Optical manipulation of 'drops on rails' in two dimensional microfluidic devices

When using single microfluidic droplets as isolated biological/chemical micro-reactors or arrays of droplets as 2D assaying tools, control over droplet placement is crucial to successful device implementation. Here we demonstrate a combined mechanical and optical approach to generate highly controllable arrays of droplets in pre-determined 'rails and anchors' patterns on a two-dimensional plane. The technique combines passive mechanical forcing with selective laser action. Passive mechanical forcing provides a vehicle for droplet transport and storage and laser induced optical forcing is employed for stopping, guiding or derailing droplets as they pass through the chip. In this way intelligent operations can be performed upon arrays of droplets such as sorting, merging to initiate chemical reactions or selective removal of droplets from a predefined array. The usergenerated array may then be held static against a mean flow for prolonged observation.