Accurate microfluidic sorting of droplets at 30 kHz

Abstract

Fluorescence-activated droplet sorting is an important tool for droplet microfluidic workflows, but published approaches are unable to surpass throughputs of a few kilohertz. We present a new geometry that replaces the hard divider separating the outlets with a gapped divider, allowing sorting over ten times faster.

Fig. 1

(a) A schematic of the fast droplet sorter with detected and selectively displaced black droplets being separated by a gapped divider (red) of reduced channel height. (b) Still from high speed video of 22kHz sorting. With a conventional hard wall divider, droplets not fully displaced are split (c), while larger applied dielectrophoretic forces pull droplets apart (d). Scale bars are 50 µm.

Fig. 2

(a) A schematic of the entire device, with shallow channels in red and green boxed regions indicating areas magnified in other figures. Microscope images of (b) irregularly spaced, reinjected droplets from an ill-designed single-layer reinjector and (c) regularly spaced droplets from the actual two-layer reinjector used to sort. (d) The droplet filter before the reinjector. (e) Equilibration channels connecting the exit outlets. Scale bars are 500 µm in (a) and 50 µm in (b–e).

Fig. 3

(a) Time series during a sort showing the PMT-detected fluorescence signal (blue) as well as the voltage applied to the electrode (red). Inset shows the frequency components from a Fourier transform of a longer time series during the same sort, as well as the range of previously reported sort rates. Fluorescence microscope images, also from the same sort, of the pre-sorted droplets (6.4% bright), the positive droplets (99.3% bright), and negative droplets (0.2% bright). Scale bars are 100 µm.