Oil droplets can be made to act as predators, chasing other droplets that escape as prey. The behavior, which is controlled by chemical signaling produced by the droplets, mimics the behavior observed among living organisms, but has not yet been reproduced in synthetic systems. This regulated chemical system could potentially serve as a model to help understand interactions in multi-body systems such as schools of fish, bacterial colonies, or swarms of insects.
An international team of researchers led by Pennsylvania scientists describes the system in an article published on November 1
“By controlling the chemistry of oil droplets, we can create a system in which the droplets behave actively and communicate with each other through chemical gradients,” said Lauren Zarzar, assistant professor of chemistry in Pennsylvania and head of the research team. “The exciting thing we found is that you can design a system of droplets that show” non-reciprocal “interactions. One droplet is attracted to the other, while the other is repelled, similar to the behavior of a predator and prey. “
Researchers put microdroplets of two different oils in a solution of water and a surfactant, a compound common in soaps that reduces the surface tension of liquids. One of the oils dissolves more easily in the surfactant solution, which causes these droplets to emit a chemical gradient of oil molecules in the surrounding environment. The drops are repelled by the dissolved oil.
“Initially, this cloud of oil around the droplets was basically symmetrical and the droplets did not move,” said Caleb Meredith, a Penn State student and co-author of the article. “But what we’ve found is that prey droplets can actually absorb some of the oil released by predator droplets, creating an oil exchange between the droplets. When the droplets get close enough, it creates an asymmetry in the chemical gradient between the two droplets and drives the predator drop to move towards the prey, creating a chase. “
The asymmetry of the oil chemical gradient generated by the source and the sink causes a difference in the surface tension on the surface of both the predator droplets and the prey. The gradient causes the predator droplet (source) to move to the prey droplet (sink). Similarly, due to the effect of the chemical gradient emitted by the predator, the prey is repelled by the approaching predator.
“One of the surprising results is that it is not necessary for the two drops of oil to be very chemically different from each other to cause this behavior,” Zarzar said. “We looked at a wide variety of chemical compositions for oils and surfactants, which allowed us to establish a set of rules that govern these interactions. We can use these rules to adjust the strength of interactions by controlling the compositions of drip oils or surfactants. “
The research team also developed a model that, based on measurements of chasing speeds between individual pairs of droplets, was able to accurately simulate the movement of many droplets and show how they are organized into larger clusters that move in different ways.
“They really look alive to me sometimes,” Meredith said. “When multiple droplets come together in clusters, they can start spinning, stopping and walking, moving in spirals, and even splitting into smaller clusters.”
Researchers say that by understanding the types of rules that govern these interactions, their system can ultimately be used to experimentally model multi-body systems, ranging from the behavior of large numbers of animals to interactions that may have played a role in evolution early life.
“What we’re doing is really basic, fundamental research, where the motivation is to understand the processes that work that can control the activity of inanimate things like oil droplets,” Zarzar said. “But these ideas could find application in other areas, such as self-assembly, group behavior, and even thinking about the origin of life on Earth, where mixtures of simple chemical components must somehow be organized into nonequilibrium structures. It is clear that we are not looking at the same chemicals, but we may be able to identify parameters or conditions that, for example, lead to similar types of interactions that have occurred. ”
Levitating droplets allow scientists to perform “non-contact” chemical reactions
Caleb H. Meredith et al, Predator-prey interactions between droplets driven by non-reciprocal oil exchange, Chemistry of nature (2020). DOI: 10.1038 / s41557-020-00575-0
Provided by Pennsylvania State University
Quote: Oil droplet predators chase prey with oil droplets (2020, November 18) extracted on November 19, 2020 from https://phys.org/news/2020-11-oil-droplet-predators-prey.html
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