Strings Of Particles Research Articles

Rheology and ordering transitions of non-Brownian suspensions in a confined shear flow: Effects of external torques

We investigate the effect of an external torque, applied in the vorticity direction, to particles in a sheared non-Brownian suspension confined by rigid walls. At volume fractions of ϕ=0.48-0.52 such suspension flows undergo an ordering transition, developing a hexagonal structure of particle strings in the velocity gradient-vorticity plane. The hexagonal structure is disturbed by negative torques, leading to an increase in the shear viscosity. Positive torque has a favorable effect on the ordered state. However, if the magnitude of the positive torque exceeds a certain threshold, the hexagonal order begins to be weakened. Due to the significant changes in suspension microstructures, rheological parameters such as the shear and vortex viscosities exhibit nonlinear responses to the external torques. On the other hand, at lower volume fractions ϕ≤0.40, where ordered structures are not developed, suspension microstructure is not sensitive to an external torque and the apparent viscosity is a linear function of the torque.

Network of polystyrene particle strings fabricated using glass slide with hydrophobic and hydrophilic periodical patterns

A dispersion of polystyrene (PS) particles was dried between two glass slides, one of which has a ∼100 μm periodical pattern surface of gold squares. The gold squares and the bare glass parts were modified with a self-assembled monolayer to produce hydrophobic and hydrophilic periodical patterns. The PS particles were confined in the hydrophilic regions, and network strings of PS particles were fabricated using the patterned glass slide with a hydrophobic gold surface.

Enhanced hardening of soft self-assembled copolymer gels under homogeneous magnetic fields

The rheological response of highly swollen physical gels obtained by self-assembling of triblock copolymers containing low remanence ferromagnetic particles was investigated in the presence of external homogeneous magnetic fields. Three different types of sample geometries with distinctive magnetic particle orderings were investigated: isotropic (no magnetic field present during synthesis), parallel to the plane of the gel film and perpendicular to the plane of the gel film. Both the storage and loss moduli exhibit a strong increase with magnetic field strength for all geometries. Dependence of the rheological response on particle volume fraction was also investigated. The strength of such rheological hardening, as well as its saturation behaviour, depend strongly on the relative orientation between particle strings, shear and external field. In some cases a very strong relative increase of storage modulus, up to 6000% was obtained. Further transient rheological studies suggest that strong rearrangement of the particle network is largely responsible for the enormous increase in elastic modulus. Parallel to that, a maximum in the loss factor was observed as a function of particle volume fraction and field strength and it was interpreted in terms of a competition between an increase in string (clusters) hardening and a decrease in their ability to deform and flow. These results suggest that magnetorheological gels are an intermediate system between magnetorheological elastomers (MREs) and magnetorheological fluids (MRFs) with directional dependent rheological response and partial rearrangement of the particle network.

Shear-induced 1-D alignment of alumina nanoparticles in coatings

Atomic Force Microscopy and Scanning Electron Microscopy were used to study shear-induced alignment of alumina and silica nanoparticles in two-component polyurethane clear coatings. 1-D strings of nanoparticles, formed in an extended pearl-necklace fashion were observed near the surfaces of cured films at nanoparticle volume fractions less than 0.05. This alignment is affected by the shear conditions of the application method. When applied by spraying, linear particle strings as long as 5 cm were observed in the direction of shear. Nanoparticle strings were also found, to a lesser extent, when coatings were applied by a drawdown method. The phenomenon was not observed in coatings applied with minimal shear. These particle string formations, in addition to affecting the performance of coatings, may have broader implications in the field of nanomaterials. Our literature searches so far have not uncovered reports of stable, 1-D nanoparticle arrangements with same degree of linearity produced under simple shear with compositions having very low particle loadings.

Phase Transformation for Primary Particles in the Surface Regions of an AA1200 Alloy

In the present investigation the particle structure in an AA1200 sheet ingot used for litho applications has been studied. Caustic etching of the as-cast material was seen to result in a zone close to the surface with a different etching response. This zone was identified as what is known as a fir-tree zone or an Altenpohl zone [1,2,3,4]. A variation in particle type over the cross section of the as-cast ingot was seen to follow the differences in etching response. After heat treatment of the material, the fir-tree zones were no longer visible, and the accompanying change in particle structure was studied. Samples from the subsurface regions and from a distance of ~20 cm from the surface has been investigated before and after heat treatment. In the as-cast material, the sample from the surface was dominated by featherlike particles with long strings of particles, identified as AlmFe. While closer to the centre Al3Fe and Al6Fe were seen to be the main phases, however, some AlmFe and probably some α-AlFeSi was also found in this sample. After heat treatment, the particle structure was seen to change, and the surface sample contained mainly Al3Fe in addition to a small amount of AlmFe. The change in particle structure during heat treatment is discussed with reference to the change in etching response.

