Shape Anisotropy Of Particles Research Articles

Shape-Anisotropic Diblock Copolymer Particles with Varied Internal Structures.

Anisotropic polymer particles have promising applications in various fields, whereas their preparation usually suffers from tedious procedures. Here, we introduce a facile strategy to fabricate novel shape-anisotropic particles with varied internal structures via self-assembly of block copolymers (BCPs), with perfluorooctane (PFO) as the liquid template in emulsion droplets. By increasing the volume ratio of PFO to polystyrene- block-poly(4-vinylpyridine) (PS- b-P4VP) or decreasing the initial concentration of the BCPs, the self-assembled polymer particles change from spherical core-shell structures to anisotropic particles. Moreover, the anisotropic shape and internal structure of the polymer particles, including cone-like particles with alternative PS and P4VP lamellas, crescent-shaped particles with cylindrical P4VP domains, and plate-like particles with spherical P4VP domains, can be obtained by changing the block ratio or molecular weight or by adding a hydrogen-bonding agent. Based on the in situ optical microscopy investigation of the morphology evolution of the emulsion droplet, we conclude that both kinetic and thermodynamic factors during emulsion evolution determine the formation of shape-anisotropic polymeric particles with controllable internal structures.

Development of Shape-Tuned, Monodisperse Block Copolymer Particles through Solvent-Mediated Particle Restructuring

Control of the shape, size, internal structure, and uniformity of block copolymer (BCP) particles is crucial for determining their utility and functionality in practical applications. Here, we demonstrate a particle restructuring by solvent engineering (PRSE) strategy that combines membrane emulsification and solvent annealing processes to produce monodisperse BCP particles with controlled size, shape, and internal structure. A major advantage of the PRSE approach is the general applicability to different families of functional BCPs, including polystyrene-block-poly(1,4-butadiene) (PS-b-PB), polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS), and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). PRSE starts with the production of monodisperse BCP spheres in a wide range of particle sizes (from hundreds of nanometers to several tens of microns) using membrane emulsification, followed by successful transformation to shape-anisotropic BCP particles by solvent annealing under neutral wetting conditions. ...

Detection and tracking of anisotropic core-shell colloids

Optical microscopy techniques with three dimensional (3D) resolution are powerful tools for the real-space imaging of the structure and dynamics of colloidal systems. While real-space imaging of spherical particles is well established, the observation of shape anisotropic particles has only recently met a lot of interest. Apart from translation, shape anisotropic particles also possess additional rotational degrees of freedom. In this manuscript, we introduce a novel technique to find the position and the orientation of anisotropic particles in 3D. It is based on an algorithm which is applicable to core-shell particles consisting of a spherical core and a shell with arbitrary shape. We demonstrate the performance of this algorithm using PMMA/PMMA (polymethyl methacrylate) core-shell ellipsoids. The algorithm is tested on artificial images and on experimental data. The correct identification of particle positions with subpixel accuracy and of their orientations with high angular precision in dilute and dense systems is shown. In addition, we developed an advanced particle tracking algorithm that takes both translational and rotational movements of the anisotropic particles into account. We show that our 3D detection and tracking technique is suitable for the accurate and reliable detection of large and dense colloidal systems containing several thousands of particles.

Bottom-Up Synthesis of Polymeric Micro- and Nanoparticles with Regular Anisotropic Shapes

Shape-anisotropic polymeric micro- and nanoparticles are of significant interest for the development of novel composite materials, lock-and-key assemblies, and drug carriers. Currently, syntheses require external confinement in microfluidic devices or lithographic techniques associated with significant infrastructure and low productivity, so new methods are necessary to scale-up such production efficiently. Here we report bottom-up polymerization of regular shape-anisotropic particles (polygonal platelets with different numbers of edges, with and without protruding asperities, and fibrilar particles with controllable aspect ratios), with size control over 4 orders of magnitude (∼50 nm–1 mm). Polymerization also enables the study of much smaller shapes than could previously be studied in water suspensions, and we study the fundamental limits of the self-shaping transition process driving these transformations for monomer oil droplets of stearyl methacrylate (SMA) monomer oil. We show the method is compatib...

