Heterogeneous Strip Research Articles

Janus Droplets and Droplets with Multiple Heterogeneous Surface Strips Generated with Nanoparticles under Applied Electric Field

A novel method of fabricating Janus droplets and droplets with heterogeneous strips is presented in this article. These droplets were made by covering the surfaces of oil droplets with different nanoparticles under an electric field in a microfluidic chip. The microfluidic chip was used to control the delivery of nanoparticles, and the electric field was employed to assemble the nanoparticles adhering on the droplet. By controlling the delivery of nanoparticles in the microfluidic chip under the electric field, different nanoparticles accumulate on the droplet surface and the desired Janus droplets and droplets with heterogeneous strips can be formed. Because of the presence of charged nanoparticles on droplets’ surfaces, the Janus droplets and droplets with heterogeneous strips have unique electrokinetic properties. The electro-osmotic flow fields around Janus droplets and droplets with heterogeneous strips in different pH solutions were visualized, and the electrokinetic velocities of these droplets in ...

Numerical modelling of waves in double-porosity Biot medium

We consider acoustic waves in fluid-saturated periodic media with large contrasts in the permeability and other poroelastic coefficients, whereby some of the mesoscopic material parameters depend on the scale parameter. The effective behaviour is described by a model obtained using the homogenization of the heterogeneous Biot continuum relevant to the mesoscopic level at which the dual porosity is featured by low permeability, whereas the primary porosity is very compliant. All material coefficients of the homogenized model depend on the frequency of incident waves. The macroscopic fields can be employed to reconstruct solutions at the mesoscopic level relevant to the heterogeneities. To validate the model, we consider heterogeneous strips with a finite scale lattice defining the heterogeneity. For such structures, the recovered mesoscopic response computed using the numerical homogenization method is compared with direct numerical simulations for various contrasts. For media featured by large contrast the double porosity model provides much better estimates of the wavelengths and other dispersion properties than the standard single porosity homogenized model. Moreover, the performed tests show computational efficiency of the two-scale simulations using the homogenized model, while the direct simulations of a heterogeneous medium become unfeasible because of requirements on the finite element discretization.

Heterogeneous strip originated from the separate enriched melt: Innate character and physical mechanism

The chemical heterogeneity, well-known as macrosegregation, is a major problem in the casting of steel ingots. Most heterogeneity generally originated from the solute partition and solute distribution in the solidification process with interdendritic convection. In this article, a new heterogeneous phenomenon originated from the movement of separate enriched melt is discovered in a steel ingot. A strip characterized as ferrite chains and MnS chains is revealed by macro-etching. The formation mechanism for the heterogeneous strip is proposed. In the mushy zone of the ingot, a large amount of separate S-enriched melts move laterally and upwards. Some S-enriched melts will remain in the moving trace. Such residual S-enriched melts produce a large amount of MnS inclusion chains. In the subsequent solid phase transition process, promoted by the MnS chains, ferrite prefers to be transited from the austenite near the MnS inclusions and shows as a large amount of separate ferrite chains. A large amount of ferrite chains align in a strip-like zone, which results in the heterogeneous strip phenomenon in macro-etching process. The physical model about the driving force for the movement of separate S-enriched melts is further theoretically analyzed. In the mushy zone, the interface tension resultant applied on the separate S-enriched melts can act as the drive force for the lateral movement of separate S-enriched melts. And the buoyance applied on the separate S-enriched melts acts as the drive force for the upwards movement. The interaction between the impurity movement and the solute segregation is also discussed.

Manifested flatness defect prediction in cold rolling of thin strips

Flatness defects in thin strip cold rolling are a consequence of roll thermo-elastic deformation, from which heterogeneous strip plastic deformation results. When flatness defects manifest on line, buckling reorganizes the stress field in the pre- and post-bite areas. Comparison with flatness roll measurement requires this effect to be taken into account. A coupled Finite Element Method (FEM) approach is used here to compute stresses and strains in-bite as well as out-of-bite. The detection of buckled (non-flat) areas is demonstrated for a very thin strip cold rolling case. The model is then applied here to two questions, namely the impact on flatness of the heterogeneous temperature field and the effect of friction on optimal setting of a flatness actuator, Work Roll Bending.

