By applying an electric field, the movement of microswimmers can be manipulated. Scientists from the Max Planck Institute for Dynamics and Self-Organization (MPI-DS), the Indian Institute of Technology (IIT) Hyderabad and the University of Twente, Netherlands, describe the underlying physical principles by comparing experiments and theoretical modeling predictions. They are able to tune the direction and mode of motion through a microchannel between oscillation, wall adherence and centerline orientation, enabling different interactions with the environment.
Microswimmers often need to independently navigate narrow environments like microchannels through porous media or blood vessels. The swimmers can be of biological origin, like algae or bacteria, but also constitute custom designed structures used for the transport of chemicals and drugs. In these cases, it is important to control how they swim in relation to walls and boundaries — as one might want them to exchange fuel or information, but also avoid them to stick where they are not supposed to.
Many swimmers are electrically charged, such that electric fields can provide a versatile method to guide them through complex environments. Scientists from MPI-DS now explored this idea in experiments on self-propelling artificial microswimmers: “We investigated the influence of a combination of electric fields and pressure-driven flow on the states of motion of artificial microswimmers in a channel,” reports Corinna Maass, group leader at MPI-DS and Associate Professor at the University of Twente. “We identified distinct modes of motion and the system parameters that control them” she summarizes. In a previous publication, the scientists already demonstrated that their artificial swimmers prefer to swim upstream, oscillating between the channel walls. With their new finding, it is now possible to control how the swimmers are moving by applying an electric field and flow through the channel.
This way, the researchers generated a broad range of possible motility patterns: The swimmers can be directed to adhere to the channel walls or follow its centerline, either in an oscillating or in a straight motion. They are also able to execute U-turns if they set off in the wrong direction. The scientists analyzed these different states using a general hydrodynamic model that is applicable to any swimmer with a surface charge. Ranabir Dey, Assistant Professor at IIT Hyderabad explains: “We show that the motility of charged swimmers can be further controlled using external electric fields. Our model can help to understand and customize artificial microswimmers, and provide inspiration for autonomous micro-robotic and other biotechnological applications.”
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