Rapid mixing of sub-microlitre drops by magnetic micro-stirring

We demonstrate rapid mixing of sub-microlitre droplets (250 nl) using miniaturized magnetic stir bars (400 mu m x 200 mu m x 15 mu m). The stir bars are fabricated using laser micromachining and placed on the substrate on which the drops are manipulated. They are activated by an externally applied magnetic field and used in combination with on-demand drop merging in enthalpy arrays. This technique results in a 10-fold increase in mixing rate, and a mixing time constant of about 2 s. Drop mixing times are measured by Forster resonance energy transfer (FRET) and verified by thermodynamic measurements of binding and enzymatic reactions.