An optical toolbox for total control of droplet microfluidics

The use of microfluidic drops as microreactors hinges on the active control of certain fundamental operations, such as droplet formation, transport, division and fusion. Recent work has demonstrated that local heating from a focused laser can apply a thermocapillary force on a liquid interface sufficient to block the advance of a droplet in a microchannel (Baroud et al., Phys. Rev. E. V 75, p.046302). Here, we demonstrate the usefulness of this optical approach by implementing the operations mentioned above, without the need for any special microfabrication or moving parts. Building blocks such as a droplet valve, sorter, fuser, or divider are shown, as is the combination of a valve and fuser using a single laser spot