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Charged colloidal particles in a charged wedge: do they go in or out?

Using real-space microscopy experiments, theory and computer simulation, we study the behaviour of highly charged colloidal particles which are confined between two highly charged plates forming a wedge geometry. Under low salt conditions it is experimentally observed that colloidal particles accumulate in the cusp of a wedge to form dense fluid or crystalline ordered structures. This behaviour is found for various cell geometries, salt concentrations and gravitational strengths, and even stays stable when additional convection is present in the system. An effort is made to understand this effect qualitatively on the basis of linear screening theory. For a single macro-ion, linear screening theory predicts an attractive ‘trapping’ force close to the cusp of the range of the Debye–Hückel screening length. The attractive force diverges logarithmically with decreasing distance of the macro-ion from the wedge cusp, while at large distances the force is repulsive. The results of the linear screening theory are confirmed by computer simulations of the primitive electrolyte model with explicit co- and counterions.