Hydrodynamics of active particles in viscosity gradients

Shaik, Vaseem A.; Elfring, Gwynn J.

doi:10.1103/physrevfluids.6.103103

Cited by 20 publications

(24 citation statements)

References 35 publications

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“…In analogy to ray optics, the refraction of the trajectory is always towards the medium of lower resistance. A similar preference was also shown by active particles in linear or diffuse viscosity gradients 24 , 40 , 41 , and as we will show below (for pullers), matches well with experimental observations of Chlamydomonas Reinhardtii algae swimming across sharp viscosity gradients 25 . In contrast, studies done where the propulsive force is fixed, both for swimmers in diffuse viscosity gradients and gliders across a frictional substrate, show reorientation towards higher viscosities as one might expect due purely to the modulation of drag.…”

Section: Resultssupporting

confidence: 90%

“…Conversely, for all orientations flow to . This means that no matter the orientation or position, the particle is always reorienting to align along and point in the direction of the lower viscosity, consistent with results for squirmers in weak viscosity gradients 40 , 41 . Because , the reorientation rate decreases very quickly with distance from the interface, and thus the reorientation process is ultimately dominated by contact with the interface.…”

Section: Resultssupporting

confidence: 85%

“…We found that pushers, pullers and neutral swimmers all interact similarly with the interface. Swimmers are generically reoriented towards the region of lower viscosity (as found in previous studies with weak gradients 40 , 41 ). As a result, if active particles approach a viscosity interface at a sufficiently shallow angle they can be reflected if swimming from low to high viscosity; otherwise, they simply cross the interface undergoing a degree of reorientation set by the relative viscosity difference.…”

Section: Discussionsupporting

confidence: 68%

“…For instance, Chlamydomonas Reinhardtii algae, with a characteristic size of m, traveling at a body length per second take to approach and cross the interface while the salinity gradients take to vanish 25 . We assume that the fluid viscosity is prescribed and steady, and not significantly disturbed by the presence and activity of the moving particle as in previous work 39 – 41 ; however, this is an uncontrolled approximation because we assume that the sharp gradient persists, nevertheless we will show that this reasonably captures the experimental observations 25 . We note that the viscosity gradients considered here are distinct from the long-lasting viscosity differences that can exist at the interface between immiscible fluids with non-negligible surface tension that can dramatically affect (and even prevent) the particle crossing 45 – 48 .…”

Section: A Model For Active Particles Crossing Sharp Viscosity Gradientsmentioning

confidence: 93%

“…Subsequent work modeled active particles as spherical squirmers, where the particle activity that generates thrust is included but abstracted to a representative surface slip, which is a particularly apt representation of active particles that do not undergo large changes of geometry such as diffusiophoretic Janus particles and ciliated organisms like Paramecium or Opalina . In this case the interaction of the spatially varying viscosity with the active slip boundary conditions on a squirmer generically resulted in negative viscotaxis 40 , 41 . A different swimmer, Taylor’s swimming sheet speeds up while moving along or against the gradients 42 .…”

Section: Introductionmentioning

confidence: 96%

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Active particles crossing sharp viscosity gradients

Gong

Shaik

Elfring

2023

Sci Rep

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Active particles (living or synthetic) often move through inhomogeneous environments, such as gradients in light, heat or nutrient concentration, that can lead to directed motion (or taxis). Recent research has explored inhomogeneity in the rheological properties of a suspending fluid, in particular viscosity, as a mechanical (rather than biological) mechanism for taxis. Theoretical and experimental studies have shown that gradients in viscosity can lead to reorientation due to asymmetric viscous forces. In particular, recent experiments with Chlamydomonas Reinhardtii algae swimming across sharp viscosity gradients have observed that the microorganisms are redirected and scattered due to the viscosity change. Here we develop a simple theoretical model to explain these experiments. We model the swimmers as spherical squirmers and focus on small, but sharp, viscosity changes. We derive a law, analogous to Snell’s law of refraction, that governs the orientation of active particles in the presence of a viscosity interface. Theoretical predictions show good agreement with experiments and provide a mechanistic understanding of the observed reorientation process.

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Section: Resultssupporting

confidence: 90%

Section: Resultssupporting

confidence: 85%

Section: Discussionsupporting

confidence: 68%

Section: A Model For Active Particles Crossing Sharp Viscosity Gradientsmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 96%

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Active particles crossing sharp viscosity gradients

Gong

Shaik

Elfring

2023

Sci Rep

Self Cite

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Elastic interactions compete with persistent cell motility to drive durotaxis

Bose,

Wang,

et al. 2024

Biophysical Journal

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Hydrodynamic particle interactions in linear and radial viscosity gradients

Ziegler

Smith

2022

J. Fluid Mech.

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We present a versatile perturbative calculation scheme to determine the leading-order correction to the mobility matrix for particles in a low-Reynolds-number fluid with spatially variant viscosity. To this end, we exploit the Lorentz reciprocal theorem and the reflection method in the far field approximation. To demonstrate how to apply the framework to a particular choice of a viscosity field, we first study particles in a finite-size, interface-like, linear viscosity gradient. The extent of the latter should be significantly larger than the particle separation. Both situations of symmetrically and asymmetrically placed particles within such an odd symmetric viscosity gradient are considered. As a result, long-range flow fields are identified that decay by one order slower than their constant-viscosity counterparts. Self-mobilities for particle rotations and translations are affected, while for asymmetric placement, additional correction appears for the latter. The mobility terms associated with hydrodynamic interactions between the particles also need to be corrected, in a placement-specific manner. While the results are derived for the system of two particles, they apply also to many-particle systems. Furthermore, we treat the viscosity gradients induced by two particles with temperatures different from that of the surrounding fluid. Assuming a linear relation between fluid temperature and viscosity, we find that both the self-mobilities of the particles as well as the mobility terms for hydrodynamic interactions increase for hot particles and decrease for cold particles.

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Hydrodynamics of active particles in viscosity gradients

Cited by 20 publications

References 35 publications

Active particles crossing sharp viscosity gradients

Active particles crossing sharp viscosity gradients

Elastic interactions compete with persistent cell motility to drive durotaxis

Hydrodynamic particle interactions in linear and radial viscosity gradients

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