Dynamics of Microbial Locomotion in Non-Newtonian Fluids
Summary
Microorganisms navigate complex environments by generating local fluid flows through flagellar rotation, ciliary beating or body undulations. In many natural and host-associated habitats, the surrounding medium departs from classical Newtonian behaviour, exhibiting shear-dependent viscosity, elastic stresses and yield thresholds. These rheological features arise in polymeric solutions, mucus gels, biofilms and soil slurries, and profoundly influence swimming kinematics, energetics and transport. Shear thinning can enhance propulsion by lowering resistance near rapidly deforming surfaces, whereas elasticity may either assist or hinder progress through normal stress differences and flow memory effects. Yield stress media impose an additional barrier, requiring microbes to plastically deform or fluidise the matrix. The interplay between microbial geometry, beat frequency and fluid microstructure governs speed, directionality and efficiency. Understanding these interactions is crucial for deciphering pathogen penetration of mucosal barriers, sperm transit through cervical mucus, bacterial dispersal in soil and the design of bio-inspired microrobotic swimmers for targeted drug delivery and environmental sensing.
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Dynamics of Microbial Locomotion in Non-Newtonian Fluids publication trend
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Technical terms
Non-Newtonian fluid: A medium whose stress–strain relationship deviates from linear viscosity, often exhibiting shear-thinning, shear-thickening or elasticity.
Shear-thinning: A decrease in apparent viscosity with increasing shear rate, common in polymer solutions and biological gels.
Viscoelasticity: Combined fluid- and solid-like response characterised by time-dependent stress relaxation and normal stress differences.
Yield stress: The critical stress below which a material behaves as an elastic solid and above which it flows plastically.
Deborah number: Dimensionless ratio of fluid relaxation time to characteristic time scale of deformation, indicating the prominence of elastic effects.
Flagellum: A helical filament rotated by a basal motor, widely used by bacteria for propulsion in viscous environments.
Cilium: A shorter, hair-like appendage that beats in coordinated waves to generate fluid flow around eukaryotic microorganisms.
Micropolar fluid: A continuum model incorporating local particle rotations and couple stresses, suitable for describing bacterial slime layers.
Bingham fluid: A viscoplastic model featuring a yield stress, above which the material flows with finite viscosity.
References
- Motility of Different Gastric Helicobacter spp.. Microorganisms (2023).
- Mud swimming: Locomotion through a viscoplastic fluid. Science Talks (2022).
- Rheological effects of micropolar slime on the gliding motility of bacteria with slip boundary condition. Results in Physics (2018).
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