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SDA stands for Static Droplet Array. Uniform droplets are chopped and immobilized into a 2-D hydrodynamic traps array from a long flowing plug. The ratio of the length versus the width of the plug is typically larger than 100. A concentration gradient is established based on the molecular exchange after coalescences of a diluting plug with the static droplets. The concentration can be varied from drop to drop in the presence of a screening target, e.g. single cells, bacteria, or worms. The pictures above (left to right, top to bottom) show a concentration gradient of black dye in 60 32-nL droplets, an SEM image of the hydrodynamic traps, and leukemia cells in one of the droplets in SDA exposed to a chemical in a concentration gradient.
Concentration gradients
Concentration gradients
The concentration gradient was discovered in the SDA thanks to the use of long plugs to capture droplets in a surfactant-free oil flow. Without a surfactant in the oil, a second plug (clear) can coalescence with the immobilized droplets (blue) sequentially, leading to producing a dynamic concentration gradient in the moving plug along with a static gradient across the SDA. A video for droplet and gradient generation in an SDA is available on the Movie page.
Collective hydrodynamics of confined drops in microfluidic parking networks
Droplet analysis
Droplet analysis
Readout of a 60-droplet array with increasing concentrations of black dye: higher gray values indicate lower concentrations.
384 2-nL droplets array
384 2-nL droplets array
Multiplexing
Multiplexing
Using long plugs and the hydrodynamic droplet trapping technique, multiplexed droplet arrays can be produced in 24 independent channels, as the picture on the top shown. All the channels share one single outlet and aspiration force. 10 drops in each channel, thus a total number of 240 droplets can be tested in a single device. The flow channel is 200-µm wide. Confined by the size of the traps, the droplet volume is 30 nL.
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