Self-assembly of arrays of micro-rings by colloidal evaporative deposition

We describe self-assembly of arrays of micro-rings in a drying process of a mixture of H2O and C2H5OH (1:1) and trace amounts of NH4OH with suspended SiO2 nano-particles of 102.5 nm average diameter. A drop of 2 mm diameter of the colloid is placed on a nearly vertical SiO2 substrate. A drop spills down and spreads into a wedge-shape thin film. Initially, the colloid wets the substrate. Due to different evaporation rates of solvent components the wettability of the colloid changes. The content of alcohol decreases and the liquid mixture does not wet the substrate any more. The colloidal thin film brakes into rhomboidal segments, which aggregate into a regular lattice of droplets. Each droplet evaporates and forms a micro-ring of about 80 μm in diameter. The micro-rings self-assemble into a rhomboid array with lattice constant ratio of 1:0.9.