Chain-like Microstructure Research Articles

Evolution mechanism of the core-shell to homogeneous microstructure in immiscible alloy by semi-solid treatment under a high magnetic field

Immiscible alloys possess broad industrial applications, but the presence of immiscible gaps makes them highly susceptible to the formation of segregated microstructures, thus, a new method for obtaining a uniform distribution of fine minor separated phases needs to be explored. The present work focuses on the evolution mechanism of a CuCo immiscible alloy with a core-shell microstructure during semi-solid isothermal processing under a high magnetic field. The results showed that the microstructure changes from the core-shell structure of the initial sample to homogeneous microstructure after semi-solid treatment, regardless of whether a magnetic field is applied. The mechanism of microstructure evolution of the alloy during semi-solid treatment was revealed through in-situ experiment, in which Ostwald ripening and coalescence mechanisms led to grain coarsening, grain boundary remelting caused grain dissociation, and ultimately the grains underwent spheroidization. However, as a high magnetic field was applied, the average radius of the Co-rich particles decreased, and the particles were aligned in the direction of the magnetic field, forming a chain-like microstructure due to dipole-dipole interactions between the grains. The CuCo immiscible alloy with a uniform microstructure after semi-solid treatment exhibited a higher microhardness and saturation magnetization. This study offers a novel perspective for designing immiscible alloys with homogeneous microstructures as well as chain-like microstructures by utilizing semi-solid treatment combined with a magnetic field.

The exopolysaccharides produced by Leuconostoc mesenteroides XR1 and its effect on silk drawing phenomenon of yoghurt

Yoghurt fermented by Leuconostoc mesenteroides XR1 from Kefir grains was found to produce a unique silk drawing phenomenon. This property was found to be associated with the exopolysaccharides (EPS), X-EPS, produced by strain XR1. In order to better understand the mechanism that produced this phenomenon, the X-EPS was extracted, purified and characterized. The molecular weight and monosaccharide composition were determined by size exclusion chromatography coupled with multi-angle laser light scattering (SEC-MALLS) and ion chromatography (IC) analysis, respectively. The results showed that its molecular weight was 4.183 × 106 g/mol and its monosaccharide composition was glucose, and glucuronic acid, with the contents of 567.6148 and 0.2096 μg/mg, respectively. FT-IR and NMR analyses showed that X-EPS was an α-pyranose polysaccharide and was composed of 92.22 % α-(1 → 6) linked d-glucopyranose units and 7.77 % α-(1 → 3) branching. Furthermore, it showed a chain-like microstructure with branches in atomic force microscopy (AFM) and scanning electron microscopy (SEM) experiments. These results suggested that the unique structure of X-EPS, gave the yoghurt a strong viscosity and cohesiveness, which resulted in the silk drawing phenomenon. This work suggested that X-EPS holds the potential for food and industrial applications.

Characterization of Dextran Biosynthesized by Glucansucrase from Leuconostoc pseudomesenteroides and Their Potential Biotechnological Applications

Glucansucrase was purified from Leuconostoc pseudomesenteroides. The glucansucrase exhibited maximum activity at pH 5.5 and 30 °C. Ca2+ significantly promoted enzyme activity. An exopolysaccharide (EPS) was synthesized by this glucansucrase in vitro and purified. The molecular weight of the EPS was 3.083 × 106 Da. Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy showed that the main structure of glucan was 97.3% α-(1→6)-linked D-glucopyranose units, and α-(1→3) branched chain accounted for 2.7%. Scanning electron microscopy (SEM) observation of dextran showed that its surface was smooth and flaky. Atomic force microscopy (AFM) of dextran revealed a chain-like microstructure with many irregular protuberances in aqueous solution. The results showed that dextran had good thermal stability, water holding capacity, water solubility and emulsifying ability (EA), as well as good antioxidant activity; thus it has broad prospects for development in the fields of food, biomedicine, and medicine.

Influence of chain-like cobalt particles on the properties of magnetorheological elastomers

The different pre-structure formed by the particles determine the performance of magnetorheological elastomers (MREs). In this study, spherical cobalt particles with a diameter of 2–5 μm and chain-like cobalt particles (CCPs) composed of spherical particles with a diameter of about 1 μm with a chain length of 10–40 μm were respectively prepared. These two kinds of particles were used to prepared MREs under different orientation magnetic fields. The effects of different chain-like microstructures on the performance of MRE are compared. The dynamic viscoelastic test results of MREs show that the chain-like particles increased the movement resistance in the matrix, thereby increasing the damping factor of CCP-MREs. The special chain-like particles improved the Payne effect factor and magnetic field-induced storage modulus of isotropic CCP-MRE. It is worth noting that the performance of anisotropic CCP-MRE is different from traditionally believed performance improvement. Based on this work, a relationship curve between the MR effect and the adjacent particles’ distance was proposed.

