Figure 7: Measurement of pair-wise colloidal interaction potentials
from digital video data.
Top: Distribution of particle separations initially at r(t) which
evolved to 
after 
msec.
In the absence of an interparticle interaction, the freely
diffusing spheres would form a distribution of points along the dashed
diagonal line.
Deviations from that line indicate the presence of interactions.
The density of points does not reflect the probability of finding
particles at a particular separation, but rather the frequency with
which initial conditions were set in that region with the
blinking optical tweezers.
Inset: Grey-scale histogram of the same data set normalized as
a propagation matrix, 
.
Bottom: Interaction potential in units of the thermal energy as a function
of center-to-center separation measured in units of sphere radii.
The solid line is a fit to the DLVO theory with corrections
for charge renormalization as described in eqns. (24)
and (25).
Inset: modified semi-logarithmic plot to emphasize the screened-Coulomb
nature of the measured interaction.