Elementary Flow Field Profiles of Micro-Swimmers in Weakly Anisotropic Nematic Fluids: Stokeslet, Stresslet, Rotlet and Source Flows

Kos, Žiga; Ravnik, Miha

doi:10.3390/fluids3010015

Cited by 16 publications

(16 citation statements)

References 60 publications

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“…To be able to make analytical progress below, we consider the director permanently and spatially homogeneously aligned along one global axis 79,105,132 . For instance, this could be achieved by a strong aligning homogeneous external electric (under insulating conditions) or magnetic field 77,92,93 .…”

Section: Low-reynolds-number Flows In the Anisotropic Mediummentioning

confidence: 99%

“…The emerging reorientation of the swimmer has been attributed to the hydrodynamic coupling between the flow field induced by the squirmer and the anisotropy described by the liquid-crystalline viscosities. In the weakly anisotropic limit, the behavior of a general axisymmetric microswimmer described as a linear combination of higher-order singularity solutions of the Stokes flow has further been considered 132 .…”

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confidence: 99%

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Dynamics of a simple model microswimmer in an anisotropic fluid: Implications for alignment behavior and active transport in a nematic liquid crystal

Daddi-Moussa-Ider

Menzel

2018

Phys. Rev. Fluids

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Several recent experiments investigate the orientational and transport behavior of self-driven bacteria and colloidal particles in nematic liquid crystals. Correspondingly, we study theoretically the dynamics of a minimal model microswimmer in a uniaxially anisotropic fluid. As a first step, the hydrodynamic Green's function providing the resulting fluid flow in response to a localized force acting on the anisotropic fluid is derived analytically. On this basis, the behavior of both puller-and pusher-type microswimmers in the anisotropic fluid is analyzed. Depending on the propulsion mechanism and the relative magnitude of different involved viscosities, we find alignment of the swimmers parallel or perpendicular to the anisotropy axis. Particularly, also an oblique alignment is identified under certain circumstances. The observed swimmer reorientation results from the hydrodynamic coupling between the self-induced fluid flow and the anisotropy of the surrounding fluid, which distorts the self-generated flow field. We support parts of our results by a simplified linear stability analysis. Our theoretical predictions are in qualitative agreement with recent experimental observations on swimming bacteria in nematic liquid crystals. They support the objective of utilizing the, possibly switchable, anisotropy of a host fluid to guide individual microswimmers and active particles along a requested path, enabling controlled active transport. I. INTRODUCTIONActive particles have the ability to move autonomously in a surrounding fluid by converting energy into directed motion. Artificial self-propelled nano-and microscale machines hold great promise for future medical research to reach otherwise inaccessible areas of the body to perform delicate and precise tasks. Prospective biomedical applications are precision nanosurgery, biopsy, and transport of radioactive substances to tumor areas and inflammation sites 1-3 . Over the last few decades, significant research efforts have been devoted to investigate the behavior of self-propelling active particles due to their importance and relevance as model systems for transport and locomotion in the micro-and nano-scale world; for recent reviews see Refs. 4-12. Unusual macroscopic signatures and intriguing spatiotemporal patterns emerge from the interaction between several active particles. For instance, the onset of collective motion 13-19 , formation of dynamic clusters 20-26 , wave patterns 27-30 , laning 31-35 , motility-induced phase separation 36-41 , swarming 42-44 , and active turbulence 45-52 are observed. In many cases, artificial self-driven particles and swimming microorganisms have to propel through complex fluids, such as polymer gels and viscoelastic microemulsions [53][54][55][56][57][58][59][60][61][62][63] . Notable examples include sperm navigation through the mammalian female reproductive tract 64,65 , bacteria locomotion in biofilm matrices composed of extracellular polymeric substances 66,67 , nematode movement in soil 68,69 , and the motion of synthetic mic...

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Section: Low-reynolds-number Flows In the Anisotropic Mediummentioning

confidence: 99%

mentioning

confidence: 99%

Dynamics of a simple model microswimmer in an anisotropic fluid: Implications for alignment behavior and active transport in a nematic liquid crystal

Daddi-Moussa-Ider

Menzel

2018

Phys. Rev. Fluids

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“…However, as demonstrated over the course of development of the field, such applied interests could often lead to fundamental insights, offering a rich landscape of unexpected dynamic manifestations, including generation [16] and sustenance [22,24] of transverse pressure gradients, anomalous viscoelasticity [25,26] and emergent biaxiality [27] and tunable flow shaping [28]. A key determinant of the effective rheological properties is the LC backflow mechanism, which only recently has been characterized in microfluidic setups [28,29], with potentially far-reaching ramifications in active and biological systems [30][31][32]. LC-s present distinct material advantages, thanks to the backflow-mediated coupling, over the isotropic counterparts.…”

