Microscopy of microdroplet volume and nanoplastic concentration.
Credit: K. Dill/NIST
Sneezes, rain clouds, and ink jet printers: They all produce or contain liquid droplets so tiny it would take several billion of them to fill a liter bottle.
Measuring the volume, motion and contents of microscopic droplets is important for studying how airborne viruses spread (including those that cause COVID-19), how clouds reflect sunlight to cool the Earth, how ink jet printers create finely detailed patterns, and even how a soda bottle fragments into nanoscale plastic particles that pollute the oceans.
By improving the calibration of a conventional optical microscope, researchers at the National Institute of Standards and Technology (NIST) have for the first time measured the volume of individual droplets smaller than 100 trillionths of a liter with an uncertainty of less than 1%. That is a tenfold improvement over previous measurements.
Because optical microscopes can directly image the positions and dimensions of small objects, their measurements can be used to determine the volume — proportional to the diameter cubed — of spherical microdroplets. However, the accuracy of optical microscopy is limited by many factors, such as how well the image analysis can locate the boundary between the edge of a droplet and the surrounding space.
To improve the accuracy of optical microscopes, NIST researchers developed new standards and calibrations for the instruments. They also devised a system in which they could simultaneously measure the volume of microdroplets in flight using microscopy and an independent technique, known as gravimetry.
Gravimetry measures volume by weighing the total mass of many microdroplets that accumulate in a container. If the number of droplets is controlled and the density — mass per unit volume — is measured, then the total mass registered on a scale ..
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