Ultrasound creates 2D arrays of droplets

Microscope image of A 2D array of droplets created by ultrasound
Rows and columns: 2D array of droplets created by ultrasound

Liquid droplets have been arranged into 2D arrays by researchers at the University of Bristol in the UK. The droplets contain entangled polymers and are created within a tank of water. A range of different chemicals can be added to the droplets, which could be used to create high-throughput analyses systems for developing new drugs or performing rapid medical diagnostics. The droplet arrays could even be used to study how living cells communicate with each other.

Although arrays of liquid droplets have been made before, previous attempts had involved either using oil-and-water mixtures or evaporating the liquid to create the array on a dry surface. Neither technique is also suitable for supporting water-based chemical reactions, which was a primary goal of Bruce Drinkwater and his team of physicists, engineers and chemists, who developed the new technology.

Coacervation and coalescence

The droplet-forming process begins with an aqueous solution of the polymer PDDA and the biomolecule adenosine triphosphate (ATP). Electrostatic interactions cause these two materials to agglomerate into tiny nanometre-sized droplets by a process of “coacervation”. When a 2D ultrasound standing wave is created in the liquid using piezoelectric transducers, the droplets move to the nodes of the standing wave, where they coalesce and grow until they reach about 50–100 μm in diameter.

The result is a square lattice of identical droplets (see image above). “The uniformity of the droplets is amazing,” says Drinkwater. “I’m convinced this technology will have many applications in the next generation of lab-on-a-chip applications.”

By adjusting the ultrasound signals, the team was able to transform a column of droplets into a solid line of PDDA/ATP and then back again into a column of droplets. The researchers could control the size of the droplets and the spacing between them. They also showed that it is possible to load the droplets with a wide range of substances including proteins, enzymes, DNA and even micron-sized solid particles.

Localized chemistry

In one set of experiments, the team introduced a dye to one side of the tank and watched as the chemical diffuses across the array to create a concentration gradient. Such experiments could be used, for example, to study the effects of different concentrations of a chemical on the contents of the droplets. The team also showed that when several different additives were introduced to different locations of the array, the substances tended to remain localized within a region of droplets. This could be used to create arrays in which different droplets contain different chemicals.

Drinkwater told physicsworld.com that the team is now looking at how to create 3D lattices of droplets using ultrasound. He also says that it is working on making the ultrasonic components of the system more robust to the chemicals used – something that must be done before the system can be commercialized. The team is also looking at how arrays of droplets could be used to simulate how living cells communicate to each other using chemicals. This would involve making the droplets more complicated by creating structures that are analogues to those found in living cells.

The research is described in Nature Communications.

  • Hamish Johnston spoke to Bruce Drinkwater about the physics of ultrasound. You can listen to that conversations and watch a video of an acoustic “tractor beam” here.

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