Creep and recovery behaviors of a polythiophene-based electrorheological fluid

We investigate the creep response of poly(3-thiopheneacetic acid) (PTAA) particles doped with perchloric acid. With increase in applied stress, these suspensions exhibit an evolution from a linear viscoelastic response, with three components of instantaneous elastic strain, retarded elastic strain and viscous strain, to a nonlinear viscoelastic response, where the retarded elastic and viscous strains monotonically decrease and a plastic contribution to the instantaneous strain grows, followed by a viscoplastic solid behavior, with fully plastic instantaneous strain, and finally a transition from plastic solid to a plastic liquid at the yield stress. With increase in electric field strength at fixed particle concentration and applied stress, the viscoplastic response diminishes, and more elastic behavior ensues. For highly doped samples, at high-electric field strengths, a fully elastic solid response is observed in the linear viscoelastic regime. The equilibrium compliance, JC and steady state recoverable compliance JR, were investigated as a function of electric field strength, particle concentration and particle conductivity. The results are interpreted in terms of the field-induced formation of thick fibrillar aggregates spanning the gap between the electrodes, each consisting of bundles of particle strings. Strings, which are fully connected to both electrodes generate an elastic response to the applied stress, whereas strings which are attached at only one end or are unattached generate a viscoplastic response. The net effect of an increase of the electric field strength, particle concentration, or particle conductivity is an increase in elasticity, i.e. predominantly creation of fully connected particle strings.

What do Japanese and Korean Have in Common?

Abstract. Many of the verb endings of modern Japanese and Korean have been created by contracting structures that consist of the stem + attached strings of particles and auxiliaries. Most of the auxiliaries have been taken from free verb stems that were grammaticalized for special purposes. Though the paradigmatic systems grew independently in the two languages, many of the ingredients go back to a common source that we can reconstruct on the basis of their shapes and meanings. Korean and Japanese share certain configurations of meaning and grammar, such as the well-known marking of focus, that are realized by markers which are not directly cognate in these structures but can be seen as cognate with forms in other structures within each language. These two languages have much more in common with each other than either has with any other language. This is why we think it is possible to reconstruct a prehistoric ancestor that can be called proto Korean-Japanese.

Highly anisotropic distribution of iron nanoparticles within MCM-41 Mesoporous Silica

Electron microscopy techniques are used to visualize the spatial distribution of iron nanoparticles inside a mesoporous MCM-41 molecular sieve. Direct observation of the iron oxide nanoparticles by STEM-HAADF imaging reveals a highly non-uniform spatial distribution inside the mesopores. These particles are retained in the pores after a reduction treatment unlike the behavior found in other similar systems. It is found that thermal treatments induce changes in its morphology, creating nanowires from particle strings.

Particle velocity and particle clustering in down-flow reactors

Particle clustering is a phenomenon of importance in many applications in gas–solid fluidized beds. Using a novel Chemical Reactor Engineering Centre Gas–Solid Optiprobe ( CREC-GS-Optiprobe) equipped with a Graded Refractive INdex ( GRIN) lens to form a 285 μm diameter high illuminated region at 5.4 mm away from the tip of the sensor, particle cluster lengths were measured with minimum probe intrusion interference. Experiments were developed in a 3 m height and 0.0263 m diameter acrylic unit using a pair of CREC-GS-Optiprobes spaced 6.05 mm with the probes placed at 1.85 m away from the gas injection port. Operating conditions were varied from 1 to 3 m/s gas superficial velocities and from 2 to 12 kg/m 2 s particle solid fluxes resulting in solid hold-ups between the 0.001 and 0.003 ranges. Acquired signals displayed distinctive double peaks showing unequivocally the simultaneous detection of particle clusters on both probes. It is concluded that in down-flow units (FCC particles of 70 μm) catalyst particles evolve as particle strings with a relatively large size distribution: average cluster size ranging between 2.0 and 6.3 times of the average particle diameter with most of the particles strings having between 2.9 and 5.5 times of the average particle diameter.