Single-Photon-Single-Electron Transition for Interpretation of Optical Spectra of Nonspherical Metal Nanoparticles in Aqueous Colloidal Solutions

Noble metal nanoparticles—especially shape anisotropic particles—have pronounced resonances in the optical spectrum. These sensitive absorption modes attract great interest in various fields of application. For nonspherical particles, no analytic description of the absorption spectra according to the commonly used Mie theory is possible. In this work, we present a semi-empirical approach for the explanation of the optical spectra of shape anisotropic particles such as silver nanoprisms and gold nanorods. We found an interpretation of the optical absorption spectra which is based on a single-photon-single-electron transition. This model is in a better agreement with the basic assumptions of quantum mechanics than the electrodynamic model of a localized surface plasmon excitation. Based on microfluidically obtained Ag nanoprisms and Au nanorods with very high ensemble homogeneities, dependencies between the geometrical properties of the shape anisotropic noble metal nanoparticles and the spectral position of the longitudinal absorption mode could be derived, which show that the assumption of a composed relative permittivity and the inclusion of the Rydberg constant is sufficient to describe the optical properties of the shape anisotropic particles. Within the scope of the measuring accuracy, the calculations furthermore lead to the value of the refractive index of the particle-surrounding medium.

Desiccation cracks in dispersion of ellipsoids: Effect of aspect ratio and applied fields

Particulate films obtained by drying colloidal dispersions often displaying a network of cracks are ubiquitous. In particulate films, drying-induced internal stresses are accumulated as a result of the consolidation of particles during solvent evaporation and are released by the formation of cracks. The morphology of crack patterns can be controlled by externally applied fields, modulation of drying kinetics, or by exploiting the shape anisotropy of the colloids. Here, we explore the role of particle shape anisotropy and externally applied thermal and magnetic fields on the evolution of cracks by performing desiccation experiments, i.e., by drying colloidal-ellipsoid-laden sessile drops on solid substrates. We determine the critical aspect ratio $({\ensuremath{\alpha}}_{c})$, where the cracks in the dried particulate deposit drastically change their orientation from a radial to circular pattern. It is observed that the drying of drops consisting of ellipsoids at an elevated temperature enhances the randomness in the particle arrangement and brings nontrivial modifications to the crack pattern. In addition, we show that the randomness in the particle arrangement due to the thermal field can be compensated by the application of an external magnetic field and the orientation of cracks can be completely restored.

The effect of non-spherical aspect of a dimer on the dynamic behaviors

Shape-anisotropic particles on vibrating plates exert diverse behaviors of interest. But there is very limited work that devoted to the effect of particle shape, let along the dynamic reasons behind. In this paper, we exhibit that a non-spherical particle called a dimer encounters double impacts with friction, superstatic problems and the Painleve paradox, which are different from those of a spherical particle, when it is put on a harmonically vibrating plate. The dimer system was firstly presented by Dorbolo et al. (Phys Rev Lett 95:044101, 2005), who found experimentally that the bouncing dimer exerted a horizontal drift on the vertically vibrating plate, forwards or backwards. We reveal that the drifting direction of the dimer is closely related to a dual singularity, a superstatic singularity during a double impact when the tangential velocities vanish simultaneously at the two impacting tips. A correlative coefficient of friction (CCoF) is defined on the basis of the Coulomb frictional law to determine stick or slip at the tips at the moment of the dual singularity. The CCoF correlates the static coefficient of friction between the dimer and the plate to its aspect ratio. It decides the bifurcation of the drifting directions. Interestingly, the CCoF is independent of the characteristics of the plate’s vibration, which is also verified by the experiment of Dorbolo. The non-spherical aspect of the dimer also leads to the Painleve paradox where the solution is inconsistent or indeterminate. A parametric study is carried out in this paper to identify the Painleve paradox in the dimer system.

Shape-Anisotropic Colloids at Interfaces.

Research in the 1980s demonstrated the formation of monolayers of particles achieved by interfacial particle trapping as a model system for investigating colloids in two dimensions. Since then, microscopy visualization of two-dimensional particle monolayers and quantification of the microstructure have led to significant fundamental understanding of a number of phenomena such as crystallization, freezing and melting transitions, dislocation dynamics, aggregation kinetics, and others. On the application front, particles at curved interfaces, as often the case in particle-stabilized emulsions and foams, have received considerable attention in the last few decades. The growing interest in the search for novel particles and new strategies to effect emulsion stabilization stems from their application in several disciplines. Moreover, particle-stabilized Pickering emulsions and foams can also be used to derive a number of advanced functional materials. Compared to several accounts of research on spherical colloids at fluid-fluid interfaces, investigations of the behavior of shape-anisotropic particles at interfaces, albeit receiving considerable attention in recent years, are still in a nascent stage. The objective of this feature article is to highlight our recent work in this area. In particular, the adsorption of shape-anisotropic particles to interfaces, wetting behavior, interfacial self-assembly, the response of nonspherical-particle-coated interfaces to compression and shear, and their ability to stabilize emulsions are discussed.

