FILLING a cup from this tap would take 40,000 years, but luckily its raison d'être has nothing to do with quenching thirst. The trickle in question flows along a silicon chip and is the slowest ever recorded. Its detection should speed up the creation of the first fully electronic lab-on-a-chip.
Such devices are too small for fluids flowing through them to be visible, but measuring the flow rate of an extremely small sample of blood, say, can help detect traces of disease. It is possible to do this using lasers and fluorescent markers, but to interpret the results, the optical signals must be converted into electrical ones, which is cumbersome.
Klaus Mathwig of the University of Twente in Enschede, the Netherlands, and his colleagues wondered whether they could detect tiny flow rates using only electronics. They carved a tunnel, 100 micrometres long, 5 micrometres wide and just 130 nanometres high, in a silicon chip and placed electrodes at each end. Then they pumped through water spiked with electrochemically active molecules, which register a characteristic electrical signal as they flow past the electrodes. This allowed the researchers to measure the fluid's flow rate (Physical Review Letters, doi.org/jcs).
The slowest flow rate recorded was 10 picolitres per minute, a third as fast as the previous lowest flow of 30 picolitres per minute, which was measured optically. "This is the smallest flow reported," confirms team member Serge Lemay.