Physical mechanisms behind the movement of microswimmers

Bacteria and other single-celled organisms have evolved sophisticated ways to actively navigate their way, although are relatively simple buildings. To reveal these mechanisms, researchers at the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) used oil droplets as a model for biological microswimmers. Corinna Maass, group leader at MPI-DS and associate professor at the University of Twente, together with her colleagues, investigated the navigational strategies of microswimmers in several studies: how they navigate against the current in narrow channels, how they affect each other’s motion, and how they collectively begin to spin to move.

To survive, biological organisms must react to their environment. While humans or animals possess a complex nervous system to sense their environment and make conscious decisions, single-celled organisms have developed different strategies. In biology, small organisms such as parasites and bacteria for example navigate through narrow channels such as blood vessels. They often do so in a regular and oscillating fashion based on hydrodynamic interactions with the channel’s confining wall. “In our experiments, we were able to confirm the theoretical model that describes the specific dynamics of the microswimmer as a function of its size and the interactions with the channel wall” comments Corinna Maass, principal investigator of the studies. These regular movement patterns could also be used to develop mechanisms for targeted drug delivery, or even countercurrent cargo transport, as also shown in a previous study.

A trail of spent fuel

In another study, researchers investigated how moving microswimmers affect each other . In their experimental model, small oil droplets in a soapy option move autonomously releasing small amounts of oil generating propulsion. Like an airplane leaving contrails behind, microswimmers generate a trail of spent fuel that can repel others. In this way, the microswimmers are able to detect if another swimmer has been in the same place shortly before. “Interestingly, this causes a self-avoidance movement in individual microswimmers, while a set of them result in droplets caging between streaks of each other,” Babak Vajdi Hokmabad reports. , first author of the study. The repulsion of the second drop on the trajectory of a previous drop depends on its angle of approach and the time spent after the first swimmer. These experimental results also confirm the theoretical work in the field, previously carried out by Ramin Golestanian, general manager of the MPI-DS. The research was conducted as part of the Max Planck Center for Complex Fluid Dynamics, a joint research center consisting of MPI-DS, MPI for Polymer Study and the University of Twente.

Collective movement through cooperation

Finally, the group also studied the collective hydrodynamic behavior of several microswimmers . They found that multiple droplets can form clumps that spontaneously begin to float like hovercraft or rise and spin like microscopic helicopters. Cluster rotation is based on cooperative coupling between individual droplets, which leads to coordinated behavior, although individual droplets alone do not understand such movement. These preparations therefore represent another physical principle of how microswimmers are able to navigate their way – without using brains or muscle groups.

BacteriumCanalEnvironmentGoutMusic BandSwimmingSingle Cell OrganismMax Planck Society

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