New Ultrafast Imaging Technique Provides Insight into Combustion Processes

Details of combustion—the chemical reactions that take place when, for example, a flame is lit—are fleeting and, therefore, difficult to study. But scientists would like to better understand the complex processes that occur in those billionths of seconds, not only to make engines more efficient but also to shed light on how candle flames, cars, and airplanes produce gases and particles that are harmful to humans and the environment.

Now a team of scientists from Caltech's campus and JPL (which Caltech manages for NASA), and the University of Erlangen–Nuremberg, in Germany, have developed an ultrafast imaging technique, called femtosecond laser sheet-compressed ultrafast photography (fsLS-CUP), that can compile videos of those incredibly transient details.

Capturing 250 billion frames per second, the new technique is 20,000 times faster than conventional high-speed imaging cameras, and about 100 times faster than state-of-the-art imaging systems. It has already revealed some of the underlying dynamics involved in the formation of soot particles during combustion.

A paper highlighting the new technique appeared online on August 29 in the journal Light: Science & Applications.

How Does Soot Form?

Soot is a major contributor to global warming, thought to be number two behind carbon dioxide in terms of its impact on climate. Yet how exactly soot particles form is still an open question. While scientists have previously suggested that the particles might be built from polycyclic aromatic hydrocarbons (PAHs), a class of chemicals that are formed during the incomplete burning of coal, oil, gasoline and even charbroiled meat, the precise mechanism by which this happens has not been fully determined, leaving room for debate.

"With the movies we have been able to capture with our new technique, we now have a very clear indication that soot particles do indeed grow from PAHs," says Yogeshwar Nath Mishra, a co-lead author of the new paper and a Swedish Research Council-funded researcher in the Caltech Optical Imaging Laboratory. "We can see that the PAHs attach together and form nascent soot particles that further grow in the flame to form very large particles."

Introducing fsLS-CUP

The new technique involves shining a femtosecond laser—one that emits a single pulse of light for just a quadrillionth of a second—on a sample flame and pairing that with a modified version of a method called compressed ultrafast photography, which was developed in the lab of Lihong Wang, the Bren Professor of Medical Engineering and Electrical Engineering at Caltech, who is also an author of the new paper.

"We are thrilled to see a high-impact application of the world's fastest camera that captures events in a single shot without repetition," says Wang, who is also the Andrew and Peggy Cherng Medical Engineering Leadership Chair and executive officer for medical engineering at Caltech. "This technology opens new doors across both biological and physical sciences, with applications ranging from microscopy to telescopy, and holds the potential for significant societal impact in understanding the underlying biology and physics."

Without disturbing the flow of the sample flame, the single femtosecond laser in fsLS-CUP accomplishes a couple of things. First, its wavelength is selected such that it excites the PAHs present, allowing their fluorescence to be detected. And second, it causes the soot nanoparticles present to rapidly heat up. As they relax back to normal temperatures, the particles emit a signal of incandescence. The fluorescence and incandescence can then be used to study the lifespan of the PAHs and soot particles created through combustion.

Still, both the fluorescence of PAHs and the laser-induced incandescence of soot particles have extremely short lifespans, lasting only nanoseconds to thousands of nanoseconds. To measure such fleeting signals in real time, the new technique uses something called a light sheet to define a two-dimensional plane within the flame and then employs a special imaging system offset by 90 degrees to capture what takes place within that plane. This imaging system includes spatial encoding by a digital micro-mirror device and temporal shearing by a streak camera. Using a special algorithm, the system can extract the ultrafast sequence of frames.

A Single Pulse is Better Than Many

Peng Wang, a co-lead author of the paper and former Caltech postdoc who completed the work at the Institute, notes that previous imaging techniques used to study combustion dynamics relied on many laser pulses, repeatedly exciting PAHs and soot particles and then capturing individual frames. "But these nanoparticles are very delicate," he says. "So, if you are exciting them continuously, you are heating them up and changing their optical properties. What we needed was a technology based on a single pulse. The single pulse induces the signal, and then the camera captures the entire decay of the signal. This is what we accomplished."

In one video captured at 1.25 billion frames per second by the new technique, a green signal representing fluorescence appears, indicating the formation of PAH molecules in a flame. The green signal fades away within 35 nanoseconds. A short while later, a cloud of red soot particles forms and grows over hundreds of nanoseconds.

"We can watch the PAH molecules form before the cloud of soot particles grows," Mishra says. Soot inception remains a highly debated topic within the combustion community, and our efforts mark just the beginning of unraveling the complexities of soot formation. To fully understand the species captured by fsLS-CUP, we need complementary optical methods for precise quantification as well as more advanced burner setups."

Many Applications

The authors note that the work is relevant beyond combustion science. "Ultimately, fsLS-CUP has not only advanced our understanding of hydrocarbon and nanoparticle formation and growth in flames, but it also holds potential across multiple fields, making it a significant milestone in ultrafast imaging technology," Mishra says. "This technique, which can capture some extremely fast phenomena in nature, has broad applications in physics, chemistry, biology, medicine, energy, and environmental science."

It is also relevant to astrophysical studies, the researchers say, given that PAHs make up some 10 to 12 percent of interstellar matter. "PAHs are robust molecules in interstellar space," says Murthy S. Gudipati, a senior research scientist at JPL and an author of the new paper. "Understanding the formation of PAHs and carbon soot expands our knowledge about their existence under astrophysical conditions as well."

Florian J. Bauer of the Friedrich-Alexander University Erlangen is also a co-lead author of the paper, "Single-pulse ultrafast real-time simultaneous planar imaging of femtosecond laser-nanoparticle dynamics in flames." The work was supported by funding from the NASA Solar System Workings program and JPL Researchers On Campus (JROC), as well as by the Swedish Research Council. Lihong Wang is an affiliated faculty member with the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech.