  In this paper, we theoretically study the electrostatic interaction between a
pair of identical colloids with constant surface potentials sitting in close
vicinity of each other at a fluid interface. By employing a simplified yet
reasonable model system, the problem is solved within the framework of
classical density functional theory and linearized as well as nonlinear
Poisson-Boltzmann (PB) theory. Apart from providing a sound theoretical
framework generally applicable to any such problem, our novel findings, all of
which contradict common beliefs, include the following: first, quantitative as
well as qualitative differences between the interactions obtained within the
linear and the nonlinear PB theories; second, the importance of the
electrostatic interaction between the omnipresent three-phase contact lines in
interfacial systems; and third, the occurrence of an attractive electrostatic
interaction between a pair of identical metal colloids. The unusual attraction
we report on largely stems from an attractive line interaction which although
scales linearly with the size of the particle, can compete with the surface
interactions and can be strong enough to alter the nature of the total
electrostatic interaction. Our results should find applications in metal or
metal-coated particle-stabilized emulsions where densely packed particle arrays
are not only frequently observed but are sometimes required.
