It's one of the hardest problems in fluid mechanics: If you have some liquid in a tube, how do you measure how fast it’s moving (or "flow rate")? It’s not easy to find an answer.
Perhaps you could put some sort of turbine in the tube, and the faster the liquid flows, the faster the turbine will spin. That works great for really big tubes, but what about when the tubes in question are very tiny? In some cases, like drug delivery in medicine, doctors or researchers need flow rates on the scale of nanoliters using tubes that are thinner than a human hair. There’s no way to put a turbine into a tube that small.
Scientists at the National Institute of Standards and Technology have for doing just that, called a microflow measurement system. Instead of a turbine, their system uses a series of lasers and fluorescent molecules to measure flow rates.
The end result is a device that can accurately measure how fast liquid is flowing through a tube, regardless of how small that tube is. This would benefit plenty of medical researchers, but it could also be used to research cell metabolism.
The system works by detecting fluorescence, and fluorescence can be tied to any number of different processes in a cell. For instance, researchers could fill cells with fluorescent molecules and send those cells through the tube, and use a bunch of detectors in a row to measure how quickly they get rid of those fluorescent molecules.
Such studies could prove invaluable when studying cancer cells, which are known to have faster metabolisms than normal cells. Perhaps a detector like this one could be used to spot cancer cells early in a patient, by picking up on their faster metabolism.