Anisotropic Forces Research Articles

Influence of stress anisotropy on stress distributions in gap-graded soils

The behaviour of gap graded soils comprising non-plastic fines (sand or silt) mixed with a coarser sand or gravel fraction has received attention from researchers interested in internal instability under seepage loading (a form of internal erosion) as well as researchers interested in load:deformation responses. Skempton and Brogan [1] postulated that resistance to seepage induced instability depends upon the proportion of the overall applied stress that is transmitted by the finer fraction. Shire et al. [2] explored Skempton and Brogan’s hypothesis using DEM simulations to look at the proportion of the applied stress transmitted by the finer fractions (α) in ideal isotropic samples. They showed that at low fines contents (FC< FC*) the average stress transmitted by the finer grains is less than the applied stress (α<1), while for FC>FC+ the fines play a key role in stress transmission (α>1); for FC*<FC< FC+, α depends on the sample density. The current contribution describes a series of constant p’ DEM triaxial test simulations carried out to assess the evolution of stress heterogeneity with shearing. The simulation data generated indicate that a sample can transition from being fines dominated (with the fines transmitting a significant proportion of the applied stress and α ≥1) to coarse or sand- dominated (with α <1) as the material dilates during shear deformation. While α reduces as the samples dilate, the relationship between the α and the sample void ratio is non-trivial. The anisotropy of the coarse-coarse contact network exceeds the overall contact force anisotropy; this indicates that the deviator stress is transmitted through a strong force network passing through the coarse-coarse contacts supported by the fine-coarse contacts.

Anisotropic cellular forces support mechanical integrity of the Stratum Corneum barrier

The protective function of biological surfaces that are exposed to the exterior of living organisms is the result of a complex arrangement and interaction of cellular components. This is the case for the most external cornified layer of skin, the stratum corneum (SC). This layer is made of corneocytes, the elementary ‘flat bricks’ that are held together through adhesive junctions. Despite the well-known protective role of the SC under high mechanical stresses and rapid cell turnover, the subtleties regarding the adhesion and mechanical interaction among the individual corneocytes are still poorly known. Here, we explore the adhesion of single corneocytes at different depths of the SC, by pulling them using glass microcantilevers, and measuring their detachment forces. We measured their interplanar adhesion between SC layers, and their peripheral adhesion among cells within a SC layer. Both adhesions increased considerably with depth. At the SC surface, with respect to adhesion, the corneocyte population exhibited a strong heterogeneity, where detachment forces differed by more than one order of magnitude for corneocytes located side by side. The measured detachment forces indicated that in the upper-middle layers of SC, the peripheral adhesion was stronger than the interplanar one. We conclude that the stronger peripheral adhesion of corneocytes in the SC favors an efficient barrier which would be able to resist strong stresses.

Toward prediction of surface tension of branched n-alkanes using ANN technique

This study features the application of artificial neural network for prediction of surface tension of branched alkanes. Surface and interfacial tensions of alkanes show a specific interaction between adjacent molecules of the higher n-alkanes which results in an anisotropic dispersion force component of the surface energy. The surface tension of branched alkanes was studied for temperatures between 283.15 and 448.15 K. Two intelligent models named multilayer perceptron model (MLP) and radial basis function (RBF) model were developed and the accuracy of two models was examined by different graphical and statistical methods. Results showed that the both models are accurate and effective in prediction of surface tension of branched alkanes. However, the results were compared with experimental data and it was found that the estimated surface tension by multi-layer perceptron neural network is more accurate than radial basis function network.

Analysis of Anisotropic Interaction Equations for Fingerprint Simulations

AbstractMotivated by the simulation of fingerprints we consider a class of interaction models with anisotropic, repulsive‐attractive interaction forces whose orientations depend on an underlying tensor field. This class of models can be regarded as a generalization of a gradient flow of a nonlocal interaction potential which has a local repulsion and a long‐range attraction structure. These anisotropic forces in our class of models cannot be derived from a potential and the underlying tensor field introduces an anisotropy leading to complex patterns which do not occur in isotropic models. This anisotropy is characterized by one parameter in the model. We study the resulting stationary solutions both analytically and numerically by considering the particle model and its continuum counterpart.

