Passive sorting of emulsion droplets with different interfacial properties using laser-patterned surfaces

Size-Based Sorting of Emulsion Droplets in Microfluidic Channels Patterned with Laser-Ablated Guiding Tracks

“…In order to characterize the performance of our droplet guiding and sorting technique, we carried out systematic experiments with aqueous emulsion droplets of varying sizes moving at different flow rates of the host liquid in microfluidic chips featuring guiding tracks of different widths. The chips used in the experiments, featuring two separate T-junctions with individually adjustable flow rates of both droplet and host liquids, allowed for controlling the speed of droplet motion through the chip independently of the droplet size . In addition, droplets produced at different T-junctions could be fused at the entrance port of the Hele-Shaw flow channel, thus creating a sorting scenario with two distinct, well-defined droplet sizes present simultaneously in the chip.…”

“…Microfluidic chips used in our sorting experiments were fabricated from PDMS using standard soft lithography. As described previously by Rashid et al, cleaned glass substrates were first coated with a thin PDMS film and, subsequently, laser-ablated to define the guiding tracks. Following the ablation, the patterned glass substrates forming the bottom part of the microfluidic devices were bonded with the top PDMS layers containing the actual flow channels.…”

“…In order to simulate the trajectories of droplets flowing through microfluidic channels featuring laser-ablated guiding tracks, we adopted the previously developed model of droplet guiding that ignores inertial effects at the low-Reynolds numbers limit (Re ≪ 1), that is, assumes a zero net force acting on the droplet at all times . Additional details of this model can be found in the Supporting Information.…”

“…Hence, precise determination of Δ A is crucial for calculating the droplet trajectory. When the droplet lies entirely outside the track, its shape has a full rotational symmetry and, thus, its surface area, A out , can be calculated analytically . On the other hand, the calculation of the surface area of a droplet partially overlapping with the track is not analytically tractable and requires a full three-dimensional CFD analysis.…”

“…We characterize the performance of our droplet guiding and sorting technique by systematic experiments, in which we vary the size of the aqueous emulsion droplets, the speed of their motion through the chip, and the width of the laser-ablated guiding track. In addition, we also simulate the droplet trajectories along the guiding track, using a previously developed theoretical model that includes the relevant forces acting on the moving droplets under the given experimental conditions . Such modeling requires realistic evaluations of changes in the shape and surface area of a droplet that is partially overlapping with the guiding track; these changes are obtained from detailed computational fluid dynamics (CFD) simulations.…”