Motor characteristics determine the rheological behavior of a suspension of microswimmers

Abstract

A suspension of motile cells exhibits complex rheological properties due to their collective motion. We measure the shear viscosity of a suspension of Escherichia coli strains varying in motor characteristics such as duration of run and tumble. At low cell densities, all strains irrespective of their motor characteristics exhibit a linear increase in viscosity with cell density suggesting that the cells behave as a suspension of passive rods with an effective aspect ratio set by the motor characteristics of the bacteria. As the cell density is increased beyond a critical value, the viscosity drops sharply signaling the presence of strongly coordinated motion among bacteria. The critical density depends not only on the magnitude of shear but also the motor characteristics of individual cells. High shear rate disrupts the coordinated motion reducing its behavior, once again, to a suspension of inactive particles.

Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows

“…Quantitative microstructural models relating particle size, shape, concentration and motility to rheological properties are just beginning to emerge. [6][7][8][9] These predictions are supported by experimental observations in shear flows with bacterial pushers [10][11][12] and algal pullers. 13 We present here the first measurements of extensional viscosities of suspensions of wild-type strains of the microalga Dunaliella tertiolecta, the bacterium Escherichia coli and mouse spermatozoa.…”

“…It does not for instance directly take into account changes in particle motility in pusher suspensions brought about by the emergence of large-scale coherent structures and collective motion due to inter-particle hydrodynamic interactions. 12,46,47 However, although U and D r in Saintillan's equations for [ Z] in principle describe the motility of isolated swimmers, it may be possible to use concentration-dependent average speeds and long-time diffusivities measured in non-dilute suspensions as particles execute collective motion.…”

Collective motion in a suspension of micro-swimmers that run-and-tumble and rotary diffuse

“…The confirmation of the above results using computations should serve as a valuable guide for experiments which can sweep through a sequence of volume fractions or mean-run-times (τ ) to detect the onset of collective behaviour. The volume fraction and swimming speeds have been varied in earlier experiments , while the experimental realization of different values of the mean-run-time has been shown to be possible by tailoring different mutant strains of E. coli (Karmakar et al 2014).…”

Modelling the mechanics and hydrodynamics of swimming E. coli

“…The alternating runs and tumbles allow the bacterium to efficiently execute a biased random walk toward favorable environments such as food-concentrated regions by adjusting run and tumble durations to the environmental conditions. [39][40][41] The complexity of the bundling and swimming processes, especially near-field hydrodynamics, poses substantial challenges for an analytical description of bacteria locomotion. 9 These comprise aspects of the bacteria flagella such as their polymorphic transformations 4,5,10-14 and bundle formation.…”

“…[25][26][27][28][29][30][31][32][33][34][35] In addition, the effects of external flows on the dynamical behaviors of bacteria suspension have been addressed [36][37][38] along with their rheological properties. [39][40][41] The complexity of the bundling and swimming processes, especially near-field hydrodynamics, poses substantial challenges for an analytical description of bacteria locomotion. Here, mesoscale hydrodynamics simulations are particularly valuable to gain insight into the microscopic aspects of swimming, because they are able to bridge the large length-and time-scale differences between the bacterium and fluid degrees of freedom.…”