Active bodies in viscous fluids interact hydrodynamically through self-generated flows. Here we study spontaneous aggregation induced by hydrodynamic flow in a suspension of stiff, apolar, active filaments. Lateral hydrodynamic attractions in extensile filaments lead, independent of volume fraction, to anisotropic aggregates which translate and rotate ballistically. Lateral hydrodynamic repulsion in contractile filaments lead, with increasing volume fractions, to microstructured states of asters, clusters, and incipient gels where, in each case, filament motion is diffusive. Our results demonstrate that the interplay of active hydrodynamic flows and anisotropic excluded volume interactions provides a generic nonequilibrium mechanism for hierarchical self-assembly of active soft matter.Filamentous structures along which chemical energy is converted to mechanical motion are found in many biological contexts such as actin filaments, molecular motors walking on microtubules, flagellar bundles and ciliary hairs. These cellular components maintain structure, influence signaling, and provide motile forces. Much recent effort has been directed at synthesising biomimetic analogues of such components. Motor-microtubule mixtures [1] and self-assembled motor-microtubules bundles capable of motility [2] are two examples where dynamic structures remarkably similar to those occurring in vivo have been synthesized in the laboratory. These emergent structures are maintained by nonequilibrium "active" forces generated by the consumption of chemical energy. Hydrodynamic flow provides a way of transmitting nonequilibrium forces and its role in creating and maintaining dynamic structures remains relatively unexplored.Our present theoretical understanding of hydrodynamics in nonequilibrium energy-converting systems is based largely on continuum [3] or kinetic [4] theories that prescribe, respectively, the spatiotemporal evolution of coarse-grained fields or distribution functions. These theories predict long wavelength collective phenomena like instabilities [5,6], the onset of flows [7] through spontaneous symmetry breaking [8] and the continuous generation of topological defects [9]. The complementary domain of short wavelength phenomena where near-field hydrodynamics, the shape of the active body and excluded volume interactions are important, has been much less studied [10,11]. This domain is relevant in situations where particle numbers are not thermodynamically large and a hydrodynamic limit is not necessarily attained, as for example in the cell. Theoretical descriptions that do not presume the existence of continuum and hydrodynamic limits, then, are required to provide insight into the physics of short wavelength phenomena in active matter.In this Letter we study short wavelength aggregation phenomena in suspensions of stiff, apolar, active filaments using a description that resolves individual filaments and includes their hydrodynamic and excluded volume interactions. Our model for an active filament consists of rigidly con...

Probe rheology experiments, in which the dynamics of a small amount of probe chains dissolved in immobile matrix chains is discussed, have been performed for the development of molecular theories for entangled polymer dynamics. Although probe chain dynamics in probe rheology is considered hypothetically as single chain dynamics in fixed tube-shaped confinement, it has not been fully elucidated. For instance, the end-to-end relaxation of probe chains is slower than that for monodisperse melts, unlike the conventional molecular theories. In this study, the viscoelastic and dielectric relaxations of probe chains were calculated by primitive chain network simulations. The simulations semi-quantitatively reproduced the dielectric relaxation, which reflects the effect of constraint release on the end-to-end relaxation. Fair agreement was also obtained for the viscoelastic relaxation time. However, the viscoelastic relaxation intensity was underestimated, possibly due to some flaws in the model for the inter-chain cross-correlations between probe and matrix chains.

Although it has not been frequently discussed, contributions of the orientational cross-correlation (OCC) between entangled polymers are not negligible in the relaxation modulus. In the present study, OCC contributions were investigated for 4- and 6-arm star-branched and H-branched polymers by means of multi-chain slip-link simulations. Owing to the molecular-level description of the simulation, the segment orientation was traced separately for each molecule as well as each subchain composing the molecules. Then, the OCC was calculated between different molecules and different subchains. The results revealed that the amount of OCC between different molecules is virtually identical to that of linear polymers regardless of the branching structure. The OCC between constituent subchains of the same molecule is significantly smaller than the OCC between different molecules, although its intensity and time-dependent behavior depend on the branching structure as well as the molecular weight. These results lend support to the single-chain models given that the OCC effects are embedded into the stress-optical coefficient, which is independent of the branching structure.

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For entangled polymers, there exist considerable contributions of the cross-correlation between different chains in the relaxation modulus, though the contribution is neglected in the single-chain models. Earlier studies have suggested that the cross-correlation is due to the inter-chain interactions such as the force balance around entanglements and the osmotic force suppressing the density fluctuations. However, the origin of the cross-correlation has been yet to be clarified. In this study, a new multi-chain slip-link model has been developed for the simulations of polymer dynamics without the inter-chain forces while the creation of entanglement is performed according to the geometrical manner and the local equilibration. The simulations reproduced the qualitative features of entangled polymer dynamics for the molecular weight dependence of the longest relaxation time and the diffusion coefficient, just as expected from the success of the earlier single-chain models without the inter-chain forces. On the other hand, the remarkable feature of the model is that the cross-correlation exists between chains, suggesting that the cross-correlation is generated via the local equilibration at the creation of entanglement even without the global equilibration by the inter-chain forces.