  We measure interaction forces between pairs of charged PMMA colloidal
particles suspended in a relatively low-polar medium (5 $\lesssim \epsilon
\lesssim$ 8) directly from the deviations of particle positions inside two
time-shared optical traps. The particles are confined to optical point traps;
one is held in a stationary trap and the other particle is brought closer in
small steps while tracking the particle positions using confocal microscopy.
From the observed particle positions inside the traps we calculate the
interparticle forces using an ensemble-averaged particle displacement-force
relationship. The force measurements are confirmed by independent measurements
of the different parameters using electrophoresis and a scaling law for the
liquid-solid phase transition. When increasing the salt concentration by
exposing the sample to UV light, the force measurements agree well with the
classical DLVO theory assuming a constant surface potential. On the other hand,
when adding tetrabutylammonium chloride (TBAC) to vary the salt concentration,
surface charge regulation seems to play an important role.
