Theory of Active Suspensions

Saintillan, David; Shelley, Michael

doi:10.1007/978-1-4939-2065-5_9

Cited by 67 publications

(59 citation statements)

References 148 publications

(216 reference statements)

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“…A common feature of many active matter systems is their ability to spontaneously self-organize into complex dynamic mesoscale structures above a certain density [1][2][3]. Such is the case of suspensions of motile bacteria [4][5][6][7], cellular extracts [8][9][10], collections of colloidal rollers [11,12], shaken grains [13,14], among many others.…”

Section: Introductionmentioning

confidence: 99%

Geometric control of active collective motion

2017

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Recent experimental studies have shown that confinement can profoundly affect selforganization in semi-dilute active suspensions, leading to striking features such as the formation of steady and spontaneous vortices in circular domains and the emergence of unidirectional pumping motions in periodic racetrack geometries. Motivated by these findings, we analyze the two-dimensional dynamics in confined suspensions of active self-propelled swimmers using a mean-field kinetic theory where conservation equations for the particle configurations are coupled to the forced Navier-Stokes equations for the self-generated fluid flow. In circular domains, a systematic exploration of the parameter space casts light on three distinct states: equilibrium with no flow, stable vortex, and chaotic motion, and the transitions between these are explained and predicted quantitatively using a linearized theory. In periodic racetracks, similar transitions from equilibrium to net pumping to traveling waves to chaos are observed in agreement with experimental observations and are also explained theoretically. Our results underscore the subtle effects of geometry on the morphology and dynamics of emerging patterns in active suspensions and pave the way for the control of active collective motion in microfluidic devices.

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Section: Introductionmentioning

confidence: 99%

Geometric control of active collective motion

2017

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“…This does not mean the HI are not important, only that their effect is quantitative, not qualitative. We use a Smoluchowski-level analysis to model the active suspension and compute the long-time diffusivity of a passive probe using generalized Taylor dispersion theory and expansions in orientational tensor harmonics [2,17,18]. The derivation and complete expressions for the active diffusivity of the probe are given in the Appendix; here we focus on limiting behaviors.…”

Section: Introductionmentioning

confidence: 99%

Tracer diffusion in active suspensions

Burkholder

Brady

2017

Phys. Rev. E

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We study the diffusion of a Brownian probe particle of size R in a dilute dispersion of active Brownian particles of size a, characteristic swim speed U 0 , reorientation time τ R , and mechanical energy k s T s = ζ a U 2 0 τ R /6, where ζ a is the Stokes drag coefficient of a swimmer. The probe has a thermal diffusivity D P = k B T /ζ P , where k B T is the thermal energy of the solvent and ζ P is the Stokes drag coefficient for the probe. When the swimmers are inactive, collisions between the probe and the swimmers sterically hinder the probe's diffusive motion. In competition with this steric hindrance is an enhancement driven by the activity of the swimmers. The strength of swimming relative to thermal diffusion is set by Pe s = U 0 a/D P . The active contribution to the diffusivity scales as Pe 2 s for weak swimming and Pe s for strong swimming, but the transition between these two regimes is nonmonotonic. When fluctuations in the probe motion decay on the time scale τ R , the active diffusivity scales as k s T s /ζ P : the probe moves as if it were immersed in a solvent with energy k s T s rather than k B T .

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“…The great example of pullers is that of algae. The rheology of suspensions containing particles that swim as pullers is similar to that of non-active colloidal suspensions, in which the suspension viscosity increases with the volume fraction of the particles [17].…”

Section: Introductionmentioning

confidence: 86%

“…Active matter is an emerging new area of research, which includes under its umbrella various types of systems such as colloidal particles and microorganisms that convert some form of energy (typically chemical) into mechanical work. Examples of such systems include motile bacteria, microscopic algae, solutions of motor proteins, human sperm, reactive driven colloidal suspensions, self-propelled nanotubes and shaken granular materials [17]. The generation of mechanical work by active matter results in a change in their microstructure, either by direct contact or by long range and complex hydrodynamic interactions.…”

Section: Introductionmentioning

confidence: 99%

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Self-propelled nanofluids a path to a highly effective coolant

Hasadi

Crapper

2017

Applied Thermal Engineering

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We propose a new self-propelled nanofluid having advantageous thermal and rheological properties at the same time. The nanofluid consists of a low volume fraction of self-propelled particles known as Artificial Bacterial Flagella (ABF), which will swim as pushers in a manner similar to the swimming of E-coil microorganisms with flagella. A theoretical model is introduced, describing the mechanisms responsible for the reduction of viscosity. The model shows that the swimming velocity of the particle and its geometry play an essential role in the reduction of the suspension viscosity. The results obtained from the theoretical model compare qualitatively with experiments in the literature. The model shows a significant decrease in viscosity at very low volume fractions, and that the viscosity of the suspension is reduced as the volume fraction of the particles increases. Using an in-house finite volume code, we numerically simulate natural convection effects in our ABF self-propelled nanofuid inside a square cavity heated from its vertical sides. Simulations are conducted at volume fractions of 0.7%, 0.8% and 0.83%, comparing the performance of a self-propelled nanofluid with conventional non-active nanofluids (i.e. carbon nanotubes in water). The results show that the heat transfer rate measured by the Nusselt number is three times higher than for the case of classical nanofluids and pure water at the same operating conditions and 0.83% volume fraction of particles. Also, due to the very dilute volume fractions of particles in the proposed nanofluid, their stability can endure for long operating times. There is also a significant decrease in the viscosity (around 25 times lower than water) which will result in a significant reduction in the pumping power.2

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Theory of Active Suspensions

Cited by 67 publications

References 148 publications

Geometric control of active collective motion

Geometric control of active collective motion

Tracer diffusion in active suspensions

Self-propelled nanofluids a path to a highly effective coolant

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