Effect of self-assembly on fluorescence in magnetic multiphase flows and its application on the novel detection for COVID-19.

In the present study, the magnetic field induced self-assembly processes of magnetic microparticles in an aqueous liquid (the pure magnetic fluid) and nonmagnetic microparticles in ferrofluid (the inverse magnetic fluid) are experimentally investigated. The microparticles are formed into chain-like microstructures in both the pure magnetic fluid and the inverse magnetic fluid by applying the external magnetic field. The fluorescence parameters of these self-assembled chain-like microstructures are measured and compared to those without the effect of magnetic field. It is found that the fluorescence in the pure magnetic fluid is weakened, because the scattering and illuminating areas are reduced in the microstructures. On the contrary, the fluorescence in the inverse magnetic fluid is enhanced, because more fluorescent nonmagnetic microparticles are enriched and become detectable under the effect of the magnetic dipole force and the magnetic levitational force, and their unnecessary scattering can be absorbed by the surrounding ferrofluid. The average enhancement of the fluorescence area ratio in the inverse magnetic fluid with 3 μm nonmagnetic microparticles reaches 112.92%. The present work shows that the inverse magnetic fluid has advantages such as low cost, no scattering effect, stable fluorescence intensity, and relatively low magnetic resistance. In the end, a prototype design for the novel detection of coronavirus disease 2019 based on the magnetic field induced self-assembly in the inverse magnetic fluid is proposed, which could support the epidemic prevention and control.

Magneto-Mechanical Coupling in Magneto-Active Elastomers.

In the present work, the magneto-mechanical coupling in magneto-active elastomers is investigated from two different modeling perspectives: a micro-continuum and a particle–interaction approach. Since both strategies differ significantly in their basic assumptions and the resolution of the problem under investigation, they are introduced in a concise manner and their capabilities are illustrated by means of representative examples. To motivate the application of these strategies within a hybrid multiscale framework for magneto-active elastomers, their interchangeability is then examined in a systematic comparison of the model predictions with regard to the magneto-deformation of chain-like helical structures in an elastomer surrounding. The presented results show a remarkable agreement of both modeling approaches and help to provide an improved understanding of the interactions in magneto-active elastomers with chain-like microstructures.

Mechanisms of spontaneous chain formation and subsequent microstructural evolution in shear-driven strongly confined drop monolayers.

It was experimentally demonstrated by Migler and his collaborators [Phys. Rev. Lett., 2001, 86, 1023; Langmuir, 2003, 19, 8667] that a strongly confined drop monolayer sheared between two parallel plates can spontaneously develop a flow-oriented drop-chain morphology. Here we show that the formation of the chain-like microstructure is driven by far-field Hele-Shaw quadrupolar interactions between drops, and that drop spacing within chains is controlled by the effective drop repulsion associated with the existence of confinement-induced reversing streamlines, i.e., the swapping trajectory effect. Using direct numerical simulations and an accurate quasi-2D model that incorporates quadrupolar and swapping-trajectory contributions, we analyze microstructural evolution in a monodisperse drop monolayer. Consistent with experimental observations, we find that drop spacing within individual chains is usually uniform. Further analysis shows that at low area fractions all chains have the same spacing, but at higher area fractions there is a large spacing variation from chain to chain. These findings are explained in terms of uncompressed and compressed chains. At low area fractions most chains are uncompressed (spacing equals lst, which is the stable separation of an isolated pair). At higher area fractions compressed chains (with tighter spacing) are formed in a process of chain zipping along y-shaped structural defects. We also discuss the relevance of our findings to other shear-driven systems, such as suspensions of spheres in non-Newtonian fluids.

The magnetic field dependent dynamic properties of magnetorheological elastomers based on hard magnetic particles

In this study, novel magnetorheological elastomers based on hard magnetic particles (H-MREs) were developed and the magnetic field dependent dynamic properties of the H-MREs were further investigated. The storage modulus of H-MREs could not only be increased by increasing magnetic field but also be decreased by the increasing magnetic field of opposite orientation. For the anisotropic H-MREs with 80 wt% NdFeB particles, the field-induced increasing and decreasing modulus was 426 kPa and 118 kPa respectively. Moreover, the dynamic performances of H-MREs significantly depended on the pre-structure magnetic field, magnetizing field and test magnetic field. The H-MREs were initially magnetized and formed the chain-like microstructure by the pre-structure magnetic field. The field-induced increasing and decreasing modulus of H-MREs both raised with increasing of the magnetizing field. When the magnetizing field increased from 400 to 1200 kA m−1, the field induced decreasing modulus of the 80 wt% isotropic H-MREs raised from 3 to 47 kPa. The magnetic field dependent curves of H-MREs’ storage modulus were asymmetric if the magnetizing field was higher than the test magnetic field. Based on the dipolar model of MREs and magnetic properties of hard magnetic material, a reasonable explanation was proposed to understand the H-MREs’ field dependent mechanical behaviors.

