Associated with gates and doorways, Janus—a god in ancient Roman mythology—is depicted with two faces- He can look both inward and outward, toward both past and future at once (see photo above). Ever since Pierre-Gilles de Gennes introduced the name into the scientific community in 1991, Janus particles have referred to objects that possess two or more sides having different chemical properties. An interesting subset of such particles have sides that are hydrophobic and hydrophilic. With that chemical anisotropy on the same object, Janus particles can densely self-assemble at those fluid interfaces to form complex colloidal structures and stabilizing emulsions. Applications abound—from the creation of water-repellent textiles to membranes with tunable surfaces. (See the Quick Study by Steve Granick, Shan Jiang, and Qian Chen, Physics Today, July 2009, page 68.)

Mohammad Hossain, Ian Gates, and Giovanniantonio Natale, all chemical engineers at the University of Calgary, have now simulated the dynamics of Janus rods at the interface of two immiscible liquids—water and benzene. The researchers modeled each rod as a cluster of 506 beads, arranged in a face-centered cubic lattice, as shown in the rod’s front and top views (see diagram). To quantify the extent to which the rods’ shape and orientation modify their energetics at the interface, Hossain and coworkers calculated their motion as a function of aspect ratio (length divided by radius). That variable turns out to affect both their diffusion along the interface and their tilt. Both factors, in turn, affect the system’s interfacial tension, stability, and rheology.

In general, the rods with larger radii exhibited a slower diffusion at the interface. But that diffusion was anisotropic and had to be decomposed into parallel and normal components. For a low aspect ratio, the rods’ so-called attachment energy had one global minimum and adopted an upright orientation perpendicular to the interface. But as the group increased the aspect ratio in subsequent simulations, the rods increasingly tilted downward into a more stable configuration. As the contact area between the rod and interface increased, so did the attachment energy, and a second, local energy minimum emerged. The existence of the two energy minima implies that the rods’ aspect ratio strongly affects their equilibrium orientation—vertical or tilted—at the interface. If the rods adopt a tilted orientation, the interfacial tension drops, and the Janus rods become more stable. The upshot is that changes to a Janus rod’s geometry alone can produce a stable liquid emulsion without having to alter the rod’s chemical properties. (M. T. Hossain, I. D. Gates, G. Natale, Phys. Fluids 34, 012117, 2022.)