Soft Matter最新文献

The evolution of granular flows generally involves solid boundaries, which add complexity to their dynamics and pose challenges to understand relevant natural and industrial phenomena. While an interesting "teapot effect" has been observed for liquid flowing over the solid surface of a teapot's spout, a similar phenomenon for discrete particles receives far less attention. In this work, we experimentally investigated the interactions between granular flows and a wedge-shaped solid edge (spout), showing that the trailing edge of the solid boundary plays a key role in causing velocity non-uniformity and splitting the flow into "dispersed" and "uniform" regions. Tuning the parameters (inclination angle, particle diameter, radii and surface roughness of the trailing edge) of the granular flow, a dimensionless number was summarized and successfully predicted the dispersion of the granular flows. Moreover, we also proved that introducing stronger cohesive forces between particles could harness the granular flows from heterogenous structures to grain clusters, which can be employed to switch between different flow regimes and regulate the dispersion behavior of particle flows. This study reveals the interaction of granular flow over complex solid boundaries, potentially offering new insights into particle-dominated flow dynamics.

Active colloids powered by self-generated gradients are influenced by nearby solid boundaries, leading to their reorientation. In this study, the tilt angles (the angle between where an active colloid moves and where it faces) were measured to be 13.3° and -33.9° for 5 μm polystyrene microspheres half coated with 10 nm Pt caps moving in 5% H₂O₂ along the bottom and top glass wall, respectively, indicating that the colloids moved with their PS (forward) caps tilted slightly toward the wall. The speeds and tilt angles of Pt Janus colloids increased consistently with increasing H₂O₂ concentration (0.5 to 10 v/v%) and Pt cap thickness (5 to 50 nm). We propose that the tilt results from a balance between gravitational torque (caused by the Pt cap's weight) and chemical activity-induced torque (from self-generated chemical gradients), qualitatively supported by finite element simulations based on self-electrophoresis. Our findings are useful for understanding how chemically active colloids move in, and interact with, their environment.

Piezoelectricity in biological soft tissues is a controversial issue with differing opinions. While there is compelling experimental evidence to suggest a piezoelectric-like response in tissues such as the aortic wall (among others), there are equally compelling experiments that argue against this notion. In addition, the lack of a polar structure in the underlying components of most soft biological tissues supports the latter. In this paper, we address the collective behavior of cells within a two-dimensional cell aggregate from the viewpoint of statistical mechanics. Our starting point is the simplest form of energy for cell behavior that only includes known observable facts e.g., the electrical Maxwell stress or electrostriction, resting potential across cell membranes, elasticity, and we explicitly exclude any possibility of electromechanical coupling reminiscent of piezoelectricity at the cellular level. We coarse-grain our cellular aggregate to obtain its emergent mechanical, physical, and electromechanical properties. Our findings indicate that the fluctuation of cellular strain (E) plays a similar role as the absolute temperature in a conventional atomistic-level statistical model. The coarse-grained effective free energy reveals several intriguing features of the collective behavior of cell aggregates, such as solid-fluid phase transitions and a distinct piezoelectric-like coupling, even though it is completely absent at the microscale. Closed-form formulas are obtained for key electromechanical properties, including stiffness, effective resting potential, critical E²-temperature (or fluctuation) for solid-fluid phase transitions, and apparent piezoelectric coupling in terms of fluctuation and electric potential regulated by active cellular processes.

Rupture dynamics and pinch-off phenomena are fundamental for understanding instabilities in fluid dynamics and biological systems. In this study, we investigate the rupture of two-dimensional, channel-like configurations in a binary mixture of particles with differential diffusivities. Through computational simulations, we analyze the evolution of this instability under various conditions, identifying key parameters such as aspect ratio, particle density, and drift strength that influence the system's stability. While its behavior resembles the Plateau-Rayleigh instability (PRI), the underlying mechanism differs fundamentally, as PRI is restricted to three-dimensional systems. Interestingly, similar instabilities have been observed in chiral fluids, further supporting the existence of a novel instability mechanism unique to two-dimensional non-equilibrium systems. Our results suggest that this phenomenon is not a finite-size effect, but rather an intrinsic property of systems with differential diffusivities, offering new insights into pattern formation and instability dynamics in active matter.

Within a framework of elasticity theory and geometry, the twister mechanism has been proposed some years ago for describing the interaction between a biofilament containing a twisted hydrophobic strip and a lipid membrane: this mechanism is capable of inducing deformations of the membrane, which can lead to its opening. The present work intends to extend this model to the interactions between a membrane and protein regions conserving their folds using coarse-grained molecular dynamics simulations. The protein region is modeled as a cylinder stabilized by a tensegrity scheme, leading to an elasticity similar to that observed in real proteins. Recording molecular dynamics trajectories of this cylinder in the presence of a fluid lipid bilayer membrane allows investigation of the effect of the positions of the hydrophobic parts on the interaction with the membrane. The entire configuration space is explored by systematically varying the hydrophobic strip width, the twisting of the strip as well as the range of hydrophobic interactions between the cylinder and the membrane. Three different states are observed: no interaction between the cylinder and membrane, the cylinder in contact with the membrane surface and the cylinder inserted into the membrane with a variable tilt angle. The variations of the tilt angle are explained using a qualitative model based on the total hydrophobic moment of the cylinder. A deformation pattern of the membrane, previously predicted for the filament-membrane interaction by the twister model, is observed for the state when the cylinder is in contact with the membrane surface, which allows estimation of the applied torques.

