In a conventional fluid such as water, molecules tumble in random directions. Physicists who study active matter have long contemplated what would happen if the molecules’ rotations were instead coordinated to create a so-called chiral fluid. Understanding how such a fluid behaves could help engineers design materials with novel transport properties. The spinning particles in a chiral fluid break the symmetry rules that govern conventional fluids. But the phenomena expected to emerge in a chiral fluid due to asymmetric stresses have not previously been measured in an experimental system.
Now researchers in William Irvine’s laboratory at the University of Chicago have developed a chiral fluid in the lab and identified the mechanisms that yield unusual surface flows. To create the two-dimensional fluid, graduate students Vishal Soni and Ephraim Bililign and postdoc Sofia Magkiriadou rotated a magnetic field around a colloidal suspension of millions of 1.6 μm hematite cubes, like the one shown in this optical micrograph. That rotation caused the particles to spin simultaneously in the same direction and ensured that they stuck together enough to behave as a liquid.
Imaging the bulk 2D fluid revealed unconventional flow patterns, such as the clockwise currents shown in the diagram below, forming at the fluid’s free surface, or 1D edge. To find out what caused those patterns, the researchers took power spectra of strips of the fluid in different configurations and measured the changing height of the free surface. Using constants derived from the individual particles’ spinning rates, the researchers solved hydrodynamic equations to determine the forces that drove the observed patterns.
For the chiral fluid on a glass substrate, the researchers observed unidirectional surface waves forming at the surface. In contrast to a conventional fluid, for which viscosity should be a damping force, the shear viscosity and substrate friction drove the propagation of surface waves. The mechanism resembles a shifting sand dune in which surface wind pushes material away from curved regions toward the flat wavefront; that motion leads to unidirectional wave motion.
Then the researchers positioned the chiral fluid on an air–water interface to reduce surface friction. Under some conditions, the fluid’s viscosity gave rise to a flow perpendicular to applied pressure. In a manner similar to surface tension, the perpendicular, or odd, viscosity flattened the surface waves. Although theorists had predicted an uneven viscosity component in chiral fluids, Irvine’s team provided the first measurement. The magnitude of the odd viscosity was 30% that of the shear viscosity.
The chiral fluid provides the first platform for probing and designing materials with properties that result from uniformly spinning particles. The model system could also help predict behaviors that may emerge in some plasmas, or in charge carriers in 2D electronic materials. (V. Soni et al., Nat. Phys., 2019, doi:10.1038/s41567-019-0603-8.)