Nonlinearity from geometric interactions: A case example

We propose a ladder model wherein dynamical nonlinearity arises from geometry. It includes two strings of particles which are set along rigid rails of a "railroad" and coupled by linear springs. Physical realizations of the model include dust-particle strings in plasma sheaths and chains of microparticles trapped in a strong optical lattice. The transverse couplings between the strings, along with the motion constraint imposed by the rails, generate nonlinearity. It gives rise to robust solitary waves, which are found analytically in the long-wavelength limit, and are obtained in simulations of the full system.

Loop Formation of Au Nanopaticles Adsorbed on Langmuir Monolayers

Abstract A hybrid monolayer of Au nanoparticles and dioctadecyl-dimethylammonium chloride (DODAC) was formed at the air–water interface. The Au nanoparticles were found to form loop structures. The loops were 0.1–1 μm in size with a width of around 5 nm, and consisted of single particle strings. The loop formation depended on the density of the DODAC monolayers, but not on the size distribution of the particles.

Dynamic transitions and oscillatory melting of a two-dimensional crystal subjected to shear flow

The effect of a delicate balance of forces on the interparticle dynamics and structure of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed as shear flow was applied to the system. A strong dipole–dipole repulsion, due to ionizable surface sulfate groups, induces the particles to arrange themselves on a hexagonal lattice under quiescent conditions. The application of a shear flow to the interface, however, forces the lattice into a new semiordered, anisotropic state over which great control is exerted by particle concentration and applied shear rate. At low particle concentrations or high shear rates, the forces applied by the flow dominate the system and cause strings of particles to align in the flow direction to facilitate their movement past each other. A remarkable contrast to this behavior is seen at high concentrations or low shear rates, where the interparticle forces gain importance and tend to more strongly keep the particles in their lattice positions. Consequently, domains of particles are forced to rotate in the flow. The transition between these two regimes and the nature of this rotation, including an associated cyclic melting and crystallization of the lattice, is discussed.

Photoinduced Alignment of Ferroelectric Liquid Crystals Using Azobenzene Polymer Networks of Chiral Polyacrylates and Polymethacrylates

Two chiral dimethacrylate and one chiral diacrylate monomer containing an azobenzene group were synthesized and dissolved in a commercial ferroelectric liquid crystal (FLC) host. With two of the monomers, thermally induced radical polymerization with the mixture exposed to linearly polarized irradiation results in a bulk alignment of FLC in the direction perpendicular to the polarization of irradiation light as a result of the photoinduced orientation of azobenzene groups related to the trans−cis isomerization. The monomer that is most effective for photoalignment of FLC displays a polymer network formed by strings of colloidal particles (∼250 nm in diameter), which exerts a commanding effect on the alignment of FLC as being able to reorient the FLC by changing the polarization direction of subsequent irradiation. The study suggests that the network of colloidal particles is isotropic and that the photoalignment of azobenzene moieties on the large surface of the network dictates the alignment of FLC.

Order and disorder in particle?liquid flows in a pipe

The spherical expanded polystyrene particle–oil two-phase flow in a vertical pipe was used to simulate the dispersed phase distribution in laminar bubbly flows. A three-dimensional particle image tracking technique was used to track the particles in the flow to study the ordered structure of dispersed phase distribution and its transition to disorder. The ordered structures behaved as particle strings aligned in the flow direction as induced by the flow shear. The structures were quite durable in high liquid velocity flows and dispersed gradually as the liquid velocity decreased. In lower velocity flows, the particles tended to form clusters in the horizontal direction, as predicted by potential theory for spherical bubbles rising in a quiescent inviscid liquid and as observed in experiments on non-shear bubbly water flows.

Microstructures of friction stir weld joints between an aluminium-base metal matrix composite and a monolithic aluminium alloy

Microstructures in friction stir welds between monolithic AA2024 and AA2014 reinforced with 20 vol% particulate Al 2O 3 reveal that the narrowest layers of each material are about 0.1 mm thick. Thus, each material retains its identity in the weld zone and convoluted macrointerfaces can be identified between material domains. When the harder material is on the advancing side of the tool the macrointerface span is larger; this can be understood qualitatively by considering how the relative hardness affects material transport. The welds also exhibit eutectic melting. The liquid phase has the traditional form of grain boundary films in the thermomechanical process zone. Particle strings and fragmentation fracture zones are observed; these may also result from eutectic melting.

