By adding magnetized particles to an oil-and-water mixture, a team of researchers, including Syracuse physicist Joseph Paulsen, have uncovered a new way to control liquid-liquid mixtures.

Key Takeaways

• Researchers discovered that adding magnetized particles to an oil-and-water mixture causes it to separate into patterns resembling a Grecian urn.
• The magnetized particles increase the surface tension at the interface of the liquids, promoting droplet merging. This discovery could influence the field of soft-matter physics and enhance energy optimization through more efficient chemical transport and processing.
• The novel findings, featured in the journal Nature Physics, highlight new avenues for controlling liquid-liquid mixtures.

The spices in salad dressing enable water and oil, which typically don't mix, to combine through emulsification. Researchers have now discovered that adding magnetized nanoparticles to an oil-water mixture produces a completely different effect.

From shaking a bottle of salad dressing to mixing a can of paint, we interact with emulsions daily. An emulsion is a blend of two liquids that typically don't mix, with a common example being oil and water. For a vast range of foods and other technologies, scientists have devised emulsifying agents which help stabilize mixtures. By incorporating small granular particles to certain foods, it can help prevent spoilage and extend shelf life, important for helping safeguard our food supply. When added to chemical mixtures, emulsifying agents can reduce viscosity, making liquids such as petroleum easier to pump and transport through pipelines, potentially leading to energy savings.

Researchers are continually investigating new emulsifiers to improve the control of liquid-liquid mixtures. Recently, Syracuse University physics professor Joseph Paulsen collaborated with scientists from the University of Massachusetts Amherst and Tufts University to make a surprising discovery. They found that when magnetized particles are added to a simple oil-and-water "salad dressing," the mixture consistently separates into patterns resembling the elegant curves of a Grecian urn immediately after being shaken. The team’s results, published in Nature Physics, uncover a novel method of using magnetic particles to control liquid-liquid mixtures.

The study, led by UMass Amherst, began when UMass graduate student Anthony Raykh was experimenting in the lab. He added magnetized nickel particles to a batch of “salad dressing” instead of spices, which are normally what allow the oil and water in dressing to remain mixed. He chose magnetized particles because fluids containing them can be engineered to exhibit unique and useful properties. After shaking his mixture, Raykh was astonished to see it consistently form a pristine urn shape. Regardless of how many times or how vigorously he shook the mixture, the urn shape always reappeared.

Watch the oil-and-water mixture form a Grecian urn shape.

To help explain this shocking phenomenon, the UMass team invited in Paulsen from Syracuse, along with colleagues from Tufts, to conduct theoretical analysis and simulations. Paulsen, whose research focuses on soft condensed matter, explores the ways in which materials like liquids and soft solids bend, deform and mix – research which lent itself well to this study.

Typically, particles added to an oil-and-water mixture, such as spices, decrease the tension at the interface between the two liquids, allowing them to mix. But in a twist, the team found that particles that are magnetized strongly enough actually increase the interfacial tension, bending the boundary between oil and water into a graceful curve.

“We turned the nature of particle-decorated interfaces on its head,” says Paulsen. “Now, you can have an emulsion droplet that you can imagine controlling in a variety of ways with a magnetic field, but the droplet will nevertheless coalesce with other droplets — something that particle-coated droplets typically resist.”

Figure a graphically depicts individual nanoparticles of magnetized nickel that form a boundary between the water and oil. Figure b shows how the magnetized particles cause the oil and water to separate into a pattern resembling a Grecian urn immediately after being shaken. (Credit: Anthony Raykh/UMass Amherst)

Their research on magnetic particles uncovered two surprising effects. First, the particles, being small magnets, form large networks with many holes due to magnetic interactions. These holes help droplets coated with the particles merge quickly into single oil and water portions. Second, the strong attraction between the magnetic particles increases the surface tension at the interface, further promoting droplet merging.

While there’s no application for this novel discovery yet, the team is excited to see how this never-before-seen state can influence the field of soft-matter physics.

“Liquid-liquid mixtures are ubiquitous in consumer products and industrial processes," notes Paulsen. "This discovery, which offers a new approach to managing these mixtures, could one day help produce better products with longer shelf lives or save energy in chemical transport and processing. I'm eager to see the future implications of this breakthrough.”

This research was funded by the US National Science Foundation and the US Department of Energy.

- Portions of this article have been adapted from a UMass Amherst press release.