Section: Introductionmentioning

confidence: 99%

Surface anchoring mediates bifurcation in nematic microflows within cylindrical capillaries

2021

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Capillary microflows of liquid crystal phases are central to material, biological and bio-inspired systems.Despite their fundamental and applied significance, a detailed understanding of the stationary behaviour of nematic liquid crystals (NLC-s) in cylindrical capillaries is still lacking. Here, using numerical simulations based on the continuum theory of Leslie, Ericksen and Parodi, we investigate stationary NLC flows within cylindrical capillaries possessing homeotropic (normal) and uniform planar anchoring conditions. By considering the material parameters of the flow-aligning NLC, 5CB, we report that instead of the expected, unique director field monotonically approaching the alignment angle over corresponding Ericksen numbers (dimensionless number capturing viscous v/s elastic effects), a second solution emerges below a threshold flow rate (or applied pressure gradient). We demonstrate that the onset of the second solution, a nematodynamic bifurcation yielding energetically degenerate director tilts at the threshold pressure gradient, can be controlled by the surface anchoring and the flow driving mechanism (pressure-driven or volume-driven). For homeotropic surface anchoring, this alternate director field orients against the alignment angle in the vicinity of the capillary center; while in the uniform planar case, the alternate director field extends throughout the capillary volume, leading to reduction of the flow speed with increasing pressure gradients. While the practical realization and utilization of such nematodynamic bifurcations still await systematic exploration, signatures of the emergent rheology have been reported previously within microfluidic environments, under both

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“…Pusher-(puller-) type force dipoles represent the leading-order flow field of Escherichia coli (Chlamydomonas) [53]. Very recently, Kos and Ravnik [29] studied analytically the flow field around a force dipole in nematic liquid crystal assuming spatially uniform director field. However, there are physical situations in which the presence of microswimmer deforms the director field not only due to the surface anchoring con- dition, but also due to the velocity-orientation coupling [54,55].…”

Section: F Force Dipolementioning

confidence: 99%

“…We also study the flow field and director orientation around pusher-type and puller-type force dipoles in nematic fluid. Very recently, Kos and Ravnik [29] studied elementary flow fields (e.g. Stokeslet, stresslet, rotlet, etc.)…”

Section: Introductionmentioning

confidence: 99%

Multiparticle collision dynamics for tensorial nematodynamics

Mandal

Mazza

2019

Phys. Rev. E

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Liquid crystals establish a nearly unique combination of thermodynamic, hydrodynamic, and topological behavior. This poses a challenge to their theoretical understanding and modeling. The arena where these effects come together is the mesoscopic (micron) scale. It is then important to develop models aimed at capturing this variety of dynamics. We have generalized the particle-based multiparticle collision dynamics (MPCD) method to model the dynamics of nematic liquid crystals. Following the Qian-Sheng theory [Phys. Rev. E 58, 7475 (1998)] of nematics, the spatial and temporal variations of the nematic director field and order parameter are described by a tensor order parameter. The key idea is to assign tensorial degrees of freedom to each MPCD particle, whose mesoscopic average is the tensor order parameter. This new nematic-MPCD method includes backflow effect, velocity-orientation coupling and thermal fluctuations. We validate the applicability of this method by testing: (i) the nematic-isotropic phase transition, (ii) the flow alignment of the director in shear and Poiseuille flows, and (iii) the annihilation dynamics of a pair of line defects. We find excellent agreement with existing literature. We also investigate the flow field around a force dipole in a nematic liquid crystal, which represents the leading-order flow field around a forcefree microswimmer. The anisotropy of the medium not only affects the magnitude of velocity field around the force dipole, but can also induce hydrodynamic torques depending on the orientation of dipole axis relative to director field. A force dipole experiences a hydrodynamic torque when the dipole axis is tilted with respect to the far-field director. The direction of hydrodynamic toque is such that the pusher-(or puller-) type force dipole tends to orient along (or perpendicular to) the director field. Our nematic-MPCD method can have far-reaching implications not only in modeling of nematic flows, but also to study the motion of colloids and microswimmers immersed in an anisotropic medium. arXiv:1903.04514v1 [cond-mat.soft]

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Elementary Flow Field Profiles of Micro-Swimmers in Weakly Anisotropic Nematic Fluids: Stokeslet, Stresslet, Rotlet and Source Flows

Cited by 16 publications

References 60 publications

Dynamics of a simple model microswimmer in an anisotropic fluid: Implications for alignment behavior and active transport in a nematic liquid crystal

Dynamics of a simple model microswimmer in an anisotropic fluid: Implications for alignment behavior and active transport in a nematic liquid crystal

Surface anchoring mediates bifurcation in nematic microflows within cylindrical capillaries

Multiparticle collision dynamics for tensorial nematodynamics

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