Two-dimensional non-close-packed arrays of polystyrene microspheres prepared by controlling the size of polystyrene microspheres

Randomly formed close-packed arrays of 3-μm-large polystyrene microspheres on a sapphire substrate by drop-casting were found to convert to two-dimensional non-close-packed ones when the microsphere-coated substrate was dipped into and taken out of a chloroform/ethanol mixed solution with specific volume fractions. The conversion was found to be associated with (1) the increased center-to-center distance between nearest neighboring microspheres caused by the increase in size and (2) the subsequently increased edge-to-edge distance between them caused by the decrease in size with their positions not much changed. The increase of microspheres in size was due to swelling by absorbing chloroform. The subsequent decrease of microspheres in size was due to deswelling caused by gradual change in the solvent environment during evaporation. The dependence of conversion from closed-packed arrays to non-close-packed ones on the volume fraction of each constituent was investigated. This type of conversion was found to occur readily on both sapphire and Si substrates.