylbromide and decalin find widespread use as model
systems in optical microscopy experiments. While the system allows simultaneous
density and refractive index matching, preparing particles with hard potentials
remains challenging and strong variations in the physical parameters of samples
prepared in the same manner are common. Here, we present data on the
measurement of forces between individual pairs of particles over the course of
tens of days using a blinking optical tweezer method. Our results show that the
variations in the particle properties are indeed caused by a temporal evolution
of the particles' charging. Additional measurements of the influence of
tetrabutylammonium bromide (TBAB) addition to the dispersions show that already
small concentrations of added TBAB drastically decrease the electrostatic
forces between colloidal particles. However, small, non-negligible contact
potentials remain even at the highest TBAB concentrations added.
