The European Physical Journal E最新文献

This study examines double-diffusive convection in a horizontal fluid layer with low thermal conductivity boundaries, where the heat flux is fixed. Using linear stability analysis and nonlinear modeling, the behavior of the system is explored under different thermal and concentration gradients. Two instability modes are identified: monotonous and oscillatory. The monotonous mode, exhibiting longwave patterns, dominates when both gradients contribute to instability. The oscillatory mode occurs when the gradients oppose each other, with stability thresholds dependent on system parameters. Nonlinear modeling confirms the linear theory, showing longwave patterns near the instability threshold and oscillatory behavior when gradients are opposed. These findings offer insights into double-diffusive convection in systems with low thermal conductivity boundaries.

Stability map on the plane Rayleigh number–solutal Rayleigh Number

Tropomyosins are central regulators of the actin cytoskeleton, controlling the binding and activity of the other actin binding proteins. The interaction between tropomyosin and actin is quite unique: single tropomyosin dimers bind weakly to actin filaments but get stabilised by end-to-end attachment with neighbouring tropomyosin dimers, forming clusters which wrap around the filament. Force spectroscopy is a powerful approach for studying protein–protein interactions, but classical methods which usually pull with pN forces on a single protein pair, are not well adapted to tropomyosins. Here, we propose a method in which a hydrodynamic drag force is applied directly to the proteins of interest, by imposing a controlled fluid flow inside a microfluidic chamber. The breaking of the protein bonds is directly visualised with fluorescence microscopy. Using this approach, we reveal that very low forces from 0.01 to 0.1 pN per tropomyosin dimer trigger the detachment of entire tropomyosin clusters from actin filaments. We show that the tropomyosin cluster detachment rate depends on the cytoplasmic tropomyosin isoform (Tpm1.6, 1.7, 1.8) and increases exponentially with the applied force. These observations lead us to propose a cluster detachment model which suggests that tropomyosins dynamically explore different positions over the actin filament. Our experimental setup can be used with many other cytoskeletal proteins, and we show, as a proof-of-concept, that the velocity of myosin-X motors is reduced by an opposing fluid flow. Overall, this method expands the range of protein–protein interactions that can be studied by force spectroscopy.

Many microswimmers are able to swim through viscous fluids by employing periodic non-reciprocal deformations of their appendages. Here we use a simple microswimmer model inspired by swimming biflagellates which consists of a spherical cell body and two small spherical beads representing the motion of the two flagella. Using reinforcement learning, we identify for different microswimmer morphologies quasi-optimized swimming strokes. For all studied cases, the identified strokes result in symmetric and quasi-synchronized beating of the two flagella beads. Interestingly, the stroke-averaged flow fields are of pusher type, and the observed swimming gaits outperform previously used biflagellate microswimmer models relying on predefined circular flagella-bead motion.

Metachronal paddling is a swimming strategy in which an organism oscillates sets of adjacent limbs with a constant phase lag, propagating a metachronal wave through its limbs and propelling it forward. This limb coordination strategy is utilized by swimmers across a wide range of Reynolds numbers, which suggests that this metachronal rhythm was selected for its optimality of swimming performance. In this study, we apply reinforcement learning to a swimmer at zero Reynolds number and investigate whether the learning algorithm selects this metachronal rhythm, or if other coordination patterns emerge. We design the swimmer agent with an elongated body and pairs of straight, inflexible paddles placed along the body for various fixed paddle spacings. Based on paddle spacing, the swimmer agent learns qualitatively different coordination patterns. At tight spacings, a back-to-front metachronal wave-like stroke emerges which resembles the commonly observed biological rhythm, but at wide spacings, different limb coordinations are selected. Across all resulting strokes, the fastest stroke is dependent on the number of paddles; however, the most efficient stroke is a back-to-front wave-like stroke regardless of the number of paddles.

