A 2D suspension of active agents: the role of fluid mediated interactions

2019

Soft Matter

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Ordered phases emerged in active suspensions of polar swimmers are under long-wavelength hydrodynamic mediated instabilities. In this letter, we show that chemical molecules dissolved in aqueous suspensions, as an unavoidable part of most wet active systems, can mediate long-range interactions and subsequently stabilize the ordered phases. Chemoattractant in living suspensions and dissolved molecules producing phoretic forces in synthesized Janus suspensions are reminiscent of such molecules. Communication between swimmers through the gradients of such chemicals generated by individual swimmers, is the foundation of this stabilization mechanism. To classify the stable states of such active systems, we investigate the detailed phase diagrams for two classes of systems with momentum conserving and non-conserving dynamics. Our linear stability analysis shows how the stabilization mechanism can work for swimmers with different dynamical properties, e.g., pushers or pullers and with various static characteristics, e.g., spherical, oblate or prolate geometries.Understanding and explaining the physics of active matter have attracted many interests recently [1][2][3][4]. Active suspensions, both living and synthetic systems, are not bounded by equilibrium laws, thus, show a variety of behaviors ranging from collective self organized motion (even in two dimension) [5-7] to nontrivial rheological properties [8][9][10][11][12]. Long-range rotational order, observed in active suspensions, are under strong dynamical instabilities mediated by hydrodynamic interactions in low Reynolds wet systems [5,13]. This instability is generic in the sense that, it is not affected by any shortrange interaction but its underlying mechanism is very sensitive to the hydrodynamic details of individual swimmers. For pushers (pullers), bend (splay) fluctuations diverge and initiate the instability. Interestingly and in contrast to this hydrodynamic mediated instability, there are examples that the ordered phases can be observed experimentally. Furthermore, studying stabilization mechanisms provides guidelines for designing micro swimmers exhibiting collective ordered motion. System-size dependent fluctuations in elastic systems [14] and 2-D film confinement [15,16] provide mechanisms that can stabilize the instability.In this article, we show that chemical signaling between swimmers is another potential mechanism that can stabilize the instabilities. Phenomena of taxis (chemo, photo and etc.) that are vital activities in most living organisms [17] and phoretic interactions between active agents in suspension of artificial swimmers [18,19] can be considered as examples of such chemical signaling. Depending on the details of system under investigation, chemotaxis itself can initiate Keller-Segel type instabilities [20, 21] but there are examples showing that phoretic interactions between active agents can lead to interesting col- * najafi@iasbs.ac.ir lective behaviors [18,22,23]. Interplay between hydrodynamic and chemotaxis is investigated previo...

mentioning

confidence: 99%

Chemotaxis mediated interactions can stabilize the hydrodynamic instabilities in active suspensions

Nejad

2019

Soft Matter

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“…As a result of that theorem, true order in low dimensional equilibrium systems is not possible [19], but a theoretical work based on renormalization group analysis by Toner and Tu has revealed a physical scenario in which, non-equilibrium nature of active systems can provide conditions for true order in lower dimensions [20]. Usually, short-range interactions are responsible for developing ordered phases but the role of long-range interactions, needs to be considered carefully [21,22,23,24]. In a system composed of active particles suspended in aqueous media, hydrodynamic interactions provide long-range forces that can propagate like 1/r 2 to long distances.…”

Section: Introductionmentioning

confidence: 99%

“…Long-wavelength instabilities in both dry and wet active systems have been studied using dierent approaches [28][29][30][31][32][33][34][35].…”

Section: Introductionmentioning

confidence: 99%

Long-wavelength instabilities in a system of interacting active particles

Fazli

2018

J. Stat. Mech.

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Based on a microscopic model, we develop a continuum description for a suspension of microscopic self propelled particles. With this continuum description we study the role of long-range interactions in destabilizing macroscopic ordered phases that are developed by short-range interactions. Long-wavelength fluctuations can destabilize both isotropic and also symmetry broken polar phase in a suspension of dipolar particles. The instabilities in a suspension of pullers (pushers) arise from splay (bend) fluctuations. Such instabilities are not seen in a suspension of quadrupolar particles.

“…Reduction or increase in the effective apparent viscosity [5,6] and non-Newtonian behavior [7] are among the main important features of such rheological responses. In addition to the rheological properties, a concentrated population of suspended active agents as a nonlinear dynamical system exhibit fascinating spatial and temporal patterns [8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Effective viscosity of a two-dimensional suspension of interacting active particles

Moradi

2017

Phys. Rev. E

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Suspensions of hydrodynamical active particles exhibit interesting rheological properties. For a dilute suspension of microswimmers, it has been shown that the effective viscosity of the suspension depends on the volume fraction of swimmers, and it behaves differently for pushers and pullers. Here we develop a theoretical framework to study the rheological properties of an interacting suspension. Taking into account the hydrodynamic interaction between swimmers and considering the small Péclet number condition, we calculate the effective viscosity of a two-dimensional suspension. For a dilute suspension, a perturbative result is obtained up to the second order of the surface fraction of swimmers. Our results show that the effective viscosity for the suspension can be very different for pushers and pullers.