Mixing minute solutions in a picoliter order droplet using electrostatic force

Mixing minute solutions in a droplet is an important technique in combinatorial chemistry. However the smaller the droplet, the more difficult it is to bring solutions into the same spot and stir the interior of the droplet homogeneously. So, we developed a novel technique for mixing minute solutions in a picoliter order droplet on a substrate using electrostatic force. In this technique, one of the solutions extracted from the nozzle utilizing electrostatic force forms a jet. This jet reaches to the substrate and initially forms a droplet. Next, a jet from the other solution is formed and collides with the initial droplet. The interior of the mixture droplet is well stirred in this procedure. This technique is also applicable to mixing solutions having high viscosity. A case of examples was demonstrated by mixing rhodamine B solution with fluorescein solution. These dye components were dissolved into a medium including 95% glycerin. The rhodamine B solution and the fluorescein solution, which have a viscosity of more than 600×10^-3 Pa·s, were mixed by the aforementioned method and the mixture droplet was produced. Observing the fluorescence images of the droplet confirmed that the two solutions were mixed more homogeneous than the conventional method which is touching the needle on a substrate.