Strings Of Particles Research Articles

Hexagonal vortices enable faster colloidal crystal grain coarsening.

We find that localized rotations of hexagonal clusters of particles occur during rapid dissolution of grain boundary loops in two-dimensional colloidal crystals. These particle vortices, or rotating "granules," are distinct from established models for grain boundary diffusion, which predict that a crystal grain enclosed within another crystal will dissolve at a constant rate. Our measurements of colloidal crystal experiments and Brownian dynamics simulations reveal grain boundary motion that is described by two distinct processes: slow dissolution due to the diffusion of individual particles, and rapid dissolution due to collective granule rotation. In the latter process, hexagonal clusters of particles rotate together in granules whose shape and position are determined by the underlying moiré pattern. Furthermore, these vortices guide cooperative strings of particles that move along the edges of the hexagonal granules. Including this vortex mechanism may improve models for grain coarsening in polycrystalline materials, ultimately offering improved predictions for the time evolution of material properties.

Self‐Assembled Robust 2D Networks from Magneto‐Elastic Bars

AbstractMagneto‐elastic materials facilitate features such as shape programmability, adaptive stiffness, and tunable strength, which are critical for advances in structural and robotic materials. Magneto‐elastic networks are commonly fabricated by employing hard magnets embedded in soft matrices to constitute a monolithic body. These architected network materials have excellent mechanical properties but damage incurred in extreme loading scenarios are permanent. To overcome this limitation, we present a novel design for elastic bars with permanent fixed dipole magnets at their ends and demonstrate their ability to self‐assemble into magneto‐elastic networks under random vibrations. The magneto‐elastic unit configuration, most notably the orientation of end dipoles, is shown to dictate the self‐assembled network topology, which can range from quasi‐ordered triangular lattices to stacks or strings of particles. Network mechanics are probed with uniaxial tensile tests and design criteria for forming stable lightweight 2D networks are established. It is shown that these magneto‐elastic networks rearrange and break gracefully at their magnetic nodes under large excitations and yet recover their original structure at moderate random excitations. This work paves the way for structural materials that can be self‐assembled and repaired on‐the‐fly with random vibrations, and broadens the applications of magneto‐elastic soft materials.

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel.

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as "particle ordering," is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the orderingefficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slippingphenomenon.

Confinement effect on the viscoelastic particle ordering in microfluidic flows: Numerical simulations and experiments

Strings of equally spaced particles, also called particle trains, have been employed in several applications, including flow cytometry and particle or cell encapsulation. Recently, the formation of particle trains in viscoelastic liquids has been demonstrated. However, only a few studies have focused on the topic, with several questions remaining unanswered. We here perform numerical simulations and experiments to elucidate the effect of the confinement ratio on the self-ordering dynamics of particles suspended in a viscoelastic liquid and flowing on the centerline of a microfluidic channel. For a fixed channel size, the particles self-order on shorter distances as the particle size increases due to the enhanced hydrodynamic interactions. At relatively low linear concentrations, the relative particle velocities scale with the fourth power of the confinement ratio when plotted as a function of the distance between the particle surfaces normalized by the channel diameter. As the linear concentration increases, the average interparticle spacing reduces and the scaling is lost, with an increasing probability to form strings of particles in contact. To reduce the number of aggregates, a microfluidic device made of an array of trapezoidal elements is fabricated and tested. The particle aggregates reduce down to 5% of the overall particle number, significantly enhancing the ordering efficiency. A good agreement between numerical simulations and experiments is found.

