Rice University postdoctoral fellow Kedar Joshi prepares an experiment at the Biswal Lab to see how magnetic fields will affect a colloid of magnetic particles. Peer-reviewed paperĮxtension of Kelvin’s equation to dipolar colloids: Image downloads McCardell Professor in Chemical Engineering, a professor of chemical and biomolecular engineering and of materials science and nanoengineering, and associate dean for faculty development. The National Science Foundation (17055703) supported the research. “Being able to see how magnetic fields can be used to control how these systems are able to achieve coexisting phases is important to designing materials that are reconfigurable or have a desired property.” “The new paper is about the idea that you can have coexistence (between the liquid and gas phases),” she said.
“We were keen to show that it would replicate what classical phases do in terms of vapor pressure, viscosity and surface tension as well.”īiswal said the study also has implications for devices like control displays that employ liquid crystals. “The system does behave like it’s being affected by temperature,” said Joshi, who recently left Rice to join the faculty at the Indian Institute of Technology, Goa.
The strength of this fast-rotating magnetic field became a knob that raised and lowered the “temperature” and controlled whether the particles condensed into a liquid or dispersed like a gas. “But if we increased the frequency, we found that it generated an isotropic attractive interaction between the particles.” Rice postdoctoral fellow Kedar Joshi prepares an experiment at the Biswal Lab to see how magnetic fields will affect a colloid of magnetic particles. “The particles followed the rotating field they look like little miniaturized stir bars,” Biswal said. In spite of that, they found the equation still held true for the interactions they observed as the particles came together or flew apart depending on the strength of the field. The researchers threw their colloid out of equilibrium by spinning it with the field. In the Rice experiments, the particles represented liquid molecules when clumped and gas molecules when dispersed, both qualities controlled by the rotating magnetic field, a stand-in for the equation’s temperature variable. The results were surprising, they wrote, because Kelvin’s equation is not intended to apply to systems kicked out of equilibrium. “Kedar decided to apply the formula to our system, in which we can see the particles, we can count them and actually track them through their ‘gas’ and condensed phases.” “This one falls under our purview of how we think about gases and liquids, but in a different way,” Biswal said. The study extends the lab’s previous work to characterize how particles interact in solutions, the most recent demonstrating how superparamagnetic colloids interact with each other in a rapidly spinning magnetic field. Our thought was these equations should explain not just one or two but every property of our colloids as well.” “They try to stay circular, rather than take an arbitrary shape. “These colloidal groups are like the droplets,” Joshi said. “Kedar likes to give the example of water droplets: how they stay a certain size, even with water and vapor phases around them.” “Kelvin’s equation comes from classical thermodynamics, and tells us how liquid and gas phases are in equilibrium with each other,” Biswal said. Their finding, detailed in the Proceedings of the National Academy of Sciences, doesn’t exactly challenge Kelvin’s equation, which describes thermodynamic interactions between liquids and gases. “It’s like trying to blow a bubble in an odd shape,” Biswal said. Brown School of Engineering found that when a colloid - in this case, a suspension of micron-sized paramagnetic particles - is jostled with a magnetic field, it still tends to seek its lowest-energy state in the same way that gas and liquid systems do. Video courtesy of the Biswal LabĬhemical and biomolecular engineer Sibani Lisa Biswal and postdoctoral fellow Kedar Joshi of Rice University’s George R.
The Rice University experiment demonstrates how gases, represented by the dispersed particles, and liquids, represented by the condensed cluster, can coexist as a vapor and liquid that follows Kelvin’s equation for molecular systems. Dipolar colloidal particles are driven out of equilibrium by a spinning magnetic field.
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