Scientists at Hull University, the CLRC in Didcot and Imperial Chemical Industries in Middlesborough, all in the UK, have measured the repulsive force between two charged polystyrene particles in an oil-water interface as it relates to their physical separation. They used a variable-power continuous-wave Nd:YAG laser operating at 1064 nm to trap the particles, which were arranged in a monolayer at the interface between water and a mixture of decane and undecane. By varying the laser's power and the distance between the particles, they determined that the repulsive force scales with distance to the negative fourth power. In addition, they found that this force is unaffected by the electrolyte content of the aqueous phase and is likely due to a small amount of charge produced by ionizable sulfate groups at the oil-particle interface. The work was reported in the June 17 issue of Physical Review Letters.