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It is known that some flagellated bacteria like Serratia marcescens, when deposited and affixed onto a surface to form a ‘‘bacterial carpet’’, self-organize in a collective motion of the flagella that is capable of pumping fluid through microfluidic channels [1]. We set up a continuum model comprising two macroscopic variables that is capable of describing this self-organization mechanism as well as quantifying it to the extent that an agreement with the experimentally observed channel width dependence of the pumping is reached as a particular challenge [2]. The activity is introduced through a collective angular velocity of the helical flagella rotation, which is an example of a dynamic macroscopic preferred direction [3,4]. Our model supports and quantifies the view that the self-coordination is due to a positive feedback loop between the bacterial flagella and the local flow generated by their rotation. Moreover, our results indicate that this biological active system is operating close to the self-organization threshold, where magnitude fluctuations of the self-organized order are substantial. This finding is rather unexpected and may hint to a general premise that biological systems are typically rather noisy --- and are therefore also quite robust against noise.


[1] M. J. Kim and K. S. Breuer, Small 4, 111 (2008).
[2] D. Svenšek, H. Pleiner, and H. R. Brand, Soft Matter 15, 2032 (2019).
[3] H. R. Brand, H. Pleiner, and D. Svenšek, Eur. Phys. J. E 34, 128 (2011).
[4] D. Svenšek, H. Pleiner, and H. R. Brand, Phys. Rev. Lett. 111, 228101 (2013).