Soft Matter最新文献

Cells have the ability to sense and respond to various mechanical cues from their immediate surroundings. One of the manifestations of such a process, which is also known as “mechanosensing”, is directed cell migration. Various biological processes have been shown to be controlled by extracellular matrix (ECM) stiffness. Substrates with a high stiffness gradient have been used as a platform to investigate cellular motion in response to mechanical cues. However, creating a cell scale stiffness gradient in such a cell adhesion friendly substrate still remains elusive. In this study, we present a simple and versatile method for fabricating substrates with a periodically varying stiffness profile at the cellular scale, featuring customizable high stiffness gradients. Fibroblast cells, when presented with such continuous yet anisotropic variation of stiffness, preferentially position their nuclei in stiffer regions of the substrate and align themselves along the direction of the lowest rigidity gradient. Furthermore, when the rigidity of the substrate is sufficiently high, cells exhibit less sensitivity to stiffness gradients, with their elongation and nuclear positioning becoming independent of stiffness variations. Overall, our experimental results reveal new insights into the process of cellular mechanosensing where the cell-scale gradient drives strong positional and orientational order.

We use coarse-grained molecular dynamics to isolate how azobenzene isomer identity (cis vs. trans) modulates polymer dynamics in a guest–host setting without covalent attachment and without explicit photoisomerization. Segmental relaxation is quantified from the incoherent intermediate scattering function F_s(k,t), with relaxation times τ(T) extracted from the F_s(k,τ) = e⁻¹ criterion, fitted by Vogel–Fulcher–Tammann, and a glass-transition temperature T_g defined by a standard operational threshold. Across compositions, global structure (density and pair correlations) is nearly isomer-invariant. In contrast, within our model, cis systems exhibit systematically shorter τ and lower T_g than trans—differences consistent with a localized dynamic facilitation near chromophores. Voronoi analysis shows that the average monomer free volume around azobenzene is essentially insensitive to isomer identity, whereas cis chromophores occupy larger Voronoi cells at low T. Isoconfigurational ensembles (propensity analysis) reveal that monomers in the first-neighbor shell of cis are more mobile than near trans, and that immobilizing the chromophores suppresses this contrast. Overall, in this fixed-isomer equilibrium setting, our results cannot support a purely homogeneous free-volume softening between isomers (and, by construction, do not test illumination-induced macroscopic stress gradients); instead they point to a local, cooperative, mobility-dependent pathway that provides a geometry-only baseline for the still-debated microscopic origin of light-driven mass transport in azobenzene materials.

During interphase, the cell nucleus exhibits spatial compartmentalization between transcriptionally active euchromatin and transcriptionally repressed heterochromatin. In conventional nuclear organization, euchromatin is enriched in the nuclear interior, while heterochromatin – approximately 50% denser – resides near the periphery. The nuclear lamina, a deformable structural shell, further modulates peripheral chromatin organization. Here, we investigate a chromatin model in which an active, crosslinked polymer is tethered to a deformable lamina shell. We show that contractile motor activity, shell deformability, and the spatial distribution of crosslinks jointly determine compartmentalization. Specifically, a radial crosslink density gradient, even with a small increase toward the periphery, coupled with motor activity, drives genomic segregation consistent with experimental observations. This effect arises as motors preferentially draw crosslinks toward the periphery, forming dense domains that promote heterochromatin formation. Our model also predicts increased stiffness of nuclear wrinkles due to heterochromatin compaction beneath the lamina, consistent with instantaneous stiffening observed under nanoindentation. We conclude by outlining potential experimental approaches to validate our model predictions.

Elastic MicroPhase separation (EMPS) provides a simple route to create soft materials with homogeneous microstructures by leveraging the supersaturation of crosslinked polymer networks with liquids. At low supersaturation, network elasticity stabilizes a uniform mixture, but beyond a critical threshold, metastable microphase-separated domains emerge. While previous theories have focused on describing qualitative features about the size and morphology of these domains, they do not make quantitative predictions about EMPS phase diagrams. In this work, we extend Flory–Huggins theory to quantitatively capture EMPS phase diagrams by incorporating strain-stiffening effects. This model requires no fitting parameters and relies solely on independently measured solubility parameters and large-deformation mechanical responses. Our results confirm that strain-stiffening enables metastable microphase separation within the swelling equilibrium state and reveal why the microstructures can range from discrete droplets to bicontinuous networks. This works highlights the critical role of nonlinear elasticity in controlling phase-separated morphologies in polymer gels.

Many functional materials, such as paints and inks used in applications like coating and 3D printing, are concentrated granular suspensions. In such systems, the contact line dynamics and the internal structure of the suspension interact through shear rate dependent viscosity and microstructural rearrangements. The local shear rate increases sharply near moving contact lines, leading to the non-Newtonian rheology of dense suspensions in this region. While hydrodynamic solutions can describe dilute suspensions, their applicability near advancing contact lines in dense suspensions remains unclear. This study quantifies the deviation from the Newtonian solution by systematically varying interparticle interactions through the choice of dispersion medium. We use silica particles suspended in two refractive index-matched fluids: (i) aqueous 2,2′-thiodiethanol (weak interactions) and (ii) aqueous sodium thiocyanate solution (strong interactions). These systems exhibit substantially different rheological responses, shear-thickening and yield-stress behaviour, respectively. Using astigmatism particle tracking velocimetry (APTV), we resolve the three-dimensional trajectories of tracer particles within a drop driven over a substrate, in an arrangement enabling tracking of the internal flows over a long travel distance of the drop. We observe distinct flow behaviours depending on the particle interactions and the resulting suspension rheology. The more the particle interactions play a role, i.e., the more pronounced the non-Newtonian effects, the more strongly the measured flow profiles differ from the Newtonian solution of the hydrodynamic equations. In the case of the shear-thickening suspension, a notable deviation from Newtonian behaviour is observed. Conversely, the yield-stress suspension exhibits plug flow over the substrate, with Newtonian-like behaviour restricted to the yielded region near the substrate.

