A bright field image of water-in-oil droplets, generated from two aqueous solutions (labeled with Lucifer yellow and Alexa 430 dyes, respectively) flowing in parallel and intersected by two mineral oil flows in a 50 × 40 μm2 microchannel. An arrow shows the cross scanning direction is perpendicular to the microchannel.
A schematic of home-built two-photon fluorescence lifetime microscopy system.
(a) Fluorescence decays of Lucifer yellow (lifetime of 5.0 ns) and Alexa 430 (lifetime of 3.1 ns). The peak and tail regions are set in identical width to define the division ratio. (b) A calibration curve of two fluid mixing based on the division method: measured (circles) from 11 pre-mixed bulk solutions and theoretically calculated (solid line) using Eq. (2).
2D computational domain and the initial dimension (75 × 40 μm2) of a droplet with two dyes in parallel arrangement.
(a) A time-tracking fluorescence signal for 10 droplets. Each point is the total photon counts integrated for 5 μs, and each periodic signal represents one droplet flowing through the detection point. (b) The autocorrelation of the time-tracking fluorescence signal shows that the reproducible mixing pattern in the droplets can be aligned and summed up.
Intradroplet time-tacking trajectories summed up using 1 droplet (a), 15 droplets (b), 150 droplets (c), and 1500 droplets (d).
(a) A time-tracking fluorescence signal (blue line with circles) after aligning-summing up 2850 periodic signals and 3× binning, and the corresponding mixing fractions of Alexa 430 (red line with squares) calculated from Eq. (2). (b) A demonstrated fluorescence decay of the mixture at the position of 0.9 ms in (a).
(a) Schematics of asymmetric and symmetric vortices formed in droplets moving through the bent and straight microchannels, respectively. Two dye mixing patterns are shown at various locations indicated in (a): experimental results (b), and numerical simulation results (c).
Mixing efficiency versus time for two dye mixing in droplets flowing through a serpentine microchannel: calculated from experiments (red line with circles) and from numerical simulations (blue line).
Physical properties of fluids.
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