Non-minimal torsion-matter coupling and wormhole solutions

Taking into account various energy density models in the framework of torsion-based modified theory, some wormhole solutions are derived and studied. We use a non-minimal coupling between matter and torsion in order to construct the wormhole geometry for constant, exponential and Lorentzian energy density models. Two sets of torsion dependent models, including (i) teleparalell case with linear forms of torsion scalar and (ii) a [Formula: see text] gravity with quadratic form of torsion scalar, have been considered throughout this paper. Our study shows that additional terms to the Einstein field equations, due to consider modified gravity, give us the opportunity of obtaining wormhole solutions which may be supported by baryonic sources. We also check the equilibrium phase of the wormhole solutions using anisotropic and hydrostatic forces. The details of the obtained results depend on the numerical values of the free parameters that the function [Formula: see text] holds.

Rheology and structure of polydisperse three-dimensional packings of spheres

We use three-dimensional contact dynamics simulations to analyze the rheology of polydisperse packings of spherical particles subjected to simple shear. The macroscopic and microstructural properties of several packings are analyzed as a function of their size span (from nearly monodisperse to highly polydisperse). Consistently with previous two-dimensional simulations, we find that the shear strength is independent of the size span despite the increase of packing fraction with size polydispersity. At the grain scale, we analyze the particle connectivity, force transmission, and the corresponding anisotropies of the contact and force networks. We show that force distributions become increasingly broader as the size span increases. In particular, stronger forces are captured by large particles, which are also better connected creating the so-called granular backbone. Throughout this backbone friction mobilization is rare and compressive forces control the stability of such structure. In return, small particles create an important population of rattlers discarded of the strength and granular structure analysis. As a consequence, the contact anisotropy declines with size span, whereas the force and branch anisotropies increase. These microstructural compensations allow us to explain the independence of the shear strength from particle size polydispersity.

Improvements to the AMOEBA Force Field by Introducing Anisotropic Atomic Polarizability of the Water Molecule.

In this work, we have developed an anisotropic polarizable model for the AMOEBA force field that is derived from electrostatic fitting on a gas phase water molecule as the primary approach to improve the many-body polarization model. We validate our approach using small to large water cluster benchmark data sets and ambient liquid water properties and through comparisons to a variational energy decomposition analysis breakdown of molecular interactions for water and water-ion trimer systems. We find that the accounting of anisotropy polarization for a single water molecule demonstrably improves the description of the many-body polarization energy in all cases. This study provides a proof of principle for extending our protocol for developing a general purpose anisotropic polarizable force field for other biological and material functional groups to better describe complex and asymmetric environments for which accurate polarization models are most needed.

A neural network (CSA-LSSVM) model for the estimation of surface tension of branched alkanes

ABSTRACTThe current study highlights the application of a model based on least square support vector machine (LSSVM) for prediction of surface tension of branched alkanes. An optimization algorithm, namely, coupled simulated annealing (CSA) was applied to the model. Surface tensions of alkanes show a specific interaction between adjacent molecules of the branched alkanes which affects the anisotropic dispersion force component of the surface energy. In this paper, surface tension of branched alkanes was studied in temperature range of 283.15 and 448.15 K. To evaluate the performance and accuracy of this model, statistical and graphical error analyses have been used simultaneously. By applying CSA-LSSVM on 600 data points and finding optimum parameters, the estimated values of surface tension of branched alkanes were compared with experimental data which showed a reasonable agreement with the experimental results. Results demonstrate that the model is precise and viable for prediction of solubility data. The model shows an overall R2 and AARD% estimations of 0.9921 and 0.89%, respectively.

