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.
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.
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.
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.
Using this method and an illumination system that is faster than the one employed by the team, scientists would have the capability of measuring the volume, motion and contents of a spray or cloud of microdroplets, the researchers said. Such measurements could play a key role in future studies for epidemiological, environmental and industrial applications.
Photo: Microscopy of microdroplet volume and nanoplastic concentration. Credit: K. Dill/NIST