Synthesis and Transformation of Hollow Rutile Titania Wires by Single Spinneret Electrospinning with Sol-Gel Chemistry

The work described below was carried out to understand how to control the morphology of nanostructured titania calcined from electrospun nanofibers. This is the first report of hollow rutile nanofibers synthesized from electrospun nanofibers with short calcination time. Titanium isopropoxide was incorporated into the nanofibers as the titania precursor. The electrospinning technique was used to fabricate ceramic/polymer hybrid nanofibers. The electrospun nanofibers were then calcined to produce rutile titania nanofibers with different morphologies (hollow or solid nanofibers), which were characterized by SEM and TEM. The initial concentration of ceramic precursor and the calcination time were shown to control the morphology of the nanofiber. The hollow morphology was only obtained with a concentration of the precursor within a certain level and with short calcination times. The heat treatment profile contributed to particle growth. At longer times, the particle growth led to the closure of the hollow core and all the nanofibers resembled strings of solid particles. A formation mechanism for the hollow nanofibers is also proposed.

Modification of Mechanical Properties of Aluminum Alloy Rods via Friction-Extrusion Method.

The elaboration of a modified friction-extrusion method aimed at obtaining 2017A aluminum rods of gradient microstructure is described. This was achieved by cutting spiral grooves on the face of the stamp used for alloy extrusion. The experiments were carried out at a constant material feed (~10 mm/min) and a range of tool rotation speeds (80 to 315 rpm). The microstructure observations were carried out using light microscopy (LM) and both scanning and transmission electron microscopy (SEM and TEM). The mechanical properties were assessed through hardness measurements and static tensile tests. The performed investigations show that material simultaneous radial and longitudinal flow, enforced by friction of the rotating tool head and extrusion, results in the formation of two zones of very different microstructures. At the perpendicular section, the outer zone stands out from the core due to circumferential elongation of strings of particles, while in the inner zone the particles are arranged in a more uniform way. Simultaneously, the grain size of the outer zone is refined by two to four times as compared with the inner one. The transfer from the outer zone to the core area is of gradient type. The hardness of the outer zone was found to be ~10% to ~20% higher than that of the core.

Electrospraying technique under pressurized carbon dioxide for hollow particle production

Fabrication of polyvinylpyrrolidone (PVP) particles by an electrospraying process under pressurized carbon dioxide (CO2) used as an anti-solvent was studied. The process was carried out at a constant temperature with CO2 pressures ranging from 1 to 6 MPa. A fixed applied voltage (17 kV) was employed to generate electric field. PVP powder dissolved in dichloromethane (DCM) at concentrations of 4, 6, and 8 wt% was used as the starting material. Scanning electron microscopy observations indicated that the electrosprayed PVP particles produced at 1–3 MPa using 8 wt% PVP solutions have spherical morphologies. At a similar concentration, increasing the CO2 pressure up to 6 MPa resulted in particle strings and strings as major products. Interestingly, hollow core particles formed when ethanol was added as a secondary solvent for controlling the evaporation rate of DCM during the electrospraying process under pressurized CO2. The FT–IR spectra revealed that the structural properties of PVP did not change after the electrospraying process. This work demonstrates that electrohydrodynamic processes under pressurized CO2 are likely to be fruitful for the fabrication of organic polymers with hollow cores.

Assembly of particle strings via isotropic potentials.

Assembly of spherical colloidal particles into extended structures, including linear strings, in the absence of directional interparticle bonding interactions or external perturbation could facilitate the design of new functional materials. Here, we use methods of inverse design to discover isotropic pair potentials that promote the formation of single-stranded, polydisperse strings of colloids "colloidomers" as well as size-specific, compact colloidal clusters. Based on the designed potentials, a simple model pair interaction with a short-range attraction and a longer-range repulsion is proposed which stabilizes a variety of different particle morphologies including (i) dispersed fluid of monomers, (ii) ergodic short particle chains as well as porous networks of percolated strings, (iii) compact clusters, and (iv) thick cylindrical structures including trihelical Bernal spirals.

2D Stokesian Approach to Modeling Flow Induced Deformation of Particle Laden Interfaces.

