A container of oil and water separated by a thin skin of magnetized particles has intrigued a team of chemical engineers by taking on an unexpected ‘Grecian urn’ shape upon agitation.
“I thought ‘what is this thing?’,” graduate student Anthony Raykh from the University of Massachusetts Amherst recalled, after doing what all chemistry students love to do, mixing materials with intriguing properties just to see what would happen.
“So, I walked up and down the halls of the Polymer Science and Engineering Department, knocking on my professors’ doors, asking them if they knew what was going on.”
An analysis of the materials revealed that interactions between the tiny magnetic beads worked alongside the surface tension between the immiscible liquids, tugging the mixture in directions that thermodynamics would normally forbid if not for a touch of magnetism.
While the discovery has no obvious applications, at least currently, the unusual shape of the mixture and its responses to an external magnetic field could provide future researchers and engineers with a new trick to tweak the structural properties of their emulsions.
Oil and water aren’t the best of friends. You can combine and shake all you like – sooner or later, the two will go their separate ways and reform into distinct layers as dictated by the thermodynamic flow of energy versus the attraction and repulsion of molecular bonds.
As any budding master chef knows, there are ways to keep fat-soluble and water-soluble fluids blended a little longer. Emulsifiers, such as the lecithin in egg yolk, can serve as a chemical diplomat to bind the materials into a mixture as satisfyingly smooth and consistent as your favorite salad dressing.
Pickering emulsions follow a method of stabilizing mixtures by inserting a thin skin between the two phases, breaking at least one of the fluids into tiny bubbles that can disperse more consistently through the other.
Technically, Raykh’s laboratory salad dressing followed the rules of being a Pickering emulsion, containing tap water and a slightly polar organic solvent as well as a dash of magnetized nickel particles, roughly 5 to 15 micrometers and 20 nanometers in size.
Dispersed through the water, the nickel particles rose to the surface to connect as a two-dimensional ‘skin’ just below the organic solvent’s interface, much as a Pickering emulsifier ought to do between an aqueous liquid and oil. Giving the mix a shake should result in a suspension of the three materials as homogenous as mayonnaise.
Only that’s not what occurred. The magnetic particles pulled into a branching web under tension dictated by the thermodynamics of the liquids, tugging the entire mixture into a lopsided hourglass-like formation at equilibrium.
A look under the microscope and a computer simulation, which modeled the forces involved, provided a clear picture of the complex mix of magnetic interactions involved in creating the unusual shape, which prevented emulsification from occurring. Using a magnet, the researchers could deform the boundary and modify the urn shape.
“When you look very closely at the individual nanoparticles of magnetized nickel that form the boundary between the water and oil, you can get extremely detailed information on how different forms assemble,” says polymer scientist David Hoagland.
“In this case, the particles are magnetized strongly enough that their assembly interferes with the process of emulsification, which the laws of thermodynamics describe.”
This research was published in Nature Physics.
