The odd free surface flows of a colloidal chiral fluid

Soni, Vishal; Bililign, Ephraim; Magkiriadou, Sofia; Sacanna, Stefano; Bartolo, Denis; Shelley, Michael; Irvine, William T. M.

doi:10.1038/s41567-019-0603-8

Cited by 258 publications

(274 citation statements)

References 42 publications

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“…, 14] and sperm cells [4,15] near surfaces, and magnetotactic bacteria in rotating fields [16,17]. Artificial chiral active systems have also been developed, such as colloids [18][19][20][21][22][23][24], millimeter-scale magnets [25,26] and rotating granular particles [27][28][29][30]. Multiple numerical and theoretical studies on chiral active fluid have been carried out [27,[31][32][33][34][35][36][37][38].…”

Section: Introductionmentioning

confidence: 99%

Robust boundary flow in chiral active fluid

et al. 2020

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We perform experiments on an active chiral fluid system of self-spinning rotors in confining boundary. Along the boundary, actively rotating rotors collectively drives a unidirectional material flow. We systematically vary rotor density and boundary shape; boundary flow robustly emerges under all conditions. Flow strength initially increases then decreases with rotor density (quantified by area fraction φ); peak strength appears around a density φ = 0.65. Boundary curvature plays an important role: flow near a concave boundary is stronger than that near a flat or convex boundary in the same confinements. Our experimental results in all cases can be reproduced by a continuum theory with single free fitting parameter, which describes the frictional property of the boundary.Our results support the idea that boundary flow in active chiral fluid is topologically protected; such robust flow can be used to develop materials with novel functions. *

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Section: Introductionmentioning

confidence: 99%

Robust boundary flow in chiral active fluid

et al. 2020

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“…particles are spinning under an external rotating magnetic field [17,18]; this can be modelled by an external body torque t in the direction of spin. Next, we shall derive the relationship between the internal body torque, s, and the stress tensor, s. Consider a co-moving finite parcel of fluid occupying a region V(t).…”

Section: Intrinsicmentioning

confidence: 99%

“…In particular, the condition on s A is equivalent to the requirement of a vanishing sum of active torques (23). An additional and somewhat separate effect of activity is to allow a term which breaks Galilean invariance  µ( · ) p pin (17). This active term describes particles' swimming in the direction of p (self-advection).…”

Section: Adding Activitymentioning

confidence: 99%

“…Furthermore, there is growing literature on systems of 'active' spinners [15][16][17][18]. In these systems, particles (of the same species) are spinning in the same direction-either clockwise or anti-clockwise on a twodimensional plane.…”

Section: Introductionmentioning

confidence: 99%

“…In these systems, particles (of the same species) are spinning in the same direction-either clockwise or anti-clockwise on a twodimensional plane. These spinners can be driven by an external field such as a rotating magnetic field [17,18] or can be active because of internal motors that interact with a substrate. Our hydrodynamic theory can capture the dynamics of 'active' spinners by including the external field as a body torque-similar to treating gravity as a body force.…”

Section: Introductionmentioning

confidence: 99%

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Chiral active matter: microscopic ‘torque dipoles’ have more than one hydrodynamic description

2019

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Many biological systems, such as bacterial suspensions and actomyosin networks, form polar liquid crystals. These systems are 'active' or far-from-equilibrium, due to local forcing of the solvent by the constituent particles. In many cases the source of activity is chiral; since forcing is internally generated, some sort of 'torque dipole' is then present locally. But it is not obvious how 'torque dipoles' should be encoded in the hydrodynamic equations that describe the system at the continuum level: different authors have arrived at contradictory conclusions on this issue. In this work, we resolve the paradox by presenting a careful derivation, from linear irreversible thermodynamics, of the general equations of motion of a single-component chiral active fluid with spin degrees of freedom. We find that there is no unique hydrodynamic description for such a fluid in the presence of torque dipoles of a given strength. Instead, at least three different hydrodynamic descriptions emerge, depending on whether we decompose each torque dipole as two point torques, two force pairs, or one point torque and one force pair-where point torques create internal angular momenta of the chiral bodies (spin), whereas force pairs impart centre of mass motion that contributes to fluid velocity. By considering a general expansion of the Onsager coefficients, we also derive a new shear-elongation parameter and crosscoupling viscosity, which can lead to unpredicted phenomena even in passive polar liquid crystals. Finally, elimination of the angular variables gives an effective polar hydrodynamics with renormalized active stresses, viscosities and kinetic coefficients. Remarkably, this can include a direct contribution of chiral activity to the equation of motion for the polar order parameter, which survives even in 'dry' active systems where the fluid velocity is set to zero.

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Magnetic Manipulation of Superparamagnetic Colloids in Droplet‐Based Optical Devices

Mattich

Sendra

Galinski

et al. 2023

Advanced Optical Materials

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Magnetically assembled superparamagnetic colloids are exploited as fluid mixers, swimmers, and delivery systems in several microscale applications. The encapsulation of such colloids in droplets may open new opportunities to build magnetically controlled displays and optical components. Here, the assembly of superparamagnetic colloids inside droplets under rotating magnetic fields is studied, and this phenomenon is exploited to create functional optical devices. Colloids are encapsulated in monodisperse droplets produced by microfluidics and magnetically assembled into dynamic 2D clusters. Using an optical microscope equipped with a magnetic control setup, the effect of the magnetic field strength and rotational frequency on the size, stability, and dynamics of 2D colloidal clusters inside droplets is investigated. The results show that cluster size and stability depend on the magnetic forces acting on the structure under the externally imposed field. By rotating the cluster in specific orientations, it is possible to magnetically control the effective refractive index and the transmission of light through the colloid‐laden droplets, thus demonstrating the potential of the encapsulated colloids in optical applications.

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The odd free surface flows of a colloidal chiral fluid

Cited by 258 publications

References 42 publications

Robust boundary flow in chiral active fluid

Robust boundary flow in chiral active fluid

Chiral active matter: microscopic ‘torque dipoles’ have more than one hydrodynamic description

Magnetic Manipulation of Superparamagnetic Colloids in Droplet‐Based Optical Devices

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