Fast camera helps characterize droplets
An ultrahigh-speed digital imaging system is letting German scientists see tiny, fast-moving instruments in action, giving them information that may lead to improvements in these microdevices. Researchers in the Department of Measurement, Control, and Micro technology at the University of Ulm (Ulm, Germany) are working with an Imacon 468 camera (DRS Hadland; Tring, England) to visualize droplet formation in ink-jet printers.
The camera is mounted on a standard optical microscope, and the object being viewed is lighted with a high-pressure xenon lamp. The system channels light through optical relays onto an eight-sided mirror pyramid beamsplitter, which passes the light to up to eight intensified charge-coupled-device sensors arrayed in a circle. Gated imaged intensifiers in front of the sensors act as high-speed shutters to set an exposure time of 10 ns. Because eight image frames are produced through a single optical input, the system provides high resolution without any displacement of the images. Images can be combined into a movie.
"The system provides the user with total flexibility of exposure duration and interframe time," said Alan Needham, engineering manager at DRS Hadland. He said that is an improvement over the company`s previous system, which worked at fixed frame rates.
The variation comes in handy for imaging droplets, allowing the researcher to use a faster sequence of images as the droplet forms and then go to a slower sequence to follow the droplet`s trajectory.
In their experiment, the Ulm re searchers looked at an ink-jet printer in which the print head consisted of 50 thermal actuators where every heating element had an area of 60 p 60 µm. The heaters warm the ink to 320°C, causing explosive vaporization. The heated liquid column is ejected from the actuator`s nozzle and forms a droplet of ink and several satellite droplets (see photo).
The researchers found that, because of tiny thermal fluctuations, the structure of each bubble of ink is unique. They could, however, average a series of images and thereby characterize the quality and uniformity of the heater. An ideal heater, the scientists said, should have uniform heat distribution.
Needham said the system, which can take exposures as short as 10 ns and magnify up to 500 times, has a variety of scientific, industrial, and military applications. Ulm scientists, for instance, also used the system to analyze the motion of a 350-µm microturbine operating at