自然和人工管理驱动下盐城海滨湿地景观格局演变特征与空间差异

Wetlands are one of most important ecological systems on earth.Their unique ecosystem function has led to them been sometimes called earth′s filtration system.Coastal wetland is an important type of wetland,which is not only a rare ecosystem but is also often found in areas disturbed human activities.Landscape changes have become the most significant feature of this fragile zone.Landscape changes inevitably have effects on material circulation and energy flow within the coastal wetland ecosystems,which alter the landscape ecological processes,as well as the coastal wetland pattern and function.In this study,Yancheng coastal wetland was chosen as the study area.The Yancheng coastal wetland,located in the east of Jiangsu Province,is one of the largest mudflats in China and represents one of the most rapidly changing landscapes in the world.Changes to coastal landscapes can have natural causes or result from human activities;the pattern of change greatly differs between the two.We focused on the core area of Yancheng Nature Reserve.This area was divided into two parts based on the type of landscape change: change in the northern area being characterized as from human activities,and that in the southern area characterized as from natural causes.We used remote sensing digitized data from 1987,1997 and 2007,and applied GIS techniques and methods of landscape ecology to analyze the temporal and spatial changes in landscape pattern caused by natural processes and human activities.In the area affected by human activities,from 1987 to 2007,the landscape pattern was characterized by greater fragmentation,higher dominance and less landscape diversity than those of the other area.The mean vegetation patch area decreased from 205.31 hm2 to 55.60 hm2 and the landscape diversity index from 1.4284 to 1.2928 while the landscape dominance index increased from 0.3634 to 0.7766.This landscape change resulted in degraded and heterogeneous strips of wetlands.The wetlands showed a one-direction succession from land to sea in the period between 1987 and 1997 but the landscape exhibited a multi-directional succession from 1997 to 2007.In the area that was affected only by natural processes,from 1987 to 2007,the landscape pattern was characterized by greater diversity and less dominance than those of the other area.The landscape dominance index decreased from 0.4844 to 0.3164 while the landscape diversity index increased from 0.9019 to 1.4754.Strips of wetland development were more evident and there was a one-direction succession from land to sea between 1987 and 2007.The geomorphic deposit process and the succession of plant communities adapting to environmental changes were fundamental factors driving the landscape dynamics of the area not affected by human activities.Whereas human activity,such as dyke construction,was the important driving force behind the changes in landscape pattern in the area with human activities.Through identifying the rules of landscape changes caused by different factors in the Yancheng coastal wetland,we can form a scientific basis for the sustainable exploitation of the coastal wetlands,ensuring protection of the wetland resources while simultaneously achieving social and economic sustainable development in the Yancheng coastal region.

A multiscale continuum model of the grain-size dependence of the stress hysteresis in shape memory alloy polycrystals

In this paper, a multiscale continuum model is proposed to study the effect of grain size on the macroscopic dissipative response of shape memory alloy polycrystals during isothermal thermoelastic phase transition. In the simplest one-dimensional (1D) heterogeneous structural hierarchy, a series of non-local and non-convex double-well continuum elements are employed to model the micro-instability and the macroscopic stress hysteresis of the material under uniaxial quasi-static stretching. Three characteristic length scales (specimen size L, grain size l and intrinsic material length g) of a bulk polycrystal are imbedded in the 1D chain model and their important roles in the macroscopic dissipation are quantified. It is shown that the specific energy dissipation or the width of the stress hysteresis is governed by two non-dimensional ratios, N(= L/ l) and l ¯ = ( l / g ) . For a given specimen of size L, the hysteresis decreases rapidly at either very large or small values of l. In particular, it vanishes when the grain size is reduced to the nano-scale where the grain size and the interface thickness become comparable. The above results of the 1D model are reproduced in a two-dimensional (2D) non-local numerical experiment on the energy dissipation during multiple domain evolution in heterogeneous strips. The predictions of the two models agree well qualitatively with the recent experimental observations of the stress hysteresis in nano-grained superelastic NiTi polycrystals.

Multi-scale second-order computational homogenization of multi-phase materials: a nested finite element solution strategy

This paper presents the detailed implementation and computational aspects of a novel second-order computational homogenization procedure, which is suitable for a multi-scale modelling of macroscopic localization and size effects. The second-order scheme is an extension of the classical (first-order) computational homogenization framework and is based on a proper incorporation of the gradient of the macroscopic deformation gradient tensor into the kinematical macro–micro scale transition. From the microstructural analysis the macroscopic stress and higher-order stress tensors are obtained, thus delivering a microstructurally based constitutive response of the macroscopic second gradient continuum. The higher-order macroscopic constitutive tangents are derived through static condensation of the microscopic global tangent matrix. For the solution of the second gradient equilibrium problem on the macrolevel a mixed finite element formulation is developed. As an example, the second-order computational homogenization approach is applied for the multi-scale analysis of simple shear of a constrained heterogeneous strip, where a pronounced boundary size effect appears.

