Note: High-precision microsphere sorting using velocity sedimentation

Monodisperse populations of microspheres are desirable for a variety of research and industrial applications, but many desirable sizes and materials can be difficult to synthesize and have limited commercial availability. In this paper, we present an effective, straightforward, and low cost method for sorting polydisperse microspheres into many separate monodisperse samples. The basic approach is to use velocity sedimentation through a density gradient in a long vertical column, followed by carefully targeted extraction. We demonstrate this technique by reducing the coefficient of variation of melamine microspheres from 13% to 1%-4% and glass microspheres from 35% to 3%-8%. This simple and inexpensive method can be used to sort microspheres of many sizes and materials, and is easily scalable, opening the possibility of cheap, monodisperse microspheres.