Active microrheology of networks composed of semiflexible polymers: Theory and comparison with simulations

Ter-Oganessian, N. V.; Pink, David A.; Boulbitch, Alexei

doi:10.1103/physreve.72.041511

“…Mesoscale simulations provide a way of investigating the underlying microscopic mechanisms relevant for colloidal motion in polymeric fluids, but currently there are very few simulations of driven colloids in simple polymer solutions [24,30]. It has been demonstrated that multiparticle collision dynamics (MPCD) is an efficient method to simulate fluctuating hydrodynamics of colloids (see, e.g.…”

Section: Introductionmentioning

confidence: 99%

Driven spheres, ellipsoids and rods in explicitly modeled polymer solutions

Zöttl¹,

Yeomans²

2019

J. Phys.: Condens. Matter

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Understanding the transport of driven nano-and micro-particles in complex fluids is of relevance for many biological and technological applications. Here we perform hydrodynamic multiparticle collision dynamics simulations of spherical and elongated particles driven through polymeric fluids containing different concentrations of polymers. We determine the mean particle velocities which are larger than expected from Stokes law for all particle shapes and polymer densities. Furthermore we measure the fluid flow fields and local polymer density and polymer conformation around the particles. We find that polymer-depleted regions close to the particles are responsible for an apparent tangential slip velocity which accounts for the measured flow fields and transport velocities. A simple two-layer fluid model gives a good match to the simulation results.

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“…Recently, using a microstructure-based continuum mechanics approach, Palmer and Boyce reproduced many of the rheological properties of actin networks observed in experiments [27] . The viscoelastic behavior of semi-flexible networks was also investigated using dissipative particle dynamics and the concept of microbead rheology, where scale-free behavior of the bead displacement was observed [28] , [29] . To date, however, most of these models neither explicitly take into account ACP mechanics nor systematically account for thermal fluctuations, nor have they been used to explore the effects of finite prestress on viscoelasticity, all of which are potentially important factors governing matrix viscoelasticity.…”

Section: Introductionmentioning

confidence: 99%

Computational Analysis of Viscoelastic Properties of Crosslinked Actin Networks

Kim

¹

,

Hwang²,

Lee³

et al. 2009

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Mechanical force plays an important role in the physiology of eukaryotic cells whose dominant structural constituent is the actin cytoskeleton composed mainly of actin and actin crosslinking proteins (ACPs). Thus, knowledge of rheological properties of actin networks is crucial for understanding the mechanics and processes of cells. We used Brownian dynamics simulations to study the viscoelasticity of crosslinked actin networks. Two methods were employed, bulk rheology and segment-tracking rheology, where the former measures the stress in response to an applied shear strain, and the latter analyzes thermal fluctuations of individual actin segments of the network. It was demonstrated that the storage shear modulus (G′) increases more by the addition of ACPs that form orthogonal crosslinks than by those that form parallel bundles. In networks with orthogonal crosslinks, as crosslink density increases, the power law exponent of G′ as a function of the oscillation frequency decreases from 0.75, which reflects the transverse thermal motion of actin filaments, to near zero at low frequency. Under increasing prestrain, the network becomes more elastic, and three regimes of behavior are observed, each dominated by different mechanisms: bending of actin filaments, bending of ACPs, and at the highest prestrain tested (55%), stretching of actin filaments and ACPs. In the last case, only a small portion of actin filaments connected via highly stressed ACPs support the strain. We thus introduce the concept of a ‘supportive framework,’ as a subset of the full network, which is responsible for high elasticity. Notably, entropic effects due to thermal fluctuations appear to be important only at relatively low prestrains and when the average crosslinking distance is comparable to or greater than the persistence length of the filament. Taken together, our results suggest that viscoelasticity of the actin network is attributable to different mechanisms depending on the amount of prestrain.

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“…After polymerization, we applied a coarse-graining procedure explained in Methods to cover a larger system size. In the resulting networks, the filament length distribution and the network morphology [35] bear a closer resemblance to reconstituted actin gels ( Figure 1) than those generated by the random placement of equal-length filaments [24][25][26]29,32]. F-actins and ACPs in our model are characterized by introducing bending and extensional stiffnesses (Table 1).…”

Section: Model Overviewmentioning

confidence: 99%

“…Therefore, the use of a geometrically identical network for all simulations enables us to systematically control and isolate the effect of a given parameter. In other studies, actin networks have been generated by the random placement of equal-length filaments [24][25][26]29,32].…”

Section: Preparation Of a Network To Estimate Viscoelastic Propertiesmentioning

confidence: 99%

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Computational Analysis of a Cross-linked Actin-like Network

Kim

¹

,

Hwang

²

,

Kamm

³

2007

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Mechanical force plays an important role in the physiology of eukaryotic cells whose dominant structural constituent is the actin cytoskeleton composed mainly of actin and actin crosslinking proteins (ACPs). Thus, knowledge of rheological properties of actin networks is crucial for understanding the mechanics and processes of cells. We used Brownian dynamics simulations to study the viscoelasticity of crosslinked actin networks.Two methods were employed, bulk rheology and segment-tracking rheology where the former measures the stress in response to an applied shear strain, and the latter analyzes thermal fluctuations of individual actin segments of the network. Bulk rheology demonstrates that the storage shear modulus (G') increases more by the addition of ACPs that form orthogonal crosslinks than for those that form parallel bundles. In networks with orthogonal crosslinks, as crosslink density increases, the power law exponent of G' as a function of the oscillation frequency decreases from 0.75, which reflects the transverse thermal motion of actin filaments, to near zero at low frequency. Under increasing prestrain, the network becomes more elastic, and three regimes of behavior are observed, each dominated by different mechanisms: bending of actin filaments, bending of ACPs, and at the highest prestrain tested (55%), stretching of actin filaments and ACPs. In the last case, only a small portion of actin filaments connected via highly stressed ACPs support the strain. We thus introduce the concept of a 'supportive framework,' as a subset of the full network, which is responsible for high elasticity. Notably, entropic effects due to thermal fluctuations appear to be important only at relatively low prestrains and when the average crosslinking distance is comparable to or greater than the persistence length of the filament. Taken together, our results suggest that viscoelasticity of the actin network is attributable to different mechanisms depending on the amount of prestrain. Author SummaryThe actin cytoskeleton provides structural integrity to a cell, is highly dynamic, and plays a central role in a wide variety of essential biological phenomena. For years, researchers have studied the mechanics of the cytoskeleton by creating reconstituted actin gels with one or more of the crosslinking proteins found in cells. These reconstituted gels, however, failed to replicate many aspects of cell behavior. Recent studies have shown that tension, or prestrain, on the cytoskeleton due to actomyosin contractility contributes to the observed stiffness. Still, our understanding of cytoskeletal mechanics is incomplete, and 3 many observed phenomena cannot be explained by existing models. Here, we introduce a Brownian dynamics simulation of a three-dimensional network containing actin filaments and crosslinking proteins. By varying model parameters, we show relative contributions of thermal fluctuations and the stiffness of filaments and crosslinks. Under conditions that replicate those in a cell, properties of the cro...

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Active microrheology of networks composed of semiflexible polymers: Theory and comparison with simulations

Cited by 17 publications

References 50 publications

Driven spheres, ellipsoids and rods in explicitly modeled polymer solutions

Driven spheres, ellipsoids and rods in explicitly modeled polymer solutions

Computational Analysis of Viscoelastic Properties of Crosslinked Actin Networks

Computational Analysis of a Cross-linked Actin-like Network

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