One of the benefits of droplet-based microfluidics is the incredible frequency with which the droplets flow through the channels. However, this also presents one of the greatest challenges, because each droplet is available in only a tiny time window for detection. To image droplet motion, high-speed cameras capable of 10-100 kHz image capture rates are required. These are the same cameras used to image bullets fired from a gun -- and they cost as much as a sports car!
Thankfully, droplets rarely need to be imaged, particularly when performing biological assays with them. Instead, in most assays all that's necessary is to measure the fluorescence intensity of dye solutions contained within the drops. This is a much simpler challenge, and one achievable using off-the-shelf photodetectors. The optical setup used to measure the fluorescence intensity of dye solutions consists of an emitter-detector pair. The emitter is usually a bright LED or focused laser beam, and the detector is a photodiode or photomultiplier tube, as illustrated in the above schematic. The drops are flowed through the focused light spot using a microfluidic device, as shown in this movie:
As the drops pass through the laser, their fluorescent dyes are excited, emitting light that is captured by the photodetector. Viewed this way, droplets appear as a bright pulse of light as a function of time, in which the pulse width is proportional to the drop size, and the pulse amplitude is proportional to the dye concentration. This allows the drop properties to be characterized using a simple, fast optical measurement. One limitation to such a detection scheme, however, is that it is intrinsically serial: each drop is scanned one at a time, one after the other. Even for relatively high droplet rates, when a large population of drops must be scanned, this can take a long time.
A way to increase the droplet detection rate is to simply parallelize the emitter-detector pairs and the microfluidic channels. However, this can be expensive and difficult, because it requires one pair per channel, and the entire optical array must be aligned with the fluidic channels. An easier way to achieve the same effect is to fabricate the optical components directly onto the microfluidic device. This can be achieved using optical lenses called zone plates, which are Fresnel lenses that work on the principle of diffraction, rather than refraction. The diffractive properties of the lenses make them easy to fabricate using the same techniques by which the microchannels are fabricated, allowing them to be built at the same time. This also makes the fabrication cost effective, for creating large arrays for massively-parallel droplet detection. Droplets imaged with such arrays appear as pulses of light as the drops pass through the focal points of the different lenses, as shown in this movie of 64 lenses monitoring drops in 64 separate channels,:
The close proximity of the lenses to the channels allows them to collect light from a wide cone-angle, for high collecting power, making the lenses very sensitive.