Epoxy resins are an important class of thermosetting resins, and their network structure, formed by the curing reaction of epoxy and amine compounds, plays a crucial role in determining material properties, including creep behavior. We here applied the time-temperature superposition (TTS) principle to analyze the creep behavior of epoxy resins with well-defined network structures that were systematically varied based on the length of the n-alkyl diamine used. The superposition of isothermal creep curves under small stress was achieved through horizontal and vertical shifting, regardless of the length of the n-alkyl diamine. The temperature dependence of the horizontal shift factor was well described by the Williams-Landel-Ferry equation. Creep rupture measurements under large stress conditions revealed specimen rupture, and the time to rupture was plotted against the imposed stress. These plots, acquired at various temperatures, could be superimposed through horizontal shifting. As the diamine length decreased-namely, the distance between cross-linking points-the temperature dependence of the horizontal shift factors deviated from the WLF equation and exhibited Arrhenius-type behavior. The deviation was associated with differences in the fracture process involving chain scission, which became more pronounced as the diamine length decreased. The insights gained in this study should be valuable for controlling creep response and predicting the long-term durability of epoxy resins.

The propulsion of many eukaryotic cells is generated by flagella, flexible slender filaments that are actively oscillating in space and time. The dynamics of these biological appendages have inspired the design of many types of artificial microswimmers. The magnitude of the filament's viscous propulsion depends on the time-varying shape of the filament, and that shape depends in turn on the spatial distribution of the bending rigidity of the filament. In this work, we rigorously determine the relationship between the mechanical (bending) properties of the filament and the viscous thrust it produces using mathematical optimisation. Specifically, by considering a model system (a slender elastic filament with an oscillating slope at its base), we derive the optimal bending rigidity function along the filament that maximises the time-averaged thrust produced by the actuated filament. Instead of prescribing a specific functional form, we use functional optimisation and adjoint-based variational calculus to formally establish the link between the distribution of bending rigidity and propulsion. The optimal rigidities are found to be stiff near the base, and soft near the distal end, with a spatial distribution that depends critically on the constraints used in the optimisation procedure. These findings may guide the optimal design of future artificial swimmers.

Understanding the structure-property relationships of semiflexible polymer networks is essential for their rational design and application across diverse fields. While classical static structural characterizations have been widely used, dynamic investigations also provide a powerful approach to analyzing these networks across multiple hierarchical levels in both time and length scales. This study presents a comprehensive methodology to dynamically determine key structural parameters in semiflexible polymer networks, characterizing time, length, volume, and molecular weight of unit segments at their respective hierarchical levels, such as Kuhn monomers, correlation blobs, and network strands. A wormlike micellar solution of sodium dodecyl sulfate and aluminum nitrate was used as a model system representing semiflexible polymers with a large Kuhn length. By combining dynamic experimental techniques, including dynamic light scattering, macrorheology, and microrheology, crucial structural information was obtained. Integrating information derived from the characteristic parameters successfully revealed the hierarchical network structure of the wormlike micelles, with results validated against static light scattering measurements. Notably, this study effectively utilizes the complex viscoelastic modulus obtained through microrheology, which has received limited attention in the literature. This approach holds potential applicability to a wide range of semiflexible polymer networks.

Directed motion up a concentration gradient is crucial for the survival and maintenance of numerous biological systems, such as sperms moving towards an egg during fertilization or ciliates moving towards a food source. In these systems, chirality-manifested as a rotational torque-plays a vital role in facilitating directed motion. While systematic studies of active molecules in activity gradients exist, the effect of chirality remains little studied. In this study, we examine the simplest case of a chiral active particle connected to a passive particle in a spatially varying activity field. We demonstrate that this minimal setup can exhibit rich emergent tactic behaviors, with the chiral torque serving as the tuning parameter. Notably, when the chiral torque is sufficiently large, even a small passive particle enables the system to display the desired accumulation behavior. Our results further show that in the dilute limit, this desired accumulation behavior persists despite the presence of excluded volume effects. Additionally, interconnected chiral active particles exhibit emergent chemotaxis beyond a critical chain length, with trimers and longer chains exhibiting strong accumulation at sufficiently high chiral torques. This study provides valuable insights into the design principles of hybrid bio-molecular devices of the future.

The dynamical properties of the local area available per particle and its relationship with the self-diffusion coefficient of colloids in quasi-2D colloidal dispersions are studied using video microscopy, supported by Brownian dynamics simulations. The local area is determined via the well-known Voronoi tessellation technique. Our findings reveal that local areas per particle are highly dispersed, exhibiting slow dynamics over time. Additionally, the evolution of the ensemble-averaged area distribution as a function of concentration shows a long tail at large and small areas for low and high concentrations, respectively, leading to a maximum in information entropy when the distribution becomes symmetric. We introduce and analyze several expressions for local area-weighted diffusion coefficients. Notably, we find that the contribution of the averaged diffusion coefficient can be expressed in terms of local areas, establishing a new framework to determine the weighted influence of each local area on particle dynamics.