For a static granular system, the constitutive equation of its stress tensor is of great significance for understanding its mechanical behaviors. Under isostatic state, it can have the form of stress-geometry equation. To investigate the force moment tensor and the stress-geometry equation of a two-dimensional (2D) granular system in theory, we propose some algebraic theories such as the decomposition formula of a second-order tensor and the cross-product of two symmetric tensors for the dyadic space (mathbb {T}^{2}(mathbb {R}^{2})). For a 2D frictionless disk packing, the local stress-geometry equation for a disk with three or four contacts is derived based on the definition of force moments tensor and the equilibrium equation of contact forces. The definition of the geometry tensor in the stress-geometry equation shows complex associations between the contact branch vectors of a disk with three or four contacts. For a disk with four contacts, its local Janssen coefficient can be given from the eigenvalues of its geometry tensor. Discrete element method (DEM) simulations for random frictionless disk packings are performed to verify two local stress-geometry equations in this paper, and the numerical results are in good agreement with the theoretical predictions. The local stress-geometry equations are convenient for obtaining some information about the stress tensors according to the contact structures without knowing the details of the deformations and the intergranular interactions.

 We study the formation of propagating large-scale density waves of mixed polar-nematic symmetry in a colony of self-propelled agents that are bound to move along the planar surface of a thin viscous film. The agents act as an insoluble surfactant, i.e. the surface tension of the liquid depends on their density. Therefore, density gradients generate a Marangoni flow. We demonstrate that for active matter in the form of self-propelled surfactants with local (nematic) aligning interactions such a Marangoni flow nontrivially influences the propagation of the density waves. Upon gradually increasing the Marangoni parameter, which characterises the relative strength of the Marangoni flow as compared to the self-propulsion speed, the density waves broaden while their speed may either increase or decrease depending on wavelength and overall mean density. A further increase in the Marangoni parameter eventually results in the disappearance of the density waves. This may occur either discontinuously at finite wave amplitude via a saddle-node bifurcation or continuously with vanishing wave amplitude at a wave bifurcation, i.e. a finite-wavelength Hopf bifurcation.

Two-dimensional monodisperse linear polymer chains are known to adopt for sufficiently large chain lengths N and surface fractions (phi ) compact configurations with fractal perimeters. We show here by means of Monte Carlo simulations of reversibly connected hard disks (without branching, ring formation and chain intersection) that polydisperse self-assembled equilibrium polymers with a finite scission energy E are characterized by the same universal exponents as their monodisperse peers. Consistently with a Flory-Huggins mean-field approximation, the polydispersity is characterized by a Schulz–Zimm distribution with a susceptibility exponent (gamma =19/16) for all not dilute systems and the average chain length (left<N right>propto exp (delta E) phi ^{alpha }) thus increases with an exponent (delta = 16/35). Moreover, it is shown that (alpha =3/5) for semidilute solutions and (alpha approx 1) for larger densities. The intermolecular form factor F(q) reveals for sufficiently large (left<N right>) a generalized Porod scattering with (F(q) propto 1/q^{11/4}) for intermediate wavenumbers q consistently with a fractal perimeter dimension (d_s=5/4).

Snapshot of a subvolume of a much larger configuration of strictly two-dimensional equilibrium polymers as considered in this study for a broad range of surface fractions and scission energies. Despite the low dimensionality and the strong polydispersity of the annealed chain systems, the same univeral scaling relations and asymptotic exponents as for their monodisperse quenched peers are found as suggested by a simple Flory-Huggins type mean-field approximation. This holds especially for semidilute solutions and dense melts where the chains are demonstrated to adopt compact configurations of a fractal perimeter dimension 5/4. The intermolecular form factor F(q) thus reveals a generalized Porod scattering with (F(q) propto 1/q^{11/4}) for intermediate wavenumbers q Snapshot of a subvolume of a much larger configuration of strictly two-dimensional equilibrium polymers as considered in this study for a broad range of surface fractions and scission energies. Despite the low dimensionality and the strong polydispersity of the annealed chain systems, the same univeral scaling relations and asymptotic exponents as for their monodisperse quenched peers are found as suggested by a simple Flory-Huggins type mean-field approximation. This holds especially for semidilute solutions and dense melts where the chains are demonstrated to adopt compact configurations of a fractal perimeter dimension 5/4. The intermolecular form factor F(q) thus reveals a generalized Porod scattering with (F(q) propto 1/q^{11/4}) for intermediate wavenumbers q