Successive Droplets Encapsulation Revealed by Optofluidics

Digital microfluidics: an easy way to encapsulate successively water droplets by oil.
Digital microfluidics is implemented to coat successively water droplets with a thin layer of silicon oil. Small water droplets are settled at three fibers nodes while a larger oil drop slides down the vertical fiber.
Optofluidics: an interesting way to reveal the droplet sliding motion and the encapsulation.
To distinguish the different fluids, quantum dots (QDs) and rhodamine labelled polystyrene beads are diluted in oil and water, respectively.These two fluorescent probes are lighted by a 532 nm radiation.The fluorescence signal is recorded by a high speed camera, perpendicularly to the laser and to the fibers network.
This set-up reveals that the oil drop passes through the water droplets without removing them from the nodes, leaving a thin oil layer around the water droplets.
The coating, clearly seen on top pictures of Fig 2, prevents water evaporation. The key parameters for a good encapsulation are the fiber diameter and the oil viscosity that determine droplets size and velocity, respectively.