Can swimming bacteria spin a perfectly symmetric disc?
A bacterium trapped under a symmetric disc makes it rotate because its body and its tail spin in opposite directions, even without pushing on any wall.
Source
The hydrodynamic torque dipole from rotary bacterial flagella powers symmetric discs
Study at a glance
- Design
- Other — Fluorescence microscopy of 3D-printed microdiscs in baths of swimming E. coli, compared against a boundary-element hydrodynamic model with one fitted parameter
- N
- No single N; many pucks and individual bacterial crossings tracked across disc designs (plain discs, four-chamber discs, open-channel discs).
- Population
- Symmetric polymer microdiscs (radius 5, 10 or 20 micrometres) in suspensions of motile E. coli MG1655
- Outcome
- Rotation angle and rotation rate of the discs versus bacterial position, body length and disc size
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The researchers 3D-printed tiny flat discs ('pucks') and let them sink to the bottom of a capillary filled with swimming E. coli. Some discs were plain, some had four dead-end chambers and some had one open channel through the middle that a single bacterium could swim through. They tracked the disc angle while fluorescent bacteria entered, and built a low-Reynolds-number fluid model in which the bacterium is two opposite point torques.
What they found
Plain discs rotated slowly clockwise because bacteria swimming in curved clockwise paths hit their edge asymmetrically, with rotation rate falling as 1/R. A bacterium inside a chamber made the disc spin about ten times faster than a simple wall-pushing estimate predicts, and each extra trapped bacterium sped it up further. In the open channel the disc turned clockwise, then reversed as the cell body left, independent of the swimming direction; longer bacteria gave bigger turns, and the model reproduced this with one fitted factor of about 1.5.
The limits
What it doesn't show
The model treats the flagellar bundle as a point torque and the channel as infinitely long, so it misplaces where the rotation reverses; the authors say near-field effects need more work. Only one bacterial species and strain in narrow, gravity-confined geometry was studied, so how important this effect is in dense or natural suspensions is untested. The dipole length is fitted rather than measured directly.
Key terms
- Torque dipole
- Two equal and opposite torques a small distance apart, here the cell body spinning one way and the flagella spinning the other, so there is no net torque on the swimmer.
- Low Reynolds number
- A flow regime where viscosity dominates inertia, so motion stops instantly when forcing stops and net displacement depends on geometry, not speed.
- Rotlet
- The flow field produced by a point torque in a viscous fluid; the basic building block of the model here.
- Active bath
- A fluid kept out of equilibrium by self-propelled particles such as swimming bacteria that continually inject energy.
- Rotational mobility
- How fast an object rotates per unit applied torque; linked to its rotational diffusion by the Stokes-Einstein relation.
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Quiz yourself
What drives the rotation of a disc when a bacterium swims through its open central channel?
Common questions
Why is it surprising that a symmetric disc rotates?
Earlier bacterial motors needed asymmetric gear shapes to rectify random pushes into steady rotation; symmetric gears only jiggled. Here confinement plus the chirality of the flagellar motor supplies the asymmetry instead.
Why does the disc reverse direction when the bacterium leaves the channel?
While both body and flagella are under the disc their opposite torques act at different positions and give a net clockwise torque; once the body exits only the flagella's torque remains, which turns the disc the other way.
Why plot rotation against bacterial position instead of time?
In Stokes flow the displacement produced depends only on how far the swimmer has moved, not how fast, so fast and slow bacteria collapse onto one curve.
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