We demonstrate a new droplet on demand (DOD) technique and an integrated system for scanning of arbitrary combinations of 3 miscible solutions in approximately 1.5 microL droplets at 3 Hz. The DOD system uses standard electromagnetic valves that are external to the microfluidic chip. This feature makes up for modularity, simplicity of assembly and compatibility with virtually any microfluidic chip and yields an on-chip footprint of less than 1 mm(2). A novel protocol for formation of DOD enables generation of an arbitrarily large range of volumes of droplets at a maximum operational frequency of approximately 30 Hz. The integrated system that we demonstrate can be used to scan up to 10,000 conditions of chemical and biochemical reactions per hour using approximately 10 mL of solutions in total.