Physicists have been interested in manipulating small particles using an electric field. The particles can be assembled into chains that can be used to control the flow of liquids. Liquid droplets can store and use various biomolecules, so replacing solid particles with liquid droplets would allow for previously impossible applications. It was not possible to use liquid droplets for electrorheology due to their tendency to coalesce or degrade.
The National Institute of Standards and Technology (NIST) and the University of Chicago have collaborated on a new research that shows a simple pathway for stabilizing polyelectrolyte coacervate droplets. The study was published in a peer reviewed journal.
The high polarizability and residual surface charge enable these "stabilized" droplets to be steered in an Aqueous environment. These droplets are known as coacervates and they contain charged polymers that allow the encapsulation of biologically relevant charged species. They have the ability to deliver cargo that is useful in the manufacturing and medical industries.
Coacervate droplets form when two polyelectrolytes co-assemble into a condensate state. The solution can convert quickly to a two-phase system with the droplets suspended in the surrounding solution. The droplets are about the size of a cell. These droplets have been shown to perform certain reactions. Coacervate droplets can form larger and larger droplets by merging with each other, but this is not a good thing.
Put a spoon of olive oil in a cup of water and shake it. When you first see the mixture, you'll see small droplets that make it cloudy, but over time these droplets will form separate oil and water layers. When droplets coalesce to form layers, bioreactors made out of coacervates fail over time.
The problem of oil-droplet coalescence was solved by adding surfactant molecule that go to the interface of oil droplets. He said that similar technology was applied to coacervate droplets where specialized polymer chains were used to coat the droplet interface. Material transport in and out of droplets can't be done with the help of such coating.
Aman Agrawal, the graduate student in the research group leading the project, wanted to stable the droplets without introducing any new molecule. When coacervate droplets are transferred from their original salt solution to distilled water, their interface tends to get a strong resilience against coalescence. The researchers think that the stability of droplets is due to a loss of ion from the droplets interface into the distilled water. Agrawal demonstrated how to form droplets under an AC field and then move them around with a DC field.
There are potential applications for the new development in the coacervate field. In basic biology, this mechanism may explain why the cells are stable. The polarizability of the coacervate droplets might have significant ramifications for the manipulation of many biological materials, as recent measurements have shown that cells of various types can be manipulated similar to the stabilizing droplets with the application of electric fields.
More information: Aman Agrawal et al, Manipulation of coacervate droplets with an electric field, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2203483119 Journal information: Proceedings of the National Academy of Sciences
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