A Stokesian dynamics simulation of the effect of surface Couette flow on the microstructure of particles irreversibly adsorbed to an interface is presented. Rather than modeling both bulk phases, the interface, and particles in a full 3D simulation, known interfacial interactions between adsorbed particles are used to create a 2D model from a top down perspective. This novel methodology is easy to implement and computationally inexpensive, which makes it favorable to simulate behavior of particles under applied flow at fluid-fluid interfaces. The methodology is used to examine microstructure deformation of monodisperse, rigid spherical colloids with repulsive interactions when a surface Couette flow is imposed. Simulation results compare favorably to experimental results taken from literature, showing that interparticle forces must be 1 order of magnitude greater than viscous drag for microstructure to transition from aligned particle strings to rotation of local hexagonal domains. Additionally, it is demonstrated that hydrodynamic interactions between particles play a significant role in the magnitude of these microstructure deformations.

Microsecond Structural Rheology.

The relationship between the local structure of complex liquids and their response to shear is generally not well understood. This concerns, in particular, the formation of particle strings in the flow direction or hydroclusters, both important for the understanding of shear thinning and thickening phenomena. Here, we present results of a microfocus X-ray scattering experiment on spherical silica colloids in a liquid jet at high shear rates. Along and across the jet, we observe direction-dependent modifications of the structure factor of the suspension, suggesting the formation of differently ordered clusters in compression lines and as particle strings. With increasing distance from the orifice, the structure relaxes to the unsheared case with a typical relaxation 10 times larger as the time scale of Brownian motion. These results provide the first experimental flow characterization of a complex fluid at high shear rates detecting cluster formation and relaxation with micrometer and microsecond resolution.

Remote Magnetic Orientation of 3D Collagen Hydrogels for Directed Neuronal Regeneration.

Hydrogel matrices are valuable platforms for neuronal tissue engineering. Orienting gel fibers to achieve a directed scaffold is important for effective functional neuronal regeneration. However, current methods are limited and require treatment of gels prior to implantation, ex-vivo, without taking into consideration the pathology in the injured site. We have developed a method to control gel orientation dynamically and remotely in situ. We have mixed into collagen hydrogels magnetic nanoparticles then applied an external magnetic field. During the gelation period the magnetic particles aggregated into magnetic particle strings, leading to the alignment of the collagen fibers. We have shown that neurons within the 3D magnetically induced gels exhibited normal electrical activity and viability. Importantly, neurons formed elongated cooriented morphology, relying on the particle strings and fibers as supportive cues for growth. The ability to inject the mixed gel directly into the injured site as a solution then to control scaffold orientation remotely opens future possibilities for therapeutic engineered scaffolds.

Analytic solutions of topologically disjoint systems

We describe a procedure to solve an up to problem where the particles are separated topologically in N groups with at most two particles in each. Arbitrary interactions are allowed between the (two) particles within one group. All other interactions are approximated by harmonic oscillator potentials. The problem is first reduced to an analytically solvable N-body problem and N independent two-body problems. We calculate analytically spectra, wave functions, and normal modes for both the inverse square and delta-function two-body interactions. In particular, we calculate separation energies between two strings of particles. We find that the string separation energy increases with N and interaction strength.

Network analysis of three-dimensional complex plasma clusters in a rotating electric field.

Network analysis was used to study the structure and time evolution of driven three-dimensional complex plasma clusters. The clusters were created by suspending micron-size particles in a glass box placed on top of the rf electrode in a capacitively coupled discharge. The particles were highly charged and manipulated by an external electric field that had a constant magnitude and uniformly rotated in the horizontal plane. Depending on the frequency of the applied electric field, the clusters rotated in the direction of the electric field or remained stationary. The positions of all particles were measured using stereoscopic digital in-line holography. The network analysis revealed the interplay between two competing symmetries in the cluster. The rotating cluster was shown to be more cylindrical than the nonrotating cluster. The emergence of vertical strings of particles was also confirmed.

