Interaction between Nearly Hard Colloidal Spheres at an Oil-Water Interface

We show that sterically stabilised ("nearly hard") colloids at a water-oil interface behave to a good approximation as charge monopoles. Interparticle potentials, $U(r)$, are extracted via a reverse Monte Carlo scheme which provides a best fit to the radial distribution functions measured by fluorescence microscopy; the results are then validated by particle tracking in a blinking optical trap. We postulate that the long range repulsion we observe arises mainly through interactions between neutral holes on a charged interface. In agreement with this interpretation, we find that the interaction can be tuned by varying salt concentration. The interaction also depends on the nature of the grafted polymer used to stabilise the colloids.