An optical toolbox for total control of droplet microfluidics

TLDR

Microfluidic droplets serve as microreactors, but require active control of operations such as formation, transport, division, and fusion; recent studies have shown that laser‑induced thermocapillary forces can block droplet motion in microchannels. The study demonstrates that laser‑based optical control can perform droplet operations—formation, transport, division, fusion—without specialized microfabrication or moving parts. The authors implement a suite of optical droplet manipulation building blocks—including valve, sorter, fuser, and divider—and show that multiple functions can be combined, for example by using a single laser spot to operate both a valve and a fuser. © 2007, Rev.

Abstract

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 (C. N. Baroud, J.-P. Delville, F. Gallaire and R. Wunenburger, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2007, 75(4), 046302). Here, we demonstrate the generality of this optical approach by implementing the operations mentioned above, without the need for any special microfabrication or moving parts. We concentrate on the applications to droplet manipulation by implementing a wide range of building blocks, such as a droplet valve, sorter, fuser, or divider. We also show how the building blocks may be combined by implementing a valve and fuser using a single laser spot. The underlying fundamentals, namely regarding the fluid mechanical, physico-chemical and thermal aspects, will be discussed in future publications.