A droplet of water held between to glass slides

A water droplet (light bulge, centre) can steadily ease itself along the channel created by two parallel glass cover slips clamped at one end. Credit: Finn Box

Fluid dynamics

Micro-droplets in minuscule channels need no help to get ahead

Channel design allows fluid droplets to self-propel in a controlled manner.

Liquid droplets placed in channels with flexible walls can whisk themselves through tiny ducts — without any help from external forces.

In ‘microfluidic’ devices, minute volumes of liquid course through microscopic channels, allowing researchers to analyse samples of one-millionth of a litre or even less. But the dominant influence of surface forces can make it tricky to control the movement of fluid in these devices.

Dominic Vella and his colleagues at the University of Oxford, UK, clamped two parallel glass coverslips at one end, leaving the other end open. When a drop of water was placed in this channel, the glass walls bulged outwards, creating a pressure difference that gradually pushed the droplet through the duct (as shown below).

A silicon oil droplet moving along between two glass slides

Credit: Alex Bradley/Univ. Oxford

In previously designed microfluidic devices that use a similar form of self-propulsion, a droplet of oily liquid moves through a channel in the opposite direction from a droplet of water-soluble liquid. But in the current design, the bending of the walls ensures that both oily and water-based substances move in the same direction, potentially increasing the technology’s practical uses.

This new mechanism of motion — which the authors dub ‘bendotaxis’ — could be used to create self-cleaning surfaces and pipes that clear themselves.