Manufacturing Bits: Dec. 28

Measuring microdroplets
The National Institute of Standards and Technology (NIST) has found a new way for microscopes to measure the volumes of microdroplets.

Using this technique, NIST has measured the volume of individual droplets smaller than 100 trillionths of a liter with an uncertainty of less than 1%. That represents a tenfold improvement compared to previous measurements, according to NIST. For this, researchers combined traditional microscopy techniques with a new method call gravimetry, which is the measurement of the strength of a gravitational field.

Microscopy of microdroplet volume and nanoplastic concentration. Credit: K. Dill/NIST.

Measuring the volume, motion and contents of microscopic droplets are an important area of research for many fields. Microscopic droplets have become a major topic of research during the Covid-19 pandemic. For example, if two people are talking loudly in a setting, they can emit thousands of oral fluid droplets per second, according to the National Institutes of Health and the University of Pennsylvania. Using a light scattering method, each droplet measured from 10μm to 100μm in size. A fraction of these droplets remained airborne after at least 30 seconds, according to researchers.

Sneezing or coughing can create even more droplets. All of this can carry airborne viruses, which can quickly spread in an environment.

Measuring microdroplets is important in other fields, such as the impact of rain clouds and ink jet printers. Droplets play a role in how clouds reflect sunlight to cool the Earth. They also determine how ink jet printers create patterns. In addition, the study of microdroplets could be important for various laboratory and fab processes.

Generally, in many settings, optical microscopes are used to measure microdroplets, but these instruments have some limitations in some but not all cases. Microscopes can image the positions and dimensions of the droplets, which in turn can “determine the volume — proportional to the diameter cubed — of spherical microdroplets,” according to NIST. “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.”

In response, NIST researchers developed new calibrations for the instrument. They also measured the volume of microdroplets in flight using an independent technique called gravimetry.

Gravimetry measures volume by weighing the microdroplets that accumulate in a container. “Gravimetry typically lacks the resolution to measure single microdroplets, whereas microscopy is often inaccurate beyond the resolution limit. To address these issues, we advance and integrate these complementary methods, introducing simultaneous measurements of the same microdroplets, comprehensive calibrations that are independently traceable to the International System of Units (SI), and Monte-Carlo evaluations of volumetric uncertainty,” said Lindsay C. C. Elliott, a researcher at NIST, in Analytical Chemistry, a technology journal. Others contributed to the research.

In an experiment, NIST took an inkjet printer. The system shot a jet of microdroplets of a viscous alcohol into a container a few centimeters away. Researchers controlled the jet to produce a known number of droplets. As the jet of microdroplets flew from the printer into a container a few centimeters away, they were backlit and imaged with the optical microscope.

The researchers then weighed the container and its accumulation of many microdroplets. “We achieve sub-picoliter agreement of measurements of microdroplets in flight with volumes of approximately 70 pL, with ensemble gravimetry and optical microscopy both yielding 95% coverage intervals of ±0.6 pL, or relative uncertainties of ±0.9%, and root-mean-square deviations of mean values between the two methods of 0.2 pL or 0.3%. These uncertainties match previous gravimetry results and improve upon previous microscopy results by an order of magnitude,” Elliott said.

“Gravimetry precision depends on the continuity of droplet formation, whereas microscopy accuracy requires that optical diffraction from an edge reference matches that from a microdroplet. Applying our microscopy method, we jet and image water microdroplets suspending fluorescent nanoplastics, count nanoplastic particles after deposition and evaporation, and transfer volumetric traceability to the number concentrations of single microdroplets,” Elliott said. “We expect that our methods will impact diverse fields involving dimensional metrology and volumetric analysis of microdroplets, including inkjet microfabrication, disease transmission, and industrial sprays.”

Nanowire-based electron microscopes
The National Institute for Materials Science (NIMS) and JEOL have developed a high-resolution transmission electron microscope (TEM) using a nanowire-based field emission gun.

The TEM performs atomic resolution observations at an energy resolution of 0.2eV—the highest resolution ever recorded for non-monochromatic electron guns—with a high current stability of 0.4%, according to NIMS and JEOL.

Found in a lab or fab, a TEM is a large system used for dimensional metrology. This involves measuring the length, height and width of a given sample at the nanoscale. TEMs are also used to explore the channel materials and other portions of a chip design.

In operation, a TEM generates electrons and sends them through a sample. The electrons interact with the sample, which provide information about the structure at the nanoscale. However, a TEM is also a destructive technique. A sample is created by cutting part of a given structure. Researchers would prefer not to cut a structure in production. That’s why TEMs are found in the lab, but they also are used in the fab.

In the system, TEMs generate electrons using a field emission gun, which are based on tungsten needle materials, according to NIMS and JEOL. For years, the industry has been trying to develop field emission guns using higher performance materials. But the industry has run into several challenges here.

Now, NIMS and JEOL have developed a lanthanum hexaboride (LaB6) nanowire-based field emission gun for high-resolution TEMs. The nanowire-based electron source has a number of advantages. It has high current stability, low extraction voltage, narrow electron beam energy distribution width and high brightness, according to researchers.

To develop this source, researchers have synthesized and grown single-crystal nanowires. They also designed an electron source capable of efficiently emitting electrons.

“After installation in an aberration-corrected transmission electron microscope, we characterized the performance of the electron source in a vacuum of 10−8 Pa and achieved atomic resolution in both broad-beam and probe-forming modes at 60 kV beam energy. The natural, un-monochromated 0.20 eV electron energy loss spectroscopy resolution, 20% probe-forming efficiency and 0.4% probe current peak-to-peak noise ratio paired with modest vacuum requirements make the LaB6 nanowire-based electron source an attractive alternative to the standard W-based sources for low-cost electron beam instruments,” said Han Zhang, a researcher at NIMS, and lead author of a paper about the topic in Nature Nanotechnology.