Bacteria hinder large-scale transport and enhance small-scale mixing in time-periodic flows

Abstract

Understanding mixing and transport of passive scalars in active fluids is important to many natural (e.g. algal blooms) and industrial (e.g. biofuel, vaccine production) processes. Here, we study the mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli in experiments using a two-dimensional (2D) time-periodic flow and in a simple simulation. Results show that the presence of bacteria hinders large scale transport and reduce overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, our results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport.

arXiv commentSupplementary Information added

References in corpus (16)

• Self-motile colloidal particles: from directed propulsion to random walk
• A bacterial ratchet motor
• Oscillatory Flows Induced by Microorganisms Swimming in Two-dimensions
• Running and tumbling with E. coli in polymeric solutions
• Precision Measurements of Stretching and Compression in Fluid Mixing
• Enhanced diffusion of nonswimmers in a three-dimensional bath of motile bacteria
• LCS Tool : A Computational Platform for Lagrangian Coherent Structures
• Turbulent fluid acceleration generates clusters of gyrotactic microorganisms
• Particle diffusion in active fluids is non-monotonic in size
• Fluid mixing by curved trajectories of microswimmers
• Accumulation of motile elongated micro-organisms in turbulence
• Preferential sampling and small-scale clustering of gyrotactic microswimmers in turbulence
• Alignment of non-spherical active particles in chaotic flows
• Gyrotactic swimmers in turbulence: shape effects and role of the large-scale flow
• Fluid Velocity Fluctuations in a Suspension of Swimming Protists
• Predicting the progress of diffusively limited chemical reactions in the presence of chaotic advection

Cited by in corpus (6)

• Harnessing elastic instabilities for enhanced mixing and reaction kinetics in porous media
• Coexistence of active and hydrodynamic turbulence in two dimensional active nematics
• Enhancing Transport Barriers with Swimming Microorganisms in Chaotic Flows
• Sedimentation dynamics of passive particles in dilute bacterial suspensions: emergence of bioconvection
• Mixing in chaotic flows with swimming bacteria
• Reference map technique for Lagrangian exploration of coherent structures