Kinematic Model of Transient Shape-Induced Anisotropy in Dense Granular Flow.

Nonspherical particles are ubiquitous in nature and industry, yet previous theoretical models of granular media are mostly limited to systems of spherical particles. The problem is that in systems of nonspherical anisotropic particles, dynamic particle alignment critically affects their mechanical response. To study the tendency of such particles to align, we propose a simple kinematic model that relates the flow to the evolution of particle alignment with respect to each other. The validity of the proposed model is supported by comparison with particle-based simulations for various particle shapes ranging from elongated rice-like (prolate) to flattened lentil-like (oblate) particles. The model shows good agreement with the simulations for both steady-state and transient responses, and advances the development of comprehensive constitutive models for shape-anisotropic particles.

Ostwald ripening of spheroidal particles in multicomponent alloys

We propose a general theory of Ostwald ripening for prolate spheroidal particles in a nonideal nondilute multicomponent alloy. The diffusion problem of a growing or shrinking particle is solved using prolate spheroidal coordinates under the assumption that the spheroidal particle has a constant Wulff shape. The result shows that the diffusional growth rate increases with an increasing particle aspect ratio due to the increased surface area per volume. The anisotropic interfacial energy necessary to guarantee that the particles are always prolate spheroids with a given aspect ratio is also determined. We find that the chemical potential decreases with an increasing particle aspect ratio under a constant volume-equivalent radius. Based on the two correction factors, asymptotic analysis reveals that the temporal exponents for the coarsening laws for spheroid particles are identical to that for spherical particles. However, as the aspect ratio increases the amplitudes of the temporal power laws of the average equivalent radius, the matrix supersaturations, and the particle composition decrease, whereas the amplitude of the number of particles per volume increases. It is also shown that the particle shape anisotropy affects the amplitudes, but not the direction of the vector representing the matrix supersaturation and particle composition.

Short-time dynamics of lysozyme solutions with competing short-range attraction and long-range repulsion: Experiment and theory.

Recently, atypical static features of microstructural ordering in low-salinity lysozyme protein solutions have been extensively explored experimentally and explained theoretically based on a short-range attractive plus long-range repulsive (SALR) interaction potential. However, the protein dynamics and the relationship to the atypical SALR structure remain to be demonstrated. Here, the applicability of semi-analytic theoretical methods predicting diffusion properties and viscosity in isotropic particle suspensions to low-salinity lysozyme protein solutions is tested. Using the interaction potential parameters previously obtained from static structure factor measurements, our results of Monte Carlo simulations representing seven experimental lysoyzme samples indicate that they exist either in dispersed fluid or random percolated states. The self-consistent Zerah-Hansen scheme is used to describe the static structure factor, S(q), which is the input to our calculation schemes for the short-time hydrodynamic function, H(q), and the zero-frequency viscosity η. The schemes account for hydrodynamic interactions included on an approximate level. Theoretical predictions for H(q) as a function of the wavenumber q quantitatively agree with experimental results at small protein concentrations obtained using neutron spin echo measurements. At higher concentrations, qualitative agreement is preserved although the calculated hydrodynamic functions are overestimated. We attribute the differences for higher concentrations and lower temperatures to translational-rotational diffusion coupling induced by the shape and interaction anisotropy of particles and clusters, patchiness of the lysozyme particle surfaces, and the intra-cluster dynamics, features not included in our simple globular particle model. The theoretical results for the solution viscosity, η, are in qualitative agreement with our experimental data even at higher concentrations. We demonstrate that semi-quantitative predictions of diffusion properties and viscosity of solutions of globular proteins are possible given only the equilibrium structure factor of proteins. Furthermore, we explore the effects of changing the attraction strength on H(q) and η.

