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.
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