Efficient Equilibrium Testing Under Adhesion and Anisotropy Using Empirical Contact Force Models

This paper presents a method for efficiently testing the stability of an object under contact that accommodates empirical models of admissible forces at individual contact points. It handles a diverse range of possible geometries of the admissible force volume, including anisotropy, adhesion, and even nonconvexity. The method discretizes the contact region into patches, performs a convex decomposition of a polyhedral approximation to each admissible force volume, and then formulates the problem as a mixed integer linear program. The model can also accommodate articulated robot hands with torque limits and joint frictions. Predictions of our method are evaluated experimentally in object lifting tasks using a gripper that exploits microspines to exert strongly anisotropic forces. The method is applied to calculate gripper loading capabilities and equilibrium predictions for a quadruped climbing robot on steep and overhanging terrain.

Pressure-induced remarkable luminescence-changing behaviours of 9, 10-distyrylanthracene and its derivatives with distinct substituents

Piezochromic luminescence behaviours of 9, 10-distyrylanthracene (DSA) derivatives with different substituents based on their single crystal structures were investigated under hydrostatic pressure via a diamond anvil cell (DAC), compared with the anisotropic force produced from grinding. Upon grinding, three single crystals show identifiable blue-shifts in fluorescent spectra because of the disturbance for ordered crystal structure to amorphous state, but nearly sightless color changes. The high-pressure fluorescent experiments applied by DAC reveal three DSA crystals all show large red-shifts. High pressure absorption spectra display that a new charge transfer state appears when -CH3 or -2CN is introduced to DSA and all of them show obvious red-shifts. Once the pressure was relieved gradually to the ambient pressure, the fluorescence and absorption spectra both entirely recovered, indicating this geometric change is reversible in a certain pressure region, which is further confirmed by Raman spectra. The relationships between molecular structures and spectroscopic behaviours were further discussed by their single crystal data. This work provides deep insights into the relationship between molecular structure and photo-physical properties of practical mechanochromic luminescent materials, and is significant in the potential applications of pressure transducers.

Modeling cell intercalation during Drosophila germband extension

Germband extension during Drosophila development is primarily driven by cell intercalation, which involves three key components: planar cell polarity, anisotropic myosin contractile forces on cellular junctions, and cellular deformation and movement. Prior experimental work probed each of these factors in depth, but the connection between them remains unclear. This paper presents an integrated chemomechanical model that combines the three factors into a coherent mathematical framework for studying cell intercalation in the germband tissue. The model produces the planar cell polarization of key proteins, including Rho-kinase, Bazooka and myosin, the development of anisotropic contractile forces, and subsequent cell deformation and rearrangement. Cell intercalation occurs through T1 transitions among four neighboring cells and rosettes involving six cells. Such six-cell rosettes entail stronger myosin-based contractile forces, and on average produce a moderately larger amount of germband extension than the T1 transitions. The resolution of T1 and rosettes is driven by contractile forces on junctions anterior and posterior to the assembly as well as the pulling force of the medial myosin in the anterior and posterior cells. The global stretching due to posterior midgut invagination also plays a minor role. These model predictions are in reasonable agreement with experimental observations.

Impact of Protein-Polymer Interactions in the Antimicrobial Activity of Lysozyme/Poly(3,4-ethylenedioxythiophene) Biocapacitors

Biocapacitors constructed by combining lysozyme (LYZ) and poly(3,4-ethylenedioxythiophene) (PEDOT) retained the bactericidal activity of the protein when this was encapsulated within the polymeric matrix but lost the antimicrobial behaviour when the LYZ was adsorbed onto the polymer. In this work we use atomistic Molecular Dynamics simulations to examine the influence of protein···polymer interactions in the bactericidal activity of LYZ-containing biocapacitors. Results show that the anisotropic forces exerted by oxidized PEDOT chains on the adsorbed protein induce small structural changes that locally affect at the active centre, breaking the intra-residue interactions associated with the antibacterial mechanism. Conversely, isotropic polymer···protein interactions in biocapacitors with encapsulated LYZ do not affect the stability of the active centre. These observations suggest that the strong repulsive or attractive forces between p-doped polymer chains and the positively or negatively charged LYZ residues, respectively, are the only ones responsible for the protein activity.

Flexible Guidance of Microengines by Dynamic Topographical Pathways in Ferrofluids.

