Studying a Splat in the Making

Hank Hogan

The splat that droplets of plasma make when they strike a substrate is similar to the shape that pancake batter forms when it hits a hot griddle. In the past, scientists investigating plasmas have had only the splat to work with — a situation that has left them hungry for more direct observation.

Researchers used the monitoring system shown here to observe the splattering of very small and very fast plasma droplets. A torch (a) sprays individual plasma droplets onto a substrate (b), with the signals from the in-flight and impacting particles triggering a high-speed video camera that captures the impact image via a long-distance microscope. Images reprinted with permission of the American Institute of Physics.

Researchers from the National Institute for Materials Science in Tsukuba and from Kinki University in Osaka, both in Japan, have directly monitored plasma-sprayed yttria-stabilized zirconia droplets as they impacted a quartz substrate at room temperature.

The monitoring system developed by the group consisted of a high-speed video camera from Shimadzu Corp. of Kyoto, Japan; a long-distance microscope from Questar Corp. of New Hope, Pa.; and custom droplet-individualization techniques. The camera enabled the researchers to capture 312 × 260-pixel images at one million frames per second; the microscope enabled a working distance of 11.4 cm.

Direct observation

A custom-built spectrometer along with photomultiplier tubes from Hamamatsu Photonics KK created a signal of the particles in flight and upon impact. The latter event triggered the camera, a necessity because the droplets impacted the substrate at random places and intervals.

These thermal images, captured at a rate of one per microsecond, show a plasma-sprayed, hot yttria-stabilized zirconia droplet approaching (1-2) and impacting (3-6) with a splat on a smooth, cold quartz glass substrate. The center cooled substantially before the liquid jet stopped expanding. Each frame had a 500-ns exposure. Scale bars equal 200 μm in each direction.

Plasma spraying — used in some industries as a coating technology — involves several parameters that influence the process. Thus, although studying splats is informative, it is difficult to figure out from the splats alone which factors affect coating properties. Researchers need direct observation to fully understand the process.

The investigators sprayed yttria-stabilized zirconia droplets onto a quartz substrate that was kept at room temperature, capturing thermal images of single-droplet impacts under constant conditions. Each hot droplet splashed off the cooler substrate, with the liquid sheet jetting out sideways and spreading without disintegration until its fullest extent was reached. The researchers found also that the center cooled very rapidly, dropping below the sensitivity of the camera as the jet was still expanding.

Splats on horizon

Kentaro Shinoda, a postdoctoral researcher at the institute, said that he was really surprised by the fact that their system captured the evolution of the droplet flattening and cooling simultaneously, because the size of a droplet is only 50 μm and the velocity is up to 200 m/s. In such a case, the deformation time would be expected to be only a few microseconds. The rapid cooling of the center part as compared with the spreading rim was also interesting to note, he added.

Shinoda noted that the team’s plans call for changing various parameters and studying the resulting effects on droplets. He added that if the droplet-imaging technique were incorporated into a process-control scheme, industry also could benefit.

“If we can monitor the droplet impact directly during the coating formation, it will produce more reliable information on controlling coating properties,” he said.

Applied Physics Letters, May 7, 2007, Vol. 90, 194103.