Shedding new light on oil slick rainbows and other thin film physics

When sunlight reflects off an oily puddle in a parking lot, it creates a swirling rainbow of colors. This is because of the thin film interference principle, which explains how light reflects off different layers, or films, in a mixture of liquids.

For David Hoffman, a research associate at the Department of Energy’s SLAC National Accelerator Laboratory, however, what happens in these mixtures is much more than pretty colors. The fine interfaces that produce these colors have major implications in biology, chemistry, and the petroleum and pharmaceutical industries, among others. A key application is chemical purification, where the separation of chemicals at their interfaces is central to the process.

Yet mastering the fundamental physics of thin interfaces like those of an oil puddle or drug interactions has been a challenge. That’s because researchers essentially need to see through stacks and stacks of film to get to the exact point where two dissimilar liquids, such as oil and water, meet.

Now, Hoffman and his colleagues may have found a solution: By throwing jets of water and oil at each other, they’ve created liquid layers just a few hundred atoms thick. As a result, they could see more clearly what was happening at the interfaces where liquids interact. They recently reported their findings in Langmuir.

“Our method will allow a bunch of new spectroscopic measurements on these interfaces, which are practically not possible with current techniques,” Hoffman said. These spectroscopic measurements allow scientists to track individual colors of light in detail that provides insight into the fundamental structures of molecules. These techniques can also allow them to observe chemical reactions occurring in real time as materials interact.

Chemical dressing

For many situations in biology and chemistry, interactions between two things occur at boundaries, or interfaces, where the two meet. Before a virus enters a cell, for example, its outer envelope must fuse with the cell membrane. A more everyday example occurs in unmixed salad dressing – there the oil and water molecules only interact at the boundaries between the pockets of the two liquids.

“The interface is where all the action for chemistry and biology takes place at the molecular level,” said Jake Koralek, lead author of the paper and researcher at SLAC.

Liquid interfaces, the focus of this study, can determine key variables like reaction rates or degrees of mixing, but the interfaces themselves are only a few atoms thick. This meant that it had been nearly impossible to study interfaces due to the large number of molecules that exist on either side of the interface between two liquids – essentially this sum of molecules was drowning out signals from the interface itself. and created a lot of noise in the experimental data.

Hoffman and his team reduced noise by reducing the mass of liquids to just a few nanometers thick. To do this, they shot jets of water and oil from tiny “microfluidic” nozzles at speeds of 1 to 10 meters per second and splashed them together to form a thin layer of liquid. Then they shined infrared light on the sheet and studied the spectrum of light passing through it – a technique called infrared spectroscopy – to understand what was happening at the interface. Next, they hope to use SLAC’s Linac Coherent Light Source (LCLS) to perform X-ray spectroscopy, which will provide insight into the chemistry itself.

“What this work demonstrates is that we can actually create very smooth, flat interfaces that cover these entire sheets of liquid, which is a perfect target for spectroscopy,” Koralek said.

Rainbow Physics

For their first tests using the microfluidic nozzles, the team fired only water – no oil – to test the feasibility of spectroscopy on a sheet of flowing liquid. By using water, the team was able to compare their results to the well-known properties of water interactions with light.

Still, they didn’t know how the experiment would work with multiple liquids, so for the central experiment the team fired three jets with two for oil and one for water, and vice versa. The flow rates were calibrated in such a way that they met as soon as they left the nozzles to spread out to form a thin layer of liquids.

“We originally thought that liquids were probably separated laterally, so one side could be a fluid and one side the other. But actually when we did infrared spectroscopy, which reveals which molecules are are found in every part of the sheet, we found to our surprise that in all cases one fluid is fully enclosed within the other fluid,” Koralek said. “It’s still a body of water completely surrounded by ‘an oil slick, or vice versa.

The team could also clearly see an undulating pattern of rainbow colors, demonstrating that they had not created an emulsion of oil and water, but rather distinctly separated layers.

“Because we see this pattern of lighter and darker colors, we know we’re not making an emulsion, but actually making these layers, which directly contribute to the rainbow patterns in our images. The pretty images are an essential part of science,” Hoffman said.

Now that they’ve established that the method works, the team has already started helping other scientists integrate microfluidic methods with multiple liquids into their own experiments. The devices researchers need to implement these methods are inexpensive and small – they can fit in the palm of your hand – and only take a few hours to set up in x-ray experiments and electron microscopes, which facilitates the study for more researchers. liquid interfaces, Koralek said. This could advance research in areas such as battery electrochemistry, he said, to improve how much energy can be stored and for how long, a crucial step on the way to a net zero carbon economy.