Collective sedimentation of squirmers under gravity

Kuhr, Jan-Timm; Blaschke, Johannes; Rühle, Felix; Stark, Holger

doi:10.1039/c7sm01180f

Cited by 44 publications

(60 citation statements)

References 70 publications

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“…In this article we address a monolayer of squirmers that forms under strong gravity at the bottom surface of the simulation box by performing parallelized simulations with up to 10 8 fluid particles and up to several thousand squirmers. In contrast to our previous work [42,46] where the squirmers can leave the bottom surface to swim upwards, in this article gravity is so strong that they are constrained to the bottom surface. Here, the squirmers either point upwards or tilt against the surface normal so that they move along the surface.…”

Section: Introductionmentioning

confidence: 64%

“…In recent articles we focussed on single microswimmers * jan-timm.kuhr@tu-berlin.de † holger.stark@tu-berlin.de in moderate gravitational fields [42] and on their collective sedimentation [46]. For our studies we used squirmers as model microswimmers [47][48][49][50], the swimmer type of which can be continuously tuned from pushers over neutral swimmers to pullers.…”

Section: Introductionmentioning

confidence: 99%

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Collective dynamics in a monolayer of squirmers confined to a boundary by gravity

2019

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We present a hydrodynamic study of a monolayer of squirmer model microswimmers confined to a boundary by strong gravity using the simulation method of multi-particle collision dynamics. The squirmers interact with each other via their self-generated hydrodynamic flow fields and thereby form a variety of fascinating dynamic states when density and squirmer type are varied. Weak pushers, neutral squirmers, and pullers have an upright orientation. With their flow fields they push neighbors away and thereby form a hydrodynamic Wigner fluid at lower densities. Furthermore, states of fluctuating chains and trimers, of kissing, and at large densities a global cluster exist. Finally, pushers at all densities can tilt against the wall normal and their in-plane velocities align to show swarming. It turns into chaotic swarming for strong pushers at high densities. We characterize all these states quantitatively.

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Section: Introductionmentioning

confidence: 64%

Section: Introductionmentioning

confidence: 99%

Collective dynamics in a monolayer of squirmers confined to a boundary by gravity

2019

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“…Interestingly, for large swimming speeds and strong bottom-heaviness inverted sedimentation profiles occur. We will also drastically increase the particle number, similar to [70], where we expect these inverted profiles to become unstable due to hydrodynamic interactions between the squirmers.…”

Section: Resultsmentioning

confidence: 96%

Gravity-induced dynamics of a squirmer microswimmer in wall proximity

et al. 2018

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We perform hydrodynamic simulations using the method of multi-particle collision dynamics and a theoretical analysis to study a single squirmer microswimmer at high Péclet number, which moves in a low Reynolds number fluid and under gravity. The relevant parameters are the ratio α of swimming to bulk sedimentation velocity and the squirmer type β. The combination of self-propulsion, gravitational force, hydrodynamic interactions with the wall, and thermal noise leads to a surprisingly diverse behavior. At a > 1 we observe cruising states, while for a < 1 the squirmer resides close to the bottom wall with the motional state determined by stable fixed points in height and orientation. They strongly depend on the squirmer type β. While neutral squirmers permanently float above the wall with upright orientation, pullers float for α larger than a threshold value a th and are pinned to the wall below a th . In contrast, pushers slide along the wall at lower heights, from which thermal orientational fluctuations drive them into a recurrent floating state with upright orientation, where they remain on the timescale of orientational persistence.

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“…Collective motion of biological and artificial microswimmers shows a broad range of interesting phenomena as demonstrated in several review articles [1][2][3][4][5]. The formation of various patterns and clustering have been investigated both experimentally and theoretically in systems of bacteria [6][7][8][9][10][11][12][13], of eukaryotic cells such as Dictyostelium discoideum or human sperm [14][15][16][17][18][19][20][21][22], as well as in suspensions of active colloids [4,[23][24][25][26][27][28][29][30][31][32]. In this article we study the collective behavior of a bacterial population, which in the concentration field of a chemoattractant forms a traveling solitary pulse.…”

Section: Introductionmentioning

confidence: 99%

Traveling concentration pulses of bacteria in a generalized Keller–Segel model

2019

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We formulate a Markovian response theory for the tumble rate of a bacterium moving in a chemical field and use it in the Smoluchowski equation. Based on a multipole expansion for the one-particle distribution function and a reaction-diffusion equation for the chemoattractant field, we derive a polarization extended model, which also includes the recently discovered angle bias. In the adiabatic limit we recover a generalized Keller-Segel equation with diffusion and chemotactic coefficients that depend on the microscopic swimming parameters. Requiring the tumble rate to be positive, our model introduces an upper bound for the chemotactic drift velocity, which is no longer singular as in the original Keller-Segel model. Solving the Keller-Segel equations numerically, we identify traveling bacterial concentration pulses, for which we do not need a second, signaling chemical field nor a singular chemotactic drift velocity as demanded in earlier publications. We present an extensive study of the traveling pulses and demonstrate how their speeds, widths, and heights depend on the microscopic parameters. Most importantly, we discover a maximum number of bacteria that the pulse can sustain-the maximum carrying capacity. Finally, by tuning our parameters, we are able to match the experimental realization of the traveling bacterial pulse.

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Collective sedimentation of squirmers under gravity

Cited by 44 publications

References 70 publications

Collective dynamics in a monolayer of squirmers confined to a boundary by gravity

Collective dynamics in a monolayer of squirmers confined to a boundary by gravity

Gravity-induced dynamics of a squirmer microswimmer in wall proximity

Traveling concentration pulses of bacteria in a generalized Keller–Segel model

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