The standard method of mixing biological and chemical compounds in test tubes has been shaken up in recent years by the emergence of microfluidics. This innovative field combines principles from micro-nanotechnology, biochemistry, engineering and physics, to manipulate the behaviour of fluids at the micron scale, performing traditionally laborious lab work on chips as small as postage stamps.

Microfluidics is providing a new avenue for real-time, high-throughput testing for point-of care diagnostics, helping to identify toxins or dangerous pathogens from tiny sample sizes. Outside the lab, microfluidics is behind a variety of technologies, from inkjet printer heads to pregnancy tests. While more streamlined than conventional methods, limitations remain that hamper the efficiency of microfluidic-based experiments.

For emulsion-based samples used in microfluidics, each individual droplet represents an experiment. Analysis can be conducted on every droplet, thus any spoiled droplets reflect a failed experiment. By minimizing droplet collision and breakup, the throughput and efficiency of the microfluidic system can be increased.

Traffic circle reduces congestion in microfluidic chip

Congestion and collisions tend to emerge when droplets are funnelled from a wide channel into a narrow one (a hopper chamber), causing the droplets to break. “It’s a traffic problem, like several lanes of cars trying to squeeze through a tollbooth,” says Sindy Tang, from Stanford School of Engineering.

To overcome this limitation, Tang and her team leveraged a known phenomenon from the behaviour of rigid particles, whereby an upstream obstacle, or “traffic circle”, suppresses particle clogging. This surprising observation can be seen in grain silos or people evacuating a room, but such behaviour had not been validated in soft particles.

Tang and her team, led by former Stanford Engineering graduate student Alison Bick, report on the transferability of this phenomena in Proceedings of the National Academy of Sciences. In their experiments, they perfused water-in-oil drops through a 2D hopper chamber of constrained geometry, with obstacles of varied size and location embedded on the chip.

When an obstacle is optimally placed, droplets are first deformed between the obstacle and side wall, and then relax before constriction through the narrow chamber. “There’s a sweet spot in the placement of the obstacles that minimizes the reduction in breakups and collisions in the droplet flow,” Tang explains. An optimally located traffic circle reduced droplet breakup frequency by one thousand times, compared with a chip without the obstacle (see video above).

This dramatic improvement in experimental efficiency, throughput and robustness could reduce the time required to perform droplet-based microfluidic assays, including digital PCR tests and antibiotic screening. The researchers’ findings may also have implications further afield, for example, enabling faster flow rates while maintaining exit droplet size and uniformity in 3D printing of emulsions or foam-based materials.

The team concludes that by placing obstacles in a “sweet spot”, orderly droplet flow can be achieved. The elegance of this solution lies in its simplicity, making it convenient to implement in other microfluidic platforms.