Determination of charge on vertically aligned particles in a complex plasma using laser excitations

Experimental studies on vertical oscillations of few particle vertical structures are described. One and two particle strings were subjected to two types of vertically driven oscillations. The first was electrode driven, which excites the structure as a whole, while the second was laser driven, which excites one particle in the structure only. The latter experiments are highly original, enabling us to excite two vertical resonances in our two particle structures. From the close agreement between our experimental data and theoretical model, several important physical parameters have been estimated, including the charge ratio and the Debye length.

Rigid-particle toughening of glassy polymers

The nature of the toughening of glassy polymers by rigid particles was investigated by using aligned assemblages of spherical glass particles. The particles were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned particle strings in three orientations with respect to the crack plane and propagation direction. The fracture toughness and fracture energy of these and of random arrangements of particles (formed without the electric field) were all higher than of the matrix alone. The increases were compared with predictions from processes on or in the immediate vicinity of the fracture plane and from those away from the fracture plane. The increases were inconsistent with those predicted by on-fracture plane processes as represented by crack pinning and bowing. But the increases did correlate with the size of the process zones, which could extend more than 100 μm away from the fracture plane. Detailed calculations showed that the increase in fracture energy arose almost completely from off-fracture plane processes of particle-matrix debonding and the accompanying local inelastic deformation of the matrix around the particles.

Structure and dynamics of particle monolayers at a liquid-liquid interface subjected to shear flow.

The effect of shear flow on the structure and dynamics of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed. While undisturbed, the particles arrange themselves on a hexagonal lattice due to strong dipole-dipole repulsion resulting from ionizable sulfate groups on their surfaces. As the interface is subjected to shear flow, however, the lattice adopts a new semi-ordered, anisotropic state for which two distinct regimes are observed. At low particle concentrations or high shear rates, nearest neighbors in the lattice align in the flow direction and create strings of particles that slip past each other fairly readily. This results in a stretching of the overall structure and achievement of a steady state orientation in the system. In contrast, at high concentrations or low shear rates, the interparticle forces gain importance and tend to keep the particles more strongly in their lattice positions. As a result, domains within the lattice are forced to rotate, thus giving rise to movement of particles perpendicular to the flow direction. Thus a rotation, in addition to stretching, of the structure is apparent in this case.

Structure formation in moderately concentrated viscoelastic suspensions in simple shear flow

We report experimental results on the evolution of the particle microstructure for noncolloidal particles that are suspended in a viscoelastic medium. For dilute suspensions consisting of particles having a monodisperse particle size distribution subjected to steady and oscillatory shear flows, as well as suspensions having a bidisperse size distribution subjected to steady shear, we verify previous results reported in the research literature. We also present new results of the particle microstructure for a dilute bidisperse system that was subjected to oscillatory shear. For moderately concentrated suspensions we report the formation of particle strings that correspond to a quantitative reduction in the simultaneously measured shear stress.

Single-walled tubes and encapsulated nanoparticles: comparison of structural properties of carbon nanoclusters prepared by three different methods

A comparative investigation of the preparation and structural properties of carbon nanoclusters was carried out by having the transition metals Fe, Co and Ni react with carbon using three different methods, with the focus on single-walled nanotubes and encapsulated nanoparticles. Carbon nanoclusters were synthesized first by the high temperature (∼3000°C) and high carbon-content process of the conventional arc-discharge, secondly by the high temperature but low carbon-content process of the modified arc discharge, and finally by the relatively low temperature (∼500°C) process of catalytic decomposition of carbon monoxide (CO). The samples were characterized with respect to morphology, internal structure, and related properties. The carbon nanoclusters prepared by the three different methods appear quite different on the surface, but have features in common that this report emphasizes. The same element can apparently serve different functions: (1) serving as catalyst under one set of conditions; and (2) being encapsulated into the growing cages in a different environment. The elements of the iron group (Fe, Co and Ni) were known as catalysts for growing the single-walled nanotubes and strings of spherical particles in conventional arc-discharge, but could be encapsulated into the graphitic particles in the modified arc discharge and the CO disproportion that this study demonstrates. It was found that variation of the metal to carbon ratio is required to make these elements assume the double roles of either catalyst or encapsulant.