Microfluidic Mixing Triggered by an External LED Illumination

The mixing of confined liquids is a central yet challenging
operation
in miniaturized devices. Microfluidic mixing is usually achieved with
passive mixers that are robust but poorly flexible, or active mixers
that offer dynamic control but mainly rely on electrical or mechanical
transducers, which increase the fragility, cost, and complexity of
the device. Here, we describe the first remote and reversible control
of microfluidic mixing triggered by a light illumination simply provided
by an external LED illumination device. The approach is based on the
light-induced generation of water microdroplets acting as reversible
stirrers of two continuous oil phase flows containing samples to be
mixed. We demonstrate many cycles of reversible photoinduced transitions
between a nonmixing behavior and full homogenization of the two oil
phases. The method is cheap, portable, and adaptable to many device
configurations, thus constituting an essential brick for the generation
of future all-optofluidic chip.