Viscoelastic fluids, such as polymer solutions, surfactant mixtures, colloidal suspensions, emulsions, and biological fluids like blood, are frequently transported in microfluidic systems using external electric fields. In such flows, two distinct types of instabilities can emerge, namely, electro-elastic instabilities (EEI), arising from the interaction between elastic stresses and streamline curvature, and electrokinetic instabilities (EKI), triggered by electrical conductivity gradients once the external electric field exceeds a critical value. Both instabilities can promote fluid mixing by inducing chaotic flow structures; however, their roles are not always complementary. Recent experimental and numerical studies have shown that increasing fluid viscoelasticity can suppress EKI, leading to reduced mixing efficiency in a microfluidic T-junction. However, this study demonstrates that while viscoelasticity initially hinders mixing by damping EKI, a further increase in the Weissenberg number (a measure of fluid elasticity) leads to the onset of EEI, which in turn again increases mixing. Therefore, a non-monotonic relationship between mixing efficiency and Weissenberg number is found in the present study. Furthermore, although both EEI and EKI promote mixing, they differ significantly in their coherent flow structures and regions of origin within the microdevice. To elucidate these differences, we employ the data-driven dynamic mode decomposition (DMD) technique to characterise the underlying instability modes and their influence on the mixing dynamics. Overall, this study provides fundamental insights into how viscoelasticity modulates flow instabilities in electrokinetically driven microflows and offers strategies to optimise mixing by tuning fluid properties and operating conditions.

We investigate the thermodynamic properties of a single inertial probe driven into a nonequilibrium steady state by random collisions with self-propelled active walkers. The probe and walkers are confined within a gravitational harmonic potential. We evaluate the robustness of the effective temperature concept in this active system by comparing values of distinct, independently motivated definitions: a generalized fluctuation–dissipation relation, a kinetic temperature, and via a work fluctuation relation. Our experiments reveal that, under specific conditions, these independent measurements coincide over a wide range of system configurations, yielding a remarkably consistent effective temperature. Furthermore, we also identify regimes where this consistency breaks down, which delineates the fundamental limits of extending equilibrium-like thermodynamic concepts to athermal, actively driven systems.

It is well established that many flagellated bacteria, such as Escherichia coli, swim in clockwise circles above rigid surfaces. However, in a cylindrical microwell with asymmetric top-bottom boundary conditions, such that bacteria segregate into two populations of differing sizes at opposing flat boundaries, the smaller bacterial vortex has been observed to rotate in the opposite direction to that expected in the absence of the other population [K. Beppu, Z. Izri, T. Sato, Y. Yamanishi, Y. Sumino and Y. T. Maeda, Proc. Natl. Acad. Sci. U. S. A., 2021, 118, e2107461118]. Motivated by these observations, we employ flow singularities to investigate the motion of a population of chiral swimmers near one flat boundary of a cylindrical geometry, subject to the flows generated by a bacterial vortex at the opposing surface. We show numerically that, purely due to hydrodynamic interactions, the rotational direction of the bacterial population reverses in the presence of a sufficiently large vortex on the opposite boundary. Our numerical results are fully explained by an analytical theory in the continuum limit, which captures the essential hydrodynamic interactions governing the observed reversal.

Phase separation of a liquid mixture embedded within an elastic network is relevant to a wide range of natural and industrial systems, including biomolecular condensates interacting with the cytoskeleton, structural colouring in bird feathers, and gas bubbles forming within soft sediments. Recent experiments in synthetic polymer gels have demonstrated that when the size of phase-separated domains is comparable to the characteristic pore size of the network, a patterned phase with a well-defined length scale may emerge. Theoretical works based on an equilibrium approach have attributed this pattern formation to non-local elastic effects arising from heterogeneity of the underlying network. Here, we extend these ideas by developing a dynamic theory in which phase separation is coupled to non-local elasticity via the framework of large-deformation poroelasticity. We study our model via both linear stability analysis and numerical simulation, identifying the parameter space in which phase separation occurs, and investigating the impact of different elasticity models. We find that although local elasticity can inhibit phase separation and affect domain count, it is unable to completely suppress coarsening. In contrast, non-local elasticity arrests coarsening to form patterned domains with a well-defined length scale that decreases with increasing stiffness. Our modelling framework thus paves the way for quantitative comparisons between simulations and experiments, for example by considering a strain-stiffening network rheology.

Liquid–liquid phase separation of coarse-grained model polymers of a given length is studied using Langevin dynamics simulations. Some pairs of monomers on each polymer are designated to interact via a short-ranged, effectively monovalent, and relatively strong (SMS) potential. We investigated the effects of the number of SMS interacting pairs, their sequence along the polymers, and polymer flexibility on the phase behavior of the solution, when the remaining monomers are hydrophobic and when they are in a good solvent condition. Our results demonstrate that monomers with SMS interactions can drive phase separation and subsequently gelation of the condensate upon lowering the temperature, even when the remaining monomers are in a good solvent condition. In this case, the phase separation and gelation temperatures increase monotonically with the number of SMS-interacting monomers. Additionally, when the remaining monomers are hydrophobic, the number of SMS monomer pairs and the polymer stiffness exhibit nonmonotonic effects on the phase separation temperature and the surface tension of the condensate. For a fixed number of SMS monomer pairs, their sequence along the polymer chain noticeably influences the phase separation temperature.