In this work, we demonstrate a simple, versatile, and real-time motion guidance strategy for artificial microengines and motile microorganisms in a ferrofluid by dynamic topographical pathways (DTPs), which are assembled from superparamagnetic nanoparticles in response to external magnetic field ( H). In this general strategy, the DTPs can exert anisotropic resistance forces on autonomously moving microengines and thus regulate their orientation. As the DTPs with different directions and lengths can be reversibly and swiftly assembled in response to the applied H, the microengines in the ferrofluid can be guided on demand with controlled motion directions and trajectories, including circular, elliptical, straight-line, semi-sine, and sinusoidal trajectories. The as-demonstrated control strategy obviates reliance on the customized responses of micromotors and applies to autonomously propelling agents swimming both in bulk and near substrate walls. Furthermore, the microengines (or motile microorganisms) in a ferrofluid can be considered as an integrated system, and it may inspire the development of intelligent systems with cooperative functions for biomedical and environmental applications.

Critical scaling near the yielding transition in granular media.

We show that the yielding transition in granular media displays second-order critical-point scaling behavior. We carry out discrete element simulations in the low-inertial-number limit for frictionless, purely repulsive spherical grains undergoing simple shear at fixed nondimensional shear stress Σ in two and three spatial dimensions. To find a mechanically stable (MS) packing that can support the applied Σ, isotropically prepared states with size L must undergo a total strain γ_{ms}(Σ,L). The number density of MS packings (∝γ_{ms}^{-1}) vanishes for Σ>Σ_{c}≈0.11 according to a critical scaling form with a length scale ξ∝|Σ-Σ_{c}|^{-ν}, where ν≈1.7-1.8. Above the yield stress (Σ>Σ_{c}), no MS packings that can support Σ exist in the large-system limit L/ξ≫1. MS packings generated via shear possess anisotropic force and contact networks, suggesting that Σ_{c} is associated with an upper limit in the degree to which these networks can be deformed away from those for isotropic packings.

α-Catenin Controls the Anisotropy of Force Distribution at Cell-Cell Junctions during Collective Cell Migration

Adherens junctions (AJs) control epithelial cell behavior, such as collective movement and morphological changes, during development and in disease. However, the molecular mechanism of AJ remodeling remains incompletely understood. Here, we report that the conformational activation of α-catenin is the key event in the dynamic regulation of AJ remodeling. α-catenin activates RhoA to increase actomyosin contractility at cell-cell junctions. This leads to the stabilization of activated α-catenin, in part through the recruitment of the actin-binding proteins, vinculin and afadin. In this way, α-catenin regulates force sensing, as well as force transmission, through a Rho-mediated feedback mechanism. We further show that this is important for stable directional alignment of multiple cells during collective cell movement by both experimental observation and mathematical modeling. Taken together, our findings demonstrate that α-catenin controls the establishment of anisotropic force distribution at cell junctions to enable cooperative movement of the epithelial cell sheet.

Vortex and glass bead interaction in nematic liquid crystal layer

An homeotropically aligned liquid crystal layer driven by external electric or magnetic fields exhibit an intricate network of defects called umbilics or vortexes. Here, we report an experimental characterization of vortexes-glass bead interaction in nematic liquid crystal layer. The glass spheres, embedded in the liquid crystal, are found to be of two type: some of them attractive, with different strength of attraction; and most of them non-attractive. The attractive glass spheres pull the vortexes so that the distance decays approximately with the square root of time. The glass bead sphere induces an anisotropic force on the vortex. This force can be well approximate by an inverse power law of the distance between vortex and glass bead with exponent 3/4. The vortexes are attracted to a polar region of the attractive glass bead.

Microscopic Origins of Shear Jamming for 2D Frictional Grains.

