On-demand mixing droplet spotter for preparing picoliter droplets on surfaces

An on-demand mixing droplet spotter for generating and mixing picoliter droplets has been developed for ultrasmall reaction vessels. The droplets were generated by applying a approximately 500-V, approximately 2-ms pulsed voltage to the tips of capillary tubes (o.d. approximately 20 microm; i.d. approximately 12 microm) filled with solution. The mixing process was achieved using electrostatic force. The initial droplet was formed by applying the pulsed voltage between one capillary and the substrate, and the second jet of the other solution was generated from the other capillary and collided with the initial droplet automatically because the electric field lines concentrated on the initial droplet. Using this mixing process, a microarray having a concentration gradient was obtained by spotting approximately 6-pL droplets on a surface with a density of one spot per 75 x 75 microm(2).