Designing shape anisotropic SmCo5 particles by chemical synthesis to reveal the morphological evolution mechanism.

In this work, we describe a new protocol to synthesize SmCo5 single crystal particles with remarkable shape anisotropy (hexagonal and rodlike), which exhibit a giant coercivity of 36.6 kOe and a high Mr/Ms value of 0.95 after an alignment. On this basis, the morphological evolution mechanism is illustrated by employing the template effect.

Shear-Induced Alignment of Anisotropic Nanoparticles in a Single-Droplet Oscillatory Microfluidic Platform.

Flow-induced alignment of shape-anisotropic colloidal particles is of great importance in fundamental research and in the fabrication of structurally anisotropic materials; however, rheo-optical studies of shear-induced particle orientation are time- and labor-intensive and require complicated experimental setups. We report a single-droplet oscillatory microfluidic strategy integrated with in-line polarized light imaging as a strategy for studies of shear-induced alignment of rod-shape nanoparticles. Using an oscillating droplet of an aqueous isotropic suspension of cellulose nanocrystals (CNCs), we explore the effect of the shear rate and suspension viscosity on the flow-induced CNC alignment and subsequent relaxation to the isotropic state. The proposed microfluidic strategy enables high-throughput studies of shear-induced orientations in structured liquid under precisely controlled experimental conditions. The results of such studies can be used in the development of structure-anisotropic materials.

Crystallization of Nanodomains in Polyethylene Latexes

Semicrystalline polyethylene (PE) nanoparticles were obtained by free radical emulsion polymerization under mild conditions of pressure and temperature. At high surfactant concentration, the obtained anisotropic nanoparticles exhibit a strong degree of supercooling. This paper explores the relationship between the morphology of such particles and their high degree of supercooling. The shape anisotropy of the semicrystalline PE particles already observed by transmission electron microscopy was confirmed by dynamic light scattering with the autocorrelation function containing both translational and rotational diffusion coefficients as well as by in situ synchrotron small-angle and wide-angle X-ray scattering (SAXS/WAXS). Temperature-resolved synchrotron scattering was used to further characterize the evolution of particles morphology and crystallinity during cooling. Paying special attention to the role of the surfactant, we propose a novel mechanism which contributes to a better understanding of the crysta...

Novel insight into the effect of disappearance of the Morin transition in hematite nanoparticles

An alternative treatment of the well-known effect of a decrease in the Morin transition temperature in hematite with a decrease in the size of crystallites to the complete disappearance of the transition for nanoparticles smaller than 20 nm is proposed. In contrast to the standard speculative explanation of this effect in terms of the effect of surface and defectiveness of grains, we suggest that the decisive factor is an increase in the contribution of the shape anisotropy of particles with a decrease in their size, which is responsible for the spread of orientations of the axes of the resulting magnetic anisotropy with respect to the crystallographic axes. Our reasons are confirmed by a numerical analysis of Mossbauer spectra of hematite nanoparticles within the continuous model of magnetic dynamics of an ensemble of antiferromagnetic nanoparticles in the two-sublattice approximation generalized to the existence of weak ferromagnetism (Dzyaloshinskii interaction).

Stress estimates for particle damage in Fe-TiB 2 metal matrix composites from experimental data and simulation

Abstract Experimental data regarding microstructural damage evolution in Fe-TiB 2 composites are examined in order to estimating the fracture stress of the TiB 2 particles in relation to their size distribution. At first, an a priori “on/off” particle breaking mode is introduced in a numerical modelling based on nonlinear homogenization methods, considering a three (matrix plus undamaged and damaged particles) phase assemblage. Specific effects on the stress estimates in the particles and on the damage evolution, due to particle shape and elasticity anisotropy, to matrix grain size and hardening, as well as to the loading mode, are examined in comparison with a reference estimate from a fully isotropic description of the undamaged material. The firstly assumed “on/off” brutal particle damage description is shown to overestimate both the damage-induced porosity in the material and the matrix hardening. Both are then adjusted with the available data using a much lower and gradual decrease of the TiB 2 particle stiffness from the undamaged state. From these adjustments, estimates of the size-related particle fracture stresses are obtained. Particle fracturing occurs with limited and gradual stiffness loss of the composite, consistently with a quite good bonding of the TiB 2 particles with the surrounding ferrite matrix.

