Interactions drive the dynamics of many microscopic biological systems, including bacteria swarms, tissue-forming cells, and molecular motor–mediated protein assemblies. Those systems can be complicated. It’s tricky to figure out how their various chemical and mechanical signals and feedback mechanisms come together to produce the observed motion, so the underlying interactions are often approximated using simplified model systems.
Researchers have developed various self-propelled particles and droplets that can exhibit attractive and repulsive interactions (see the article by Jeffrey Moran and Jonathan Posner, Physics Today, May 2019, page 44). Nature, on the other hand, doesn’t stop there: Nonreciprocal interactions, in which one object is attracted to another but the second object repels the first, underlie such chase scenes as a white blood cell pursuing a bacterium. Now Caleb Meredith at the Pennsylvania State University, Pepijn Moerman at Utrecht University in the Netherlands, and their coworkers have found oil-in-water droplets that can mimic that behavior.
The experimental system is illustrated in the figure. Droplets of two miscible oils, 1-bromooctane (BOct) and ethoxynonafluorobutane (EFB), are dispersed in water. Both oils are immiscible in water, but when surfactant molecules with hydrophilic heads and hydrophobic tails are added (black in the figure), they form swollen micelles that shuttle small amounts of oil between the droplets.
The Utrecht researchers had previously observed micelle-mediated transport with other oil droplets, but it had only produced reciprocal interactions. Adding a surfactant lowers a droplet’s surface tension, and surface tension gradients produce Marangoni flows. If two nearby droplets are both feeding oil into micelles, the free surfactant between them gets depleted faster than in the surrounding areas, and the resulting Marangoni flow pushes them apart. If the droplets instead absorb oil from micelles, the effect is reversed.
Nonreciprocal interactions, the researchers realized, would require two oils with disparate micelle-transport properties. The Penn State group identified such a system: BOct oil goes into micelles with Triton X-100 surfactant much more readily than EFB does. So when a pair of those droplets comes together as in the figure, the BOct droplet acts as a net source of oily micelles, and the EFB droplet serves as a sink. The source droplet gets pulled toward the sink, but the sink moves away from the source, and a chase ensues.
Replacing the Triton surfactant with one that preferentially forms EFB-filled micelles reversed the direction of the chase. The behavior could also be turned off, either by using a mixture of the two surfactants, which balances the oil-exchange rates, or by using less-miscible oils, which prevents oil uptake by the sink. Tailoring the interactions further could produce complex networks for chemical transfer and elucidate the design rules employed by biological chasers. (C. H. Meredith et al., Nat. Chem., 2020, doi:10.1038/s41557-020-00575-0; thumbnail micrograph credit: C. H. Meredith et al., Nat. Chem., 2020, doi:10.1038/s41557-020-00575-0.)