A 10 mm x 10 mm piece of silicon wafer (#16006, Ted Pella) was treated by glow discharge (Ted Pella, model PELCO easiGlowTM) to render the surface hydrophilic. A 30-µl drop of the particle suspension was placed on the silicon and stored for 3 hours at 4 °C in a wet chamber. To avoid drying artifacts, the samples were immersed in distilled water shortly to remove unbound particles. Excess water was blotted off with a piece of filter paper and dried in air.