Schlieren imaging enables us to visualize index of refraction gradients in transparent media. Changes in index of refraction deflect rays of light, some of which are obscured by a knife edge, generating contrast in the image. The history of the technique, dating back to the 17th century, is described wonderfully by Settles in his text on the topic,1 detailing the cast of scientists who had a role in understanding, implementing, and using schlieren imaging throughout the last 300 year. Toepler is credited with conceiving the first practical schlieren set up, consisting of a light source, an adjustable knife edge, and a telescope for viewing the image.

Here, we use a schlieren system not very different from Toepler’s design, but we also incorporate a high-repetition-rate camera to acquire images at up to 10 000 frames/s. We use our system to visualize the routine life of a strike-anywhere match. We strike a match, wait for the flame to stabilize, and immediately attempt to blow out the flame. The entire sequence of events happens in about 2 seconds.

In the beginning of the video, a match is struck against its box, generating heat via friction, prompting reactions between chemicals in the match head (Figure 1). As these chemicals react, they produce hot gases that jet out and away from the match. The reactions progress until eventually the match is lit, and the original small scales of the jetting gases give way to the larger scales in the density driven flow of hot combustion products.

To blow out the flame, our character enters from the right (Figure 2). We see the expired breath traverse the distance from his mouth to the match, and when the jet reaches the flame, it erupts in the smaller scales of the jet-driven flow. As we watch, we see the jet of exhaled air fails to mix enough cool air into the flame to extinguish the persisting reactions, and the match eventually reignites.

This work was supported by the Air Force Office of Scientific Research with Dr. Chiping Li as technical monitor. V. A. Miller was supported by the Claudia and William Coleman Foundation Stanford Graduate Fellowship.

Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transparent Media
Springer Science & Business Media
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