Abstract
Interaction of swimming bacteria with flows controls their ability to explore complex environments, crucial to many societal and environmental challenges and relevant for microfluidic applications such as cell sorting. Combining experimental, numerical, and theoretical analysis, we present a comprehensive study of the transport of motile bacteria in shear flows. Experimentally, we obtain with high accuracy and, for a large range of flow rates, the spatially resolved velocity and orientation distributions. They are in excellent agreement with the simulations of a kinematic model accounting for stochastic and microhydrodynamic properties and, in particular, the flagella chirality. Theoretical analysis reveals the scaling laws behind the average rheotactic velocity at moderate shear rates using a chirality parameter and explains the reorientation dynamics leading to saturation at large shear rates from the marginal stability of a fixed point. Our findings constitute a full understanding of the physical mechanisms and relevant parameters of bacteria bulk rheotaxis.
Highlights
The interaction of swimming microorganisms with flows determines their ability to move in complex environments such as biological channels, soils, or medical conducts
A dilute suspension of E. coli bacteria is injected at a given flow rate Q into a microchannel of width W = 600 m and height H = 100 m (Fig. 1A), which imposes to a very good approximation a planar Poiseuille flow Vx(z) = 4Vmaxz(H − z)/H2 sufficiently away from the side lTahteeralol cwaallslsh,ewairthraVtemiasx,tthheenfl ȯ w = ve∂l Vocxi(tzy )i /n ∂t zh e=c e4n Vtemraox(fHth −e c2hza )n /n He 2l
Since we focus on bulk dynamics, bacterial trajectories close to the top and bottom channel walls are not included in our analysis
Summary
The interaction of swimming microorganisms with flows determines their ability to move in complex environments such as biological channels, soils, or medical conducts. Elongated passive objects in shear flows perform so-called Jeffery orbits [8, 9], periodically changing their orientation while being transported downstream along stream lines. These orbits have been observed in different experimental systems with and without Brownian noise [10,11,12], and the role of fluctuations on the orbits has been addressed theoretically [12]
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