Magnetic particles turn water droplets into tightrope-walking acrobats

Putting tiny magnetic particles inside ordinary water droplets can turn them into liquid acrobats – the droplets can climb steps, leap over obstacles and jump-start chemical reactions. This level of control could be useful in drug delivery or to make more complex lab-on-a-chip technologies.

Shilin Huang at Sun Yat-sen University in China and his colleagues made a surface with tiny grooves and covered it in a varnish that is superhydrophobic, or nearly impossible to wet. They knew water droplets sitting on top of such grooves can spontaneously jump up because of the pressure difference between a droplet’s bottom, which is deformed by the small channel, and its rounder and less restrained top.

The researchers wanted to create this pressure difference on demand. They added a tiny magnetic particle into each droplet and placed an electromagnet underneath the groove. When they turned on the electromagnet, it pulled the particle – and therefore some of the droplet – into the groove. When they switched it off, the droplet’s shape rebounded and it flew upwards as if flying from a slingshot.

With this technique, the team made liquid droplets hop up millimetre-scale stairs and over miniature obstacles. The researchers even steered a droplet into a narrow space between two wires, thus connecting a circuit and lighting a light bulb.

Xiao Yan at Chongqing University in China says this is a creative way to take control of pressure-based droplet jumping, and it could be a valuable tool for precisely transporting droplets of chemicals.

In one experiment, researchers caused a droplet to jump into and mix with a liquid chemical sample under a microscope lens, enabling them to watch the resulting chemical reaction from start to finish. In another, they made two droplets mix with a third inside a closed box, remotely starting a reaction that would have been ruined if a researcher had needed to open the box and let air in.

Such precise chemical control has applications for drug delivery. Huang hopes the technique will also advance “lab-on-a-chip” technologies, efforts to miniaturise complex biochemistry experiments that usually require lots of space and glassware. He proposes “lab-on-stacked-chips”, where droplets vertically jump between levels to allow many reactions to happen in parallel.

Topics:

• chemistry /
• fluid dynamics