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(a) Schematic of flow-focus drop maker used to create drops. The flow-focus junction is indicated by the arrow and shown at higher magnification in (b) and (d). Images of drops flowing through a wide delay line for (c) low-volume fraction and (e) high-volume fraction emulsions. (f) Velocity profile across the delay line for both cases; y and L, respectively, represent the distance from one wall of the channel where the velocity is measured and the width of the channel. corresponds to the center of the channel. The profile is more plug-like for the high-volume fraction emulsion than for the low-volume fraction emulsion due to close-packed plug flow. The scale bars denote .
Images of (a) low-volume fraction and (b) high-volume fraction emulsions being reintroduced into a microfluidic device; the close-packing of the high-volume fraction emulsion causes the drops to organize into a regular array. Measurement of drop frequency at the spacing junction for (c) low-volume fraction and (d) high-volume fraction emulsions. The frequency probability distributions for each volume fraction are plotted inset in the figures. The standard deviation of the distribution normalized by the average frequency is 0.5 for LVF and 0.1 for HVF. The scale bars denote .
Fluorescence intensity distributions of droplets containing -galactosidase and fluorescein-di(β-D-galactopyranoside) for low-volume fraction and high-volume fraction emulsions. The broadness of the low-volume fraction emulsion is an artifact of the time dispersion due to the variation in droplet incubation time. With close-packing, time dispersion is diminished, resulting in a narrower distribution that more accurately reflects the true activity of the enzyme.
Droplet diameter d as a function of absolute flow rate for flow-focus drop makers with different nozzle widths, as labeled; the nozzle widths are in units of microns. d represents the diameter of a spherical drop with the same volume than the squashed droplet formed in the device. The flow rate ratio is 0.5. Inset: droplet diameter divided by nozzle height as a function of the capillary number. The data for all drop makers collapse to .
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