Spin Coated Plasmonic Nanoparticle Interfaces for Photocurrent Enhancement in Thin Film Si Solar Cells

Nanoparticle (NP) arrays of noble metals strongly absorb light in the visible to infrared wavelengths through resonant interactions between the incident electromagnetic field and the metal’s free electron plasma. Such plasmonic interfaces enhance light absorption and photocurrent in solar cells. We report a cost‐effective and scalable room temperature/pressure spin‐coating route to fabricate broadband plasmonic interfaces consisting of silver NPs. The NP interface yields photocurrent enhancement (PE) in thin film silicon devices by up to 200% which is significantly greater than previously reported values. For coatings produced from Ag nanoink containing particles with average diameter of 40 nm, an optimal NP surface coverage ϕ of 7% is observed. Scanning electron microscopy of interface morphologies revealed that for low ϕ, particles are well separated, resulting in broadband PE. At higher ϕ, formation of particle strings and clusters causes red‐shifting of the PE peak and a narrower spectral response.

Bio-inspired, topologically connected colloidal arrays via wrinkle and plasma processing

We investigated the fabrication of various topologically patterned 1D arrays of polystyrene colloidal particles, including single, helical, zigzag, triple-line, and random arrays integrated in sinusoidal wrinkle grooves, through simple spin-coating. In an additional processing step, we used the wrinkled substrates as stamps to transfer the formed polystyrene patterns onto planar supports. Subsequent plasma etching of the particle strings yielded anisotropic, topologically connected particles. Interestingly, these colloidal chains can be easily released and dispersed to maintain the topological alignment and long length (>100μm) with free-standing capability. Depending on the topological structures, these connected particle arrays would enable us to create new bio-inspired materials and devices such as artificial muscles and soft robotics with flexible motion.

Isolation of intraflagellar transport trains

The intraflagellar transport (IFT) system was first identified in situ by electron microscopy in thin sections of plastic-embedded flagella as linear arrays of electrondense particles, located between the B tubules of the outer doublets and the flagellar membrane. These arrays of particles are referred to as IFT trains. Upon membrane rupture, IFT trains are thought to easily dissociate to yield soluble IFT particles, which are commonly purified through sucrose gradients as ∼16-17S complexes. The latters easily dissociate into two subcomplexes, named A and B. We report here the isolation, visualization, and identification by immunolabeling of flexible strings of IFT particles, which are structurally similar to in situ IFT trains and appear to be formed by both complex A and complex B polypeptides. Moreover, the particles forming isolated IFT trains are structurally similar to the individual particles found in the ∼17S gradient peak. Our results provide the first direct evidence that ∼17S particles do indeed compose the IFT trains. The paper also represents the first isolation of the IFT trains, and opens new possibilities for higher resolution studies on their structure and how particles are attached to each other to form the particle trains.

The relationship of dynamical heterogeneity to the Adam-Gibbs and random first-order transition theories of glass formation

We carefully examine common measures of dynamical heterogeneity for a model polymer melt and test how these scales compare with those hypothesized by the Adam and Gibbs (AG) and random first-order transition (RFOT) theories of relaxation in glass-forming liquids. To this end, we first analyze clusters of highly mobile particles, the string-like collective motion of these mobile particles, and clusters of relative low mobility. We show that the time scale of the high-mobility clusters and strings is associated with a diffusive time scale, while the low-mobility particles' time scale relates to a structural relaxation time. The difference of the characteristic times for the high- and low-mobility particles naturally explains the well-known decoupling of diffusion and structural relaxation time scales. Despite the inherent difference of dynamics between high- and low-mobility particles, we find a high degree of similarity in the geometrical structure of these particle clusters. In particular, we show that the fractal dimensions of these clusters are consistent with those of swollen branched polymers or branched polymers with screened excluded-volume interactions, corresponding to lattice animals and percolation clusters, respectively. In contrast, the fractal dimension of the strings crosses over from that of self-avoiding walks for small strings, to simple random walks for longer, more strongly interacting, strings, corresponding to flexible polymers with screened excluded-volume interactions. We examine the appropriateness of identifying the size scales of either mobile particle clusters or strings with the size of cooperatively rearranging regions (CRR) in the AG and RFOT theories. We find that the string size appears to be the most consistent measure of CRR for both the AG and RFOT models. Identifying strings or clusters with the "mosaic" length of the RFOT model relaxes the conventional assumption that the "entropic droplets" are compact. We also confirm the validity of the entropy formulation of the AG theory, constraining the exponent values of the RFOT theory. This constraint, together with the analysis of size scales, enables us to estimate the characteristic exponents of RFOT.