The stressed state of a laminated elastic composite with a thin linear inclusion

The plane deformation of a piecewise-uniform body is investigated using the method of homogenization of an elastic medium with macroperiodic structure [1, 2]. The body consists of a periodic system of joined heterogeneous strips, and there is a thin elastic inclusion of finite length in one of the straight interfaces of the materials. By representing the stresses and displacements in terms of complex potentials [3] and using the conditions for the interaction of a thin elastic inclusion with the matrix [4–6] a system of four integrodifferential equations of the problem is obtained which has a solution which is appropriate for inclusions of any stiffness: from absolutely pliable, which simulates a cut, to absolutely rigid. Unlike existing solutions for defects at the interface of materials [7, 8], the solution obtained does not oscillate in the region of the tip of the inclusion.

Three-dimensional perturbation solution for a dynamic planar crack moving unsteadily in a model elastic solid

A half-plane crack propagates dynamically, nominally in the x direction, along the plane y = 0 in an unbounded solid subjected to remote loading equivalent to a static stress intensity factor K∗. The crack front at time t lies along the arc x = v 0t + ϵƒ(z, t) where ƒ v 0 is a constant velocity, ( z, t) is an arbitrary function, and ϵ is a small parameter. The crack front speed thus varies along the z axis and its shape deviates from straightness. We address this problem within a model 3D elastodynamic theory involving a single displacement variable u, satisfying a scalar wave equation, and representing tensile opening or shear slippage, with associated tensile or shear stress σ = M δu δy across planes parallel to the crack, where M is an elastic modulus. The problem is then one of finding a solution to the scalar wave equation satisfying σ = 0 on y = 0 within the rupture. When ϵ = 0 the solutions for u, σ, dynamic stress intensity factor K and energy release rate G are familiar 2D results. We develop corresponding 3D solutions to first order in σ, for arbitrary ƒ( z, t). The solutions are used to address in some elementary cases how a crack front moves unsteadily through regions of locally variable fracture resistance. When a straight crack front approaches a slightly heterogeneous strip, lying parallel to the crack tip along an otherwise homogeneous fracture plane, it may be blocked by asperities after some advancement into the heterogeneous region if it has a relatively small incoming velocity. If, however, the incoming crack velocity is relatively high, the asperities give way and the, now curved, crack front propagates into the bordering homogeneous region. There, the moving crack front recovers a straight configuration through slowly damped space-time oscillations. The oscillatory crack tip motion results from constructive-destructive interferences of stress intensity waves, initiated by encounters of the crack front with asperities, and then propagating along the front. Oscillations in response to a heterogeneity that is spatially periodic in the direction along the crack front decay as t t- 1 2 at large t. The slowness of the decay suggests that the straight crack front configuration may be sensitive to small sustained heterogeneity of the fracture resistance. This is consistent with results of a related analysis ( Perrin and Rice, 1994, in press, J. Mech. Phys. Solids) based upon a strictly linearized form of our equations. The persistence of unsteady crack tip motion beyond the immediate region of heterogeneities provides an explanation for high frequency seismic radiation, using a lesser amount of heterogeneity than what might be naively assumed by strict correspondence of all curved and variable velocity portions of a propagating rupture front to asperities. Also, oscillations of crack tip velocity in the presence of sustained small heterogeneities, suggested by features of our 3D results for the model theory, may provide a mechanism for the generation of rough tensile fracture surfaces when the average (macroscopic) propagation speed of the crack is relatively small.

Surface heterogeneity and contact angle hysteresis

The effect of surface heterogeneity on contact angle hysteresis is studied by using the model of Neumann and Good of a vertical plate with horizontal heterogeneous strips. The results of this study explain well known, but not understood patterns of contact angle behaviour: On the one hand, the advancing contact angle on a carefully prepared solid surface is generally reproducible; on the other hand, even a very small amount of surface heterogeneity may cause the receding contact angle to be less reproducible and to depend on several non-thermodynamic factors.