Magneto-induced anisotropy in magnetic colloids of superparamagnetic magnetite nanoparticles in an external magnetic field.

Here we report a study of chain formation and the magnetic anisotropy induced by them in suspensions of slightly anisotropic Fe3O4 nanoparticles in water and in aqueous V2O5 suspensions. An investigation into the magnetization of the fluid and frozen suspensions, the application of dynamic light scattering techniques and the observation of the magnetic anisotropy in the frozen magnetically aligned samples allowed us to confirm the existence of chains of Fe3O4 in both suspensions. Our study shows that the magneto-induced anisotropy appearing in magnetic fields in colloids with Fe3O4 particles is mainly due to many particle (chain) magneto-induced anisotropy, but not due to single particle magneto-induced anisotropy connected with particle shape anisotropy. In other words, the single particle magneto-induced anisotropy is much smaller than the many particle (chain) anisotropy. The anisometry of the chains provides effective coupling with the nonmagnetic V2O5 component of the suspension and results in its strong sensitivity to the magnetic field.

Dielectrophoretic assembly of dimpled colloids into open packing structures.

Reversible solid-state phase transitions between open- and close-packed structures in two-dimensional colloidal crystals comprising 1.8 μm dimpled spherical colloids were observed using negative dielectrophoresis. These asymmetrically-shaped colloids adopted lattices with cmm plane group symmetry and a packing fraction, ϕ, of 0.68 at low electric field strengths. At high electric field strengths, the close-packed p6m symmetry was observed, with ϕ = 0.90. The transition between open and close-packed structures was found to be reversible, depending on the applied electric field strength and frequency. Finite Fourier transform analysis and COMSOL simulations revealed the existence of repulsive interactions between colloids perpendicular to the electric field lines due to a concentration of the electric field at the edges of the dimpled regions of the colloids. The repulsive interactions resulted in a stretching of the hexagonal lattice perpendicular to the electric field lines, the magnitude of which depended on the electric field strength. By screening the colloids from the electric field in local potential wells, the entropically favored close-packed hexagonal lattice with ϕ = 0.91 was recovered.

Synthesis, magnetic and microstructural properties of Alnico magnets with additives

The phase formation, crystal structure, crystallographic texture, microstructure and magnetic properties of Alnico-8 alloys with varying Co and Nb content have been investigated and presented. Alnico-8 alloys were fabricated by induction melting and casting techniques. Magnetic properties in the alloys were induced by optimized thermomagnetic treatment and subsequent aging. The 37.9Fe-32Co-14Ni-7.5Al-3.1Cu-5.5Ti alloy exhibits coercivity of 110kA/m, remanence of 0.66T and energy product of 31.2kJ/m3. The addition of 35wt% Co in conjunction with 1.5wt%Nb to 37.9Fe-14Ni-7.5Al-3.1Cu-5.5Ti alloys led to increase the magnetic properties, especially coercivity. The enhancement of the coercivity is attributed to ideal shape anisotropy and optimum mass fraction of ferromagnetic Fe-Co rich particles, which are 25–30nm in diameter and 300–350nm in length. The 33.4Fe-35Co-14Ni-7.5Al-5.5Ti-3.1Cu-1.5Nb alloy yields the optimum magnetic properties of coercivity of 141.4kA/m, remanence of 0.83T and energy product of 42.4kJ/m3. The good magnetic properties in the studied alloys are attributed to the nanostructured microstructure comprising textured Fe-Co-Nb rich α1 phase and Al-Ni-Cu rich α2 phase.

Plasmonic Lattice Lenses for Multiwavelength Achromatic Focusing

This paper describes an evolutionary approach to design flat multiwavelength achromatic lenses based on subwavelength plasmonic nanoparticles. Our lattice evolution algorithm achieved desired optical responses by tuning the arrangement of the phase units on a discrete square lattice. Lattice lenses consisting of a single type of nanoparticle could operate at any wavelength in the visible to near-infrared regime (540-1000 nm) by tailoring the localized surface plasmon resonance. When the unit cells were expanded to anisotropic particle shapes, the planar optics could selectively focus light depending on the polarization of incident light. Finally, the algorithm realized efficient multiobjective optimization and produced achromatic lattice lenses at up to three wavelengths (λ = 600 nm, λ = 785 nm, and λ = 980 nm) using multiple different nanoparticle shapes.