Microelectromechanical strain and pressure sensors based on electric field aligned carbon cone and carbon black particles in a silicone elastomer matrix

Methods for developing microelectromechanical strain and pressure sensors based on aligned carbon particle strings within dielectric elastomer matrices are presented. Two different types of carbon particles were used: a mixture of carbon cone and carbon disk particles and spherical carbon black particles. The particles were assembled and aligned into strings by an alternating electric field with a strength of 4 kV/cm and a frequency of 1 kHz, utilizing the dielectrophoretic effect. The particle fraction was about 0.1 vol. %, which is an order of magnitude lower than their percolation threshold (∼2 vol. %). The aligned strings were produced in a couple of minutes. The matrices were subsequently cured thus stabilizing the strings. Micromechanical strain sensors with a capacitive readout were produced by aligning the particles into a single string-like formation in the in-plane direction, the string dimensions being 3 μm width and 30 μm length. The pressure sensors with piezoresistive readout were made by aligning the particles into multiple unidirectional strings in the out-of-plane direction, the thickness of the sensors being of the order of 100 μm and the lateral area of 1.5 cm2. The strain and the pressure sensors show reversible piezocapacitive and piezoresistance effects when stretched and compressed, respectively.

Direct observation of particle rearrangement during cyclic stress hardening of magnetorheological gels

Swollen physical magneto-responsive gels were obtained by self-assembly of triblock copolymers containing dispersed soft magnetic particles. When subject to simple shear cyclic loading–unloading cycles in the presence of a magnetic field, magnetorheological (MR) gels show a stress hardening behavior. The stress-hardening was characterized by the increase of the maximum stress against the number of cycles. Most of the stress increases are achieved in the first and second cycles and only minor effects take place in the subsequent ones. The cyclic behaviour of MR gels with dilute particle volume fraction is also studied with the help of a homemade shear cell mounted on an optical microscope. Despite the difference of particle volume fraction, we are able to relate cyclic stress hardening to the change in particle–particle distribution observed directly at the microscopic scale. While anisotropic particle strings created by a pre-structuring process (original sample alignment) still has many defects, the history of simple shear in the presence of the magnetic field gradually removes the defects of the particle strings and helps them to relax towards a zipped cluster. We show here the first directly observed example of the shear-annealing effect in MR gels. The shear induced rearrangement of the particle network serves as the microscopic origins of the above mentioned cyclic stress hardening behavior of MR gels and seems to be fully reversible after the removal of the magnetic field. Results show that a slow quiescent process also takes place and it is suggested to be related to solvent flow outwards of the clusters, while they get more compact. Interestingly, all directly observed rearrangements including string breaking seem to be fully reversible after removal of the magnetic field.

String-fluid transition in systems with aligned anisotropic interactions

Systems with aligned anisotropic interactions between particles exhibit numerous phase transitions. A remarkable example of the fluid phase transition occurring in such systems is the formation of particle strings--the so-called "string" or "chain" fluids. We employ an approach based on the Ornstein-Zernike (OZ) equation, which allows us to calculate structural properties of fluids with aligned anisotropic interactions. We show that the string-fluid transition can be associated with the bifurcation of the "isotropic" correlation length into two distinct scales which characterize the longitudinal and transverse order in string fluids and, hence, may be used as a fingerprint of this transition. The comparison of the proposed OZ theory with the Monte Carlo simulations reveals fairly good agreement.