Depletion potentials near geometrically structured sub - strates

– Using the recently developed so-called White Bear version of Rosenfeld's Fundamental Measure Theory we calculate the depletion potentials between a hard-sphere colloidal particle in a solvent of small hard spheres and simple models of geometrically structured sub-strates: a right-angled wedge or edge. In the wedge geometry, there is a strong attraction beyond the corresponding one near a planar wall that significantly influences the structure of colloidal suspensions in wedges. In accordance with an experimental study, for the edge geometry we find a free energy barrier of the order of several kBT which repels a big colloidal particle from the edge. When a colloidal suspension contains particles of different size, depletion forces arise between big particles due to excluded volume effects. If a colloidal suspension is prepared such that electrostatic interactions are screened and only hard-core repulsive interactions are left, these forces are attractive at small distances [1] and can exhibit a potential barrier and an oscillatory tail for large separations [2, 3, 4, 5]. The same forces also arise if a big colloidal particle b (diameter σ b) suspended in a solvent of small particles s (diameter σ s) approaches a wall. The depletion forces for hard-sphere models of bulk colloidal suspensions, as well as colloidal suspensions close to structureless walls have been the subject of many theoretical studies. Technical developments during the last decade made it possible to prepare well-defined, monodisperse colloidal suspensions and with the help of video-microscopy [6] and total internal reflection microscopy [7, 8, 9] such depletion potentials can be now measured directly and compared quantitatively with theoretical predictions. On the other hand, the behavior of colloidal suspensions close to structured walls and resulting depletion potentials have not yet been investigated in depth. Such situations arise naturally for example close to a corner in a container for colloidal suspensions. Geometrically structured substrates with a dedicated design have been used to control the growth of colloidal crystals [10]. In this context Dinsmore et al. [11] studied the motion of colloidal " hard-spheres " c EDP Sciences