Tech Briefs Magazine - March 2022 - PIT-21

NIST Improves the Ability of Optical Microscopes to Measure the
Volume of Microdroplets
National Institute of Standards and Technology, Gaithersburg, MD
S
neezes, 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 can be used to calculate
the average volume of one droplet.
Although this is valuable information,
because droplets can vary in size, imagPhotonics
& Imaging Technology, March 2022
Microscopy of microdroplet volume and nanoplastic concentration. (Image: K. Dill/NIST)
ing single droplets by optical microscopy
enables a more direct and complete
measurement.
To improve the accuracy of locating
the microdroplet edges, the researchers
tested two standard objects to mimic a
microdroplet and calibrate the image
boundaries. For each standard object, a
precisely and accurately measured distance
between its edges allows calibration
of the corresponding image boundaries.
The first standard object consisted
of sharp metal edges separated by a
calibrated distance to represent the diameter
of a microdroplet. Such " knife
edges, " which assume a flat boundary
between the edge of a microdroplet and
surrounding space, are commonly used
to test optical systems but bear only a
passing resemblance to microdroplets.
The other standard object consists
of plastic spheres with calibrated diam21
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Tech Briefs Magazine - March 2022

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