Micro-gap flow behavior and micro-structure of stored electro-rheological nano-suspensions in the presence of sinusoidal electric field

Simultaneous observations of the electro-rheological (ER) effect and flow behavior within micro-gap were performed for a nano-suspension based on nano-particles of titanium dioxide and a chemically modified silicone oil stored for a long time. The ER effect and flow behavior in the presence of sinusoidal electric field simultaneously observed were discussed. In the presence of the sinusoidal electric field of 1 kHz, a chain-like microstructure spanning the two electrodes was induced for the nano-suspension stored for 28 months. The stability of the flow and the ER response was fairly well even for the stored nano-suspension in the presence of the sinusoidal electric field of 1 kHz. However, simultaneous observations showed substantial wall slip of the nano-suspension. The ER effect was also discussed in terms of a dimensionless relationship between the induced stress and Mason number, which is a characteristic ratio of viscous forces to polarization forces acting on a particle.

Modelling of iron-filled magneto-active polymers with a dispersed chain-like microstructure

Magneto-active polymers are a class of smart materials commonly manufactured by mixing micron-sized iron particles in a rubber-like matrix. When cured in the presence of an externally applied magnetic field, the iron particles arrange themselves into chain-like structures that lend an overall anisotropy to the material. It has been observed through electron micrographs and X-ray tomographs that these chains are not always perfect in structure, and may have dispersion due to the conditions present during manufacturing or some undesirable material properties. We model the response of these materials to coupled magneto-mechanical loading in this paper using a probability based structure tensor that accounts for this imperfect anisotropy. The response of the matrix material is decoupled from the chain phase, though still being connected through kinematic constraints. The latter is based on the definition of a ‘chain deformation gradient’ and a ‘chain magnetic field’. We conclude with numerical examples that demonstrate the effect of chain dispersion on the response of the material to magnetoelastic loading.

Magneto-induced stress enhancing effect in a colloidal suspension of paramagnetic and superparamagnetic particles dispersed in a ferrofluid medium.

The magneto-induced stress and relative microstructure in a colloidal suspension of paramagnetic and superparamagnetic particles dispersed in a ferrofluid medium is studied using particle-level dynamics simulation. It shows that the stress perpendicular to the direction of an external uniaxial magnetic field can be strongly enhanced by increasing the ratio of paramagnetic particles to approaching that of superparamagnetic particles. The magnetic field-induced net-like or embedded chain-like microstructures formed by paramagnetic and superparamagnetic particles contribute to this stress enhancing effect.

Mechanical properties of silicone composites reinforced with micron- and nano-sized magnetic particles

Silicone composites filled with different-sized nickel particles were prepared. By applying a permanent magnet, both the micron- and nano-sized particles were found to distribute along the magnetic field direction, resulting in chain-like microstructures, which improved the key mechanical properties of the resultant samples effectively, compared to the sam- ples with randomly-distributed particles. The composites were also tested under various magnetic field strengths. The sam- ples with aligned particles showed larger improvements in shear storage modulus than those with random particles.

Synthesis and Self-Assembly of Ultrathin NixFe1−x(OH)2 Nanodiscs via a Wet-Chemistry Method

Self-assembled ultrathin Ni(x)Fe(1-x)(OH)2 nanodiscs have been synthesized by using a wet-chemistry method. The uniformity and the assembly of Ni(x)Fe(1-x)(OH)2 nanostructures are sensitive to the iron ion concentration in the precursor. An optimum iron concentration of 10% results in the formation of uniform ultrathin Ni(x)Fe(1-x)(OH)2 nanodiscs with a typical side length of 50 nm and a thickness of 10 nm. They are also self-assembled by connecting their (001) facets with in-plane orientation and form a chain-like microstructure. Lattice relaxation is present within several atomic layers at the interfaces between two adjacent nanodiscs, which introduces about 6 degrees misalignment of these nanocrystals. Analytical electron microscopy analysis reveals that the iron additive atoms distribute uniformly in the nanodiscs and they are substitution atoms of Ni atoms. It has been found that the iron species is critical to the formation and assembly of the hexagonal nanodiscs.

