Energetic Properties of Colloids in Spinning Magnetic Field

If a merry-go-round rotates very fast, the riders will be thrown off in all directions. However, at Rice University lab, spinning particles do just the opposite.

Experiments in the Rice lab of chemical engineer Sibani Lisa Biswal, reveal micron-sized spheres coming together under the impact of a speedily spinning magnetic field. That’s not novel as the particles themselves are magnetized.

But how they come together is of importance - the particles first collect into a disorganized aggregated cluster and then into a crystal-like regimen as the magnetic field gains more strength.

Results of the research, led by Biswal and graduate student Elaa Hilou, have been published in Physical Review Materials. The researchers believe it will inspire ways to study, model and create novel 2D materials like colloids or tunable catalysts that can alter their surface area on demand.

Experiments exposed shapes, boundaries, phase transitions and the creation and resolution of crystal-like defects as between 300 and 1,500 magnetized spheres followed their energetic impulses under the moving field’s impact.

“I have been presenting this as a miniaturized version of a fidget spinner where we use the magnetic field to generate an isotropic interaction around the particles,” Biswal said. “We can create particle ensembles that are loosely to very tightly packed by the strength of that interaction.”

That fascinated Biswal and Hilou, but not as much as what they observed happening near the edges, where line tension formed by the outermost particles defined the ultimate shape of the arrays.

Think about a soap bubble. It always forms a sphere, even when you try to deform it. That’s because surface tension wants to minimize its surface area. It’s the same for our system but in two dimensions. The interactions are always trying to minimize what we call the line tension.

Elaa finds the Gibbs interface and measures the energy at that interface where it goes from many particles thick (at low magnetic field strengths) to nearly a single particle thick by changing the strength of the interaction. She’s done a lot of analysis of the line tension and how it relates to the energetics of the system.

Sibani Lisa Biswal

The subsequent step is to develop physical, movable models for real systems to observe how the constituents react when disturbed.

There’s a lot of interest in trying to create models for atomic and molecular systems. Most of that has been done through computational simulations, but here we have an experimental system that can realize structure and processes such as coalescence.

Sibani Lisa Biswal

For example, in catalysis, if you want to increase the surface area, you want more voids to facilitate contact between a catalyst and a reaction. By increasing the concentration and controlling the field, we can start to see voids and control the interface relative to the bulk.

Elaa Hilou

The method could model emulsions, she said. “Say you have oil and water, and you want to phase-separate them,” Hilou said. “In the case of cosmetics and the food industry, you want the emulsions to be stable. We want to be able to mimic their dynamics by controlling particle size and the field strength.”

Biswal stated that the technique might also be used to model systems in which temperature, instead of electromagnetism, is the driver. In fields like metallurgy, defects are eliminated “by turning up the temperature to give molecules more freedom to move grain boundaries and voids,” she said. “Then they decrease the temperature to lock in the structures."

“What we have is a dial that not only mimics the effects of temperature with a magnetic field but also offers the ability to watch through a microscope what happens in an actual system,” Biswal said.

Rice graduate alumnus Di Du, currently a statistical research analyst at the University of Texas MD Anderson Cancer Center, and graduate student Steve Kuei are the paper’s co-authors. The National Science Foundation supported the research.

Particles rotated in a spinning magnetic field of 8 gauss, a measure of magnetic strength, stay loosely connected, simulating a droplet dissipating into a gas at its edges. Courtesy of Biswal Lab.