Star Wars Microfluidics

Researchers have found a way to move liquid droplets using the force…of light.

The new technique, dubbed ‘chemopropulsion’, could open up new vistas in drug delivery by using the droplets as ‘vehicles’ to transport therapeutics in the body. It could also lead to light-activated chemical sensors.

Droplets floating in water suddenly move when light, from a hand-held LED, is shone nearby. The droplets flee the light-source, moving from the light-side to the dark-side of the petri-dish, at speeds of up to 4 mm/s.

Professor David Officer, who co-led the research, says the work was inspired by biology. “A cell can move chemicals against a gradient, basically uphill,” he says. “The big challenge is to be able to do that remotely and in a controlled way. How do you move something from point A to point B without actually pumping it in one direction?”

The team solves this problem using some clever chemistry. The key was to load the droplet with a chemical ‘fuel’ that could be released by a light-activated trigger.

When the fuel is released, it lowers the surface tension of the water behind the droplet. Since there is now a surface tension difference between the front and the back of the droplet, the water in the front of the droplet can pull it forward.

The process works by the well-known Marangoni Effect, which also causes the ‘tears of wine’ phenomena in which wine creeps up the side of the glass causing droplets to fall back down the walls of the glass.

Though self-propelled droplets were invented by a group from Chicago in 2009, those droplets could only follow an acid gradient, meaning their movement was one-way only.  As the video shows, Officer’s team can reverse the motion of the droplet, something impossible with previous technology.

The work bubbled forth from a collaboration between the University of Wollongong and Dublin City University, in Ireland and the Microtechnologies Cooperative Research Center in Spain.

Prof Officer and his Irish collaborators have recently been awarded an ARC Discovery Project grant to further develop the technology.  They hope to load the droplet with chemicals to create a light-activated, remote-control ‘vehicle’ for chemical transport.  In future the same principles could be used to direct anti-cancer drugs to the site of a tumour in the body.

This research was published in the November 19 issue of Advanced Materials.