Uncover the physical mechanics behind the micro-swimmer's movement

Chemical trace visualization. (A) Schematic of the experimental setup for fluorescent microscopy of charged micelle traces. CB15 droplet diameter a_drop=50 was injected into a quasi-2D microfluidic cell (height = 50 m) and observed using a bright-field or fluorescent microscope. (B) A fluorescence micrograph of a chemical trace droplet, with the surfactant concentration increased to 15% by weight to increase the visibility of the trace (increased solubility level). (C) Enlarged view of B. (D) Schematic of the drip propulsion mechanism. Black arrows indicate the direction of movement. (E) The flow field generated by the Marangoni flow at the droplet interface visualized by the streak lines of a 0.5 m fluorescent tracer colloid (droplet reference frame). (F) Time evolution of fluorescent intensity (in arbitrary units [a.u.]) profile along AA (Inset) superimposed with Gaussian fit (5 wt% surfactant concentration). (G) Peak intensity vs. time. The zero point in time is shifted 20 seconds from the droplet trajectory time to account for the fact that the droplet is not a point source I₀(Materials and Methods). The green circle marks the boundary of the droplet in the overexposed area. (Bar scale: 50 m.). Credit: Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2122269119

Bacteria and other unicellular organisms developed sophisticated ways to actively navigate their way, despite their simple structure. To uncover this mechanism, researchers from the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) used oil droplets as a model for biological micro-swimmers. Corinna Maass, group leader at MPI-DS and professor at the University of Twente, together with her colleagues, investigated the navigational strategies of micro-swimmers in several studies: how they navigate against the current in narrow channels, how they affect each other’s movements, and how they collectively began to spin to move.

To survive, biological organisms must react to their environment. Whereas humans or animals have complex nervous systems to sense their environment and to make conscious decisions, unicellular organisms develop different strategies. In biology, small organisms like parasites and bacteria for example navigate through narrow channels like blood vessels. They often do so regularly, oscillating based on hydrodynamic interactions with the channel boundary walls.

“In our experiments, we were able to confirm a theoretical model that describes the specific dynamics of microswimmers based on their size and interactions with the channel wall,” said Corinna Maass, lead author of the study. This regular movement pattern can also be used to develop targeted drug delivery mechanisms, even transporting cargo against the current as has also been shown in previous studies.

Traces of spent fuel

In another study, researchers investigated how moving micro-swimmers influenced one another. In their experimental model, tiny oil droplets in a soap solution move independently by growing a small amount of oil-producing propulsion. Like an airplane leaving a trail, micro-swimmers produce a trail of spent fuel that can repel others. In this way, microswimmers can detect whether other swimmers have been in the same place a moment before.

“Interestingly, this causes evasive motion for individual microswimmers, whereas assemblages of them produce droplets confined between each other’s tracks,” said Babak Vajdi Hokmabad, first author of the study. Rejection of the second drop on the previously passed trajectory depends on the angle of approach and the time elapsed after the first swimmer. These experimental findings also confirm theoretical work in the field, previously carried out by Ramin Golestanian, managing director of MPI-DS.

Collective movement through cooperation

Finally, the group also investigated the collective hydrodynamic behavior of some micro-swimmers. They found that some droplets can form clusters that spontaneously begin to float like hovercraft or rise and spin like microscopic helicopters. Cluster rotation is based on cooperative merging between individual droplets leading to coordinated behavior — although individual droplets alone do not consist of such movement. This arrangement represents another physical principle of how microswimmers can navigate their way — without using brains or muscles.

---

Modeling the behavior and dynamics of micro-swimmers

---

Further information:
Babak Vajdi Hokmabad et al, Self-caging chemotactics in active emulsions, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2122269119

Ranabir Dey et al, Reotaxis of artificial swimmer oscillations in microchannels, Nature Communication (2022). DOI: 10.1038/s41467-022-30611-1

Babak Vajdi Hokmabad et al, Spontaneously rotating clusters of active droplets, Soft Material (2022). DOI: 10.1039/D1SM01795K

Provided by Max Planck Society

Quote: Uncovering the physical mechanisms behind the movement of micro swimmers (2022, August 1) retrieved August 1, 2022 from https://phys.org/news/2022-08-revealing-physical-mechanisms-movement-microswimmers.html

This document is subject to copyright. Apart from any fair dealing for purposes of personal study or research, no part may be reproduced without written permission. Content is provided for informational purposes only.

#Uncover #physical #mechanics #microswimmers #movement