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Improved Accuracy in Measuring Microdroplets

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Improved Accuracy in Measuring Microdroplets
Microdroplet Microscopy

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

Scientists boost the Gravimetry Illustration

Gravimetry, microscopy and deposition of microdroplets. Credit: K. Dill/NIST

Nonetheless, weighing the contents of a container is a tried-and-true method, and gravimetric measurements are readily linked to the International System of Units (SI) with high confidence. Such measurements are the most reliable because the units are based on fundamental constants of nature, which do not change over time. Therefore, the team used gravimetry to check the reliability of microscopy in determining droplet dimensions.

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 consisted of plastic spheres with calibrated diameters, which produce images in the microscope very similar to those of microdroplets. Indeed, the scientists found that when they used the plastic spheres to calibrate their measurements of image boundaries, the microdroplet volume derived from microscopy precisely matched that from gravimetry. (The researchers found that the knife edges resulted in a poorer match.) The scientists also calibrated several other aspects of the optical microscope, including focus and distortion, maintaining the links to the SI throughout.

With these improvements, optical microscopy resolved the volume of microdroplets to one trillionth of a liter. The standards and calibrations are practical and can be applied to many types of optical microscopes employed in basic and applied research, the researchers noted. In fact, the less advanced the microscope optics, the more a microscopy measurement can benefit from standards and calibrations to improve the accuracy of image analysis.

The NIST researchers, in collaboration with the University of Maryland in College Park, reported the findings on December 20, 2021, online in Analytical Chemistry.

In their main experiment, the researchers used a printer to shoot a jet of microdroplets of cyclopentanol, a viscous alcohol that evaporates slowly. They precisely controlled the jet to produce a known number of microdroplets. 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.

With the optical microscope calibrated and checked by comparing it with the gravimetry method, the team embarked on another experiment, replacing the cyclopentanol with water microdroplets containing nanoparticles of DOI: 10.1021/acs.analchem.1c02640

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