Investigate phase separation in liquids at the microscopic scale when adding energy

oil and water

Credit: Pixabay/CC0 Public domain

A team of physicists from the University of California, Santa Barbara have discovered some of the dynamics involved with active liquid surfaces at the microscopic level when energy is added. In their article published in the journal Science, the group describes the use of high-speed cameras to capture the action as they activated mixed liquids by adding a power source. Jérémie Palacci of the Austrian Institute of Science and Technology published a Perspective article in the same journal issue highlighting the importance of their work.

As Palacci notes, when certain liquids are mixed, such as alcohol and fruit juice, they stay mixed. And when others are mixed, like oil and vinegar, they quickly separate until some kind of energy source (like shaking) forces them back together. In this new effort, the researchers noted that little work has been done on the properties of liquids as they mix (or unmix) at the micro level and set themselves the task of learning more.

The job involved placing two types of liquids together and studying how they behaved when forced together. Specifically, they combined poly(ethylene glycol) also known as PEG, with dextran, which is an extract of sucrose. When mixed under typical conditions, the two separate, but not like oil and vinegar: instead, droplets of PEG are formed that hover or float in the dextran. The researchers chose the two liquids because they both have very low surface tensions, making it easier to study how they react to each other. When experimenting with the two liquids, the researchers also sometimes added kinesin, a protein that helped the two liquids bond together.

Credit: Science (2022). DOI: 10.1126/science.abo5423

To learn more about the behavior of the two liquids when an energy source was added (agitation), the researchers placed them in microtubules, which allowed them to see more clearly what was happening.

The researchers observed that the tremors created chaotic flows. In the mixtures without added kinesin, the droplets formed slowly – adding kinesin accelerated the process. And when they added a lot of kinesin, the droplets became animated, continuously merging and then separating again. The researchers noted that at the interface between liquids, they could see undulating waves without the need for a microscope. They also found that the liquid mixture sometimes became so lively that it scaled the walls of the microtubules a little.

The researchers suggest that there are characteristics of combined fluids that are still unknown and that continued study could lead to a better understanding of fluid applications.

Research makes the physics of glass formation clearer

More information:
Raymond Adkins et al, Dynamics of active liquid interfaces, Science (2022). DOI: 10.1126/science.abo5423

Jérémie Palacci, A soft active ingredient that can climb walls, Science (2022). DOI: 10.1126/science.adc9202

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