Linear viscoelastic properties of electro-rheological nano-suspension confined to narrow gap between electrodes

The linear viscoelastic properties of a suspension composed of titanium dioxide nanoparticles were measured under the direct current (dc) electric field with narrow gap distances between the electrodes. The yielding behavior under no external electric fields was also discussed. The wall slip at the interface between the parallel plates and the nano-suspension was briefly discussed. Under the dc electric field, a fine chain-like microstructure was optically found within a narrow gap of 50 μm between the electrodes in the quiescent state. The nano-suspension confined to a narrow gap of 65 μm between the parallel plates was rather viscoelastic even at the highest strength of the electric field of 16 kV·mm−1. Furthermore, fast and slow relaxations of the dynamic moduli were found after removal of the electric field. It was pointed out that the linear viscoelasticity was an appropriate measure of the microstructure before yielding.

Electrically developed morphology of carbon nanoparticles in suspensions monitored by in situ optical observations under sinusoidal electric field

In situ optical observations were performed for suspensions composed of carbon nanoparticles under the sinusoidal electric field with an amplitude around 20 kV/mm (volt per micrometer) and various frequencies. For extremely diluted suspensions of mixed fullerenes or multiwalled carbon nanotubes (MWNTs) in a silicone oil, the dark-field optical microscopy was effective for the in situ observation of the particle behavior under the electric field. The nanoparticles in a fullerene suspension under the sinusoidal electric field with a frequency of 100 Hz (in short, 100 Hz electric field) were aggregated to form a rigid spherical microstructure around the halfway between the electrodes. On the other hand, the nanoparticles in an MWNT suspension under 100 Hz electric field were also aggregated but aligned to form a chain-like microstructure which spans the electrodes. Both of the aggregated particles were stable even after the removal of the electric field, and they were redispersed by application of 10 Hz electric field.

Microstructures in Strongly Interacting Dipolar Fluids

The formation of ring- and chain-like microstructures inferrofluids consisting of interacting magnetic particles coated by asurfactant layer is studied by Monte Carlo simulations. For thincoating layers, it is found that ring- and chain-like structurescoexist. The ring-like structures are suppressed by thicker coatinglayers. These observations are in agreement with recent experiments.Also, the ring-like structures are formed by dynamical aggregation ofparticles, instead of bending of linear chains. The latter process isforbidden by a substantial energy barrier. More generally, it is foundthat the ring structures exist when the parameter α = μ2/[kBT(d+2δ)3] is higher than a critical value αc, where μ is themagnitude of magnetic moment, d is the diameter, and δ is thecoating thickness of a particle. We find that when α<αc, the ringstructures cannot be formed spontaneously, while the chain-likestructures still exist. Furthermore, the critical value αc isalmost independent of magnetic particle volume density in the lowdensity range.

Rheological responses of oil-in-oil emulsions in an electric field

In the present article, the rheological responses of oil-in-oil emulsions in a dc electric field were investigated experimentally. Specifically, the dispersed phase of the emulsions considered in this work was less conducting than the continuous phase. Depending on the relative strength between the shear flow and electric fields, three distinctive responses of the emulsions were observed in steady and dynamic oscillatory shear tests. First, the apparent viscosity enhancement (positive electrorheological effect) was produced when the electric field was predominant. Second, when the shear flow was strong and dominant, the electric field played a trivial role on the rheological behavior of emulsions. Finally, the viscosity reduction (negative electrorheological effect) was generated when the shear flow and electric fields were competitive. The viscosity enhancement was induced by the formation of chain-like microstructures of the dispersed droplets as in a typical electric-field responsive particle suspension. Meanwhile, the viscosity reduction was closely associated with the electric-field-induced rotation of the dispersed droplets, which was confirmed by the electrohydrodynamics of a single conducting drop in a more conducting ambient fluid. Finally, the rheological responses of the model emulsions to a dynamic small-amplitude oscillatory shearing were considered in conjunction with the morphology evolution under the action of the electric and flow fields. The results showed that the interfacial contribution to the rheological response appeared quite differently depending on the conductivity ratio of the two contiguous fluids.

VISUALIZATION OF ER FLUID MICROSTRUCTURE IN OSCILLATORY SLIT FLOW

The dynamic viscoelasticity of ER suspensions depends on the fluid strain amplitude, the oscillation frequency and the applied electric field strength. This behavior can be explained based on the formation and deformation of ER particle clusters that span the electrodes. In this work, changes of the fibrous structure of an ER fluid in an oscillatory slit flow are analyzed (by direct observation using an optical microscopic probe) when the electric field, the strain amplitude and the oscillation frequency are varied. The ER fluid studied in this work is composed of sulfonated polymer particles (5 vol%) in a silicone oil. The hysteresis loop of pressure difference as a function of strain is explained based on the visualization of the chain-like microstructure.