Chemotaxis mediated interactions can stabilize the hydrodynamic instabilities in active suspensions

Abstract

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...

Chemotactic smoothing of collective migration

“…It therefore provides a counterpoint to previous studies investigating the ability of perturbations to instead disrupt collective migration ( Sándor et al, 2017 ; Morin et al, 2016 ; Yllanes et al, 2017 ; Bera and Sood, 2020 ; Chepizhko and Peruani, 2013 ; Chepizhko et al, 2013 ; Chepizhko and Peruani, 2015 ; Toner et al, 2018 ; Wong et al, 2014 ; Alert and Trepat, 2020 ; Alert et al, 2019 ; Driscoll et al, 2016 ; Doostmohammadi et al, 2016 ; Williamson and Salbreux, 2018 ; Miles et al, 2019 ; Subramanian et al, 2011 ; Lushi et al, 2012 ; Lushi et al, 2018 ; Ben Amar and Bianca, 2016 ; Ben Amar, 2016 ; Funaki et al, 2006 ; Brenner et al, 1998 ; Mimura and Tsujikawa, 1996 ; Stark, 2018 ). It also complements recent theoretical work describing how chemotaxis can stabilize the hydrodynamic instabilities that arise in unconfined populations of self-propelled particles ( Nejad and Najafi, 2019 ).…”

Pairing, waltzing and scattering of chemotactic active colloids

“…These results are expected to hold in higher dimensions as well. In recent works, chemical [60] and magnetic [61] interactions have been shown to compete against instabilities originating from hydrodynamic interactions. Therefore, it is important to examine how hydrodynamic effects modify the dynamics (especially in the case of bound states) that we have described in this work under more realistic geometries such as a Hele-Shaw cell.…”

Hydrochemical interactions in dilute phoretic suspensions: From individual particle properties to collective organization

“…Under these circumstances, the correlation between the flow field and the local mean orientation of the particles, u • n , averages to zero over time (Fig. 5) because geometrically isotropic swimmers do not experience shear alignment (unlike rodlike pushers or disklike pullers [42,53]) and polarize only in the far-field chemical signature of other particles.…”