Despite their relatively simple structure, bacteria and other single-celled organisms have developed sophisticated methods for active navigation. To uncover these mechanisms, researchers at the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) used oil droplets as a model for biological microswimmers. MPI-DS group leader Corinna Maass, associate professor at the University of Twente, and her colleagues investigated the navigation strategies of microswimmers in several studies: how they navigate against the current in narrow channels, how they influence each other on their movements, and How together they start spinning to move.
To survive, biological organisms must respond to their environment. While humans or animals have complex nervous systems to sense their surroundings and make conscious decisions, single-celled organisms have developed different strategies. In biology, small organisms such as parasites and bacteria travel through narrow passages such as blood vessels, for example. They typically do so in a regular, oscillatory manner based on hydrodynamic interactions with the channel walls. “In our experiments, we were able to confirm a theoretical model describing the specific dynamics of microswimmers based on their size and interaction with the channel walls,” commented Corinna Maass, lead researcher on the study. These regular movement patterns can also be used to develop mechanisms for targeted drug delivery, and even to transport cargoes countercurrently, as pointed out in previous studies.
Trail of used fuel
In another study, the researchers investigated how moving microswimmers interacted with each other. In their experimental model, small oil droplets in a soap solution moved autonomously by producing a small amount of oil to generate a propulsive force. Like a plane leaving a trail, microswimmers produce a trail of spent fuel that can repel others. In this way, the miniature swimmers were able to detect whether other swimmers were in the same spot not long ago. “Interestingly, this leads to self-avoiding movements of individual microswimmers, while their ensemble causes droplets to get trapped between each other’s trails,” reports Babak Vajdi Hokmabad, first author of the study. The repulsion of a drop on a previously passed trajectory depends on its angle of approach and the elapsed time after the first swimmer. These experimental results also corroborate theoretical work in the field, previously conducted by MPI-DS managing director Ramin Golestanian. The research was carried out within the confines of the Max Planck Centre for Complex Fluid Dynamics, a joint research centre comprising MPI-DS, MPI Polymer Research and the University of Twente.
collective movement through cooperation
Finally, the group also investigated the collective hydrodynamic behavior of multiple microswimmers. They found that multiple droplets can form clusters that spontaneously start to float like a hovercraft, or rise and spin like a miniature helicopter. The rotation of the clusters is based on cooperative coupling between the individual droplets, which leads to coordinated behavior—although individual droplets do not include this motion. These arrangements thus represent another physical principle of how miniature swimmers can navigate without using their brains or muscles.
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