Shear jamming (SJ) occurs for frictional granular materials with packing fractions ϕ in ϕ_{S}<ϕ<ϕ_{J}^{0}, when the material is subject to shear strain γ starting from a force-free state. Here, ϕ_{J}^{μ} is the isotropic jamming point for particles with a friction coefficient μ. SJ states have mechanically stable anisotropic force networks, e.g., force chains. Here, we investigate the origins of SJ by considering small-scale structures-trimers and branches-whose response to shear leads to SJ. Trimers are any three grains where the two outer grains contact a center one. Branches occur where three or more quasilinear force chain segments intersect. Certain trimers respond to shear by compressing and bending; bending is a nonlinear symmetry-breaking process that can push particles in the dilation direction faster than the affine dilation. We identify these structures in physical experiments on systems of two-dimensional frictional discs, and verify their role in SJ. Trimer bending and branch creation both increase Z above Z_{iso}≃3 needed for jamming 2D frictional grains, and grow the strong force network, leading to SJ.

Jamming of packings of frictionless particles with and without shear**Project supported by the National Natural Science Foundation of China (Grant Nos. 11702289, 11734014, and 11574278) and the Anhui Provincial Natural Science Foundation (Grant No. 1708085QA07).

By minimizing the enthalpy of packings of frictionless particles, we obtain jammed solids at desired pressures and hence investigate the jamming transition with and without shear. Typical scaling relations of the jamming transition are recovered in both cases. In contrast to systems without shear, shear-driven jamming transition occurs at a higher packing fraction and the jammed solids are more rigid with an anisotropic force network. Furthermore, by introducing the macrofriction coefficient, we propose an explanation of the packing fraction gap between sheared and non-sheared systems at fixed pressure.

A path planning approach for crowd evacuation in buildings based on improved artificial bee colony algorithm

This paper proposes a new path planning approach for emergency evacuation simulation. This technique combines the Extended Social Force Model (ESFM) and the Improved Artificial Bee Colony (IABC) algorithm to enhance the visual realism and improve the efficiency of crowd evacuation. In the ESFM, we introduce a visual parameter to the original SFM and obtain the anisotropic psychological force rather than the isotropic one in the SFM so as to better fit crowd behaviors, such as long-range obstacle avoidance and self-organizing group formation. In addition, the IABC algorithm is proposed to improve the evacuation efficiency and provide support for building design and evacuation management by employing the strategies of grouping and exit selection. The algorithm uses the evacuation time of the individuals as the evaluation metric. If an exit is overcrowded and congested, the individuals will assess the degree of congestion, estimate the time needed to escape, and determine whether to select a farther exit for escape. By selecting the optimal exit and avoiding congestion, the evacuation efficiency can be improved. We have simulated the crowd evacuation with our new path planning approach via a crowd evacuation simulation system. The results show the effectiveness of our method.

Rheology of granular materials composed of crushable particles.

We investigate sheared granular materials composed of crushable particles by means of contact dynamics simulations and the bonded-cell model for particle breakage. Each particle is paved by irregular cells interacting via cohesive forces. In each simulation, the ratio of the internal cohesion of particles to the confining pressure, the relative cohesion, is kept constant and the packing is subjected to biaxial shearing. The particles can break into two or more fragments when the internal cohesive forces are overcome by the action of compressive force chains between particles. The particle size distribution evolves during shear as the particles continue to break. We find that the breakage process is highly inhomogeneous both in the fragment sizes and their locations inside the packing. In particular, a number of large particles never break whereas a large number of particles are fully shattered. As a result, the packing keeps the memory of its initial particle size distribution, whereas a power-law distribution is observed for particles of intermediate size due to consecutive fragmentation events whereby the memory of the initial state is lost. Due to growing polydispersity, dense shear bands are formed inside the packings and the usual dilatant behavior is reduced or cancelled. Hence, the stress-strain curve no longer passes through a peak stress, and a progressive monotonic evolution towards a pseudo-steady state is observed instead. We find that the crushing rate is controlled by the confining pressure. We also show that the shear strength of the packing is well expressed in terms of contact anisotropies and force anisotropies. The force anisotropy increases while the contact orientation anisotropy declines for increasing internal cohesion of the particles. These two effects compensate each other so that the shear strength is nearly independent of the internal cohesion of particles.