Researchers have created the fluid analogue of an electrostatic quadrupole. The discovery, reported this week in Nature Communications, may find use in cell biology as a means to culture stem cells.

Quadrupoles have many engineering applications, most notably as magnets used to focus beams of charged particles in particle accelerators. However, experimental applications of fluid quadrupoles have not been reported until now. The microfluidic quadrupole demonstrated by David Junker and colleagues consists of flowing liquids rather than magnetic fields. A stagnation point at the centre of the quadrupole can be readily adjusted in both size and position from which a concentration gradient can form, allowing solutions to move between poles. This could provide a platform for cultivating cell cultures that are normally hard to breed inside closed channels, such as stem cells.