Imaging a Nebulizer with a High-Speed Camera

Hank Hogan

Nebulizers aren't used just for dispensing drugs or treating respiratory diseases. They also are used in analytical instruments as a way to get the chemical components being studied — the analytes — into a form that makes measurement possible. Now, by employing a high-speed CMOS camera, scientists from Universidade Estadual de Campinas in Brazil have demonstrated an analytical method that could help any nebulization technique.

Despite being used for very diverse applications, all nebulizers contain a bulk liquid that holds the compound of interest — a drug or other analyte. Cohesive forces such as surface tension act to keep the liquid together, while the nebulizer works to break it apart. Using pressure, heat or mechanical means, the nebulizer disintegrates the liquid into droplets that can be dispersed into the air or other gas.

In a study of low-flow thermospray, investigators injected water droplets into a furnace and captured the result using a high-speed CMOS camera. Because the furnace was at 1000 °C, the camera and optics had to be cooled by a fan. TS-FF-AAS = thermospray flame furnace atomic absorption spectrometry. Images reprinted with permission of the American Chemical Society.

One of the simplest nebulizer setups, at least as far as its design is concerned, is used in thermospray flame furnace atomic absorption spectrometry. In this technique, a sample droplet is introduced into a heated tube, where it vaporizes and forms smaller droplets that are then ready for measurement of their absorption spectra. The method produces good results, especially in terms of sensitivity and ability to deal with the more volatile analytes.

However, despite the simplicity of the design, there are aspects in the operation of these devices that are not well understood. The researchers wanted to investigate these, particularly in the case where the sample flow was low, where, they believed, the thermospray process would be different from that in high-flow situations.

The problem they faced was that nebulization typically takes place very rapidly. Hence, they needed an equally fast method to capture what was going on. For this, they turned to a high-speed CMOS camera from Photron Ltd. of Tokyo. The camera provides speeds of up to 18,000 fps, and the company supplied software capable of acquiring data at these high frame rates.

In this sequence, captured at 6000 fps, a water droplet enters the furnace from the right (0), travels across the furnace (0.83 ms) and vanishes. The droplet survives because evaporation forms a protective skin of water vapor around it. After impacting on the far side, the droplet disintegrates into smaller droplets that then evaporate.

As described in the Sept. 1 issue of Analytical Chemistry, the researchers set up the camera a distance away from a metallic furnace, using Nikon zoom lenses to bring the interior of the open-ended cylindrical furnace into focus.

After heating the furnace to >1000 °C, they injected deionized water into it through a capillary with a 0.5-mm inside diameter. They measured the temperature with a thermometer and captured images ranging from 1000 to 18,000 fps. They also varied the water flow from 0.1 to 1.5 ml/min. They used the same flow parameters in thermospray flame furnace atomic absorption spectrometry of varying cadmium concentrations so that they could correlate what they observed to instrument performance.

When they examined the images, they found that the action of the low-flow thermospray was quite different from that of the reported high-flow case. Instead of a stream of continuous drops, at low-flow conditions the capillary produced one at a time — or a few drops at most. These drops survived entry into, and transit of, the 10-mm-diameter furnace. Upon impact into the opposite wall, the liquid broke up in a chaotic manner into a number of smaller droplets, which then rapidly evaporated.

The low flow of the setup ensured that nebulization was the result of thermal and not mechanical action. As for how the water survived the trip across the hot tube, the researchers attributed this to the Leidenfrost effect, the same mechanism that allows water to dance on a hot skillet: When a liquid comes into contact with a hot surface, vaporization produces a skin of gas that protects the liquid.

As for the thermospray formation itself, the investigators concluded that the carrier flow rate was the most important parameter.

They also noted that the same imaging technique could be used to study other nebulizers and processes. Laser-based analytical techniques, for example, could be followed almost in real time, noted associate professor of analytical chemistry Marco Aurélio Zezzi Arruda.

“Such studies and applications could result in better nebulizer designs as well as lead to an even better understanding of the transport phenomena,” he said.

Contact: Marco Aurélio Zezzi Arruda, Universidade Estadual de Campinas;
e-mail: zezzi@iqm.unicamp.br.