Dipolar Particles Research Articles

A note on dielectrophoresis

A story about dipole molecules in liquid (Physics Today, April 2020, page 17) has prompted me to share some of my own experiences.When an electrically polarized object, like a dipole, is in a nonuniform electric field, it experiences a force. That phenomenon is known as dielectrophoresis. It is possible to create a field with constant, nonzero second derivative. A dipole particle in such a field would experience a constant force that can thus be made to move in a fixed direction under a constant force.I created such a field in a shallow device I had made at glass manufacturer Owens-Illinois, my employer, and used yeast and blood cells to study it. The cells did not move unless the field had a frequency of at least 1 MHz. I did not try much higher frequencies. In 1975–76 I took my experiment to the hematology laboratory of Massachusetts General Hospital. Unfortunately, the hoped-for dispersion of velocities among cell types was not observed, and the study ended.However, water molecules themselves have a dipole moment. When polarized light was passed vertically through the horizontal device, applying the 1 MHz field would switch the transparency on and off.Apparently, the yeast or blood cells were passively carried along a current of water, driven by the nonuniform field. It is a mystery why there was no motion of the cells (yeast or blood) until the frequency was increased to 1 MHz. Perhaps clusters of water molecules, which are imagined to explain the high boiling point of water, are not dipoles, and the field at or above 1 MHz breaks up some of the aggregates, allowing the dipole moment of H2O to be sensed. Section:ChooseTop of page <<© 2021 American Institute of Physics.

A novel experimental approach for direct observation of magnetic field induced structuration in ferrofluid

This article proposes a novel approach to investigate the field-induced structuration in ferrofluids (FFs) at room temperature. The in-house experimental setup has been fabricated to perceive the influence of dipolar interaction and particle size of dispersed magnetic nanoparticles (MNPs) in FFs on the static magneto-optical (MO) properties under the influence of an external magnetic field. The diffraction fringes are observed in FF when a high-power laser passing from the diluted fluid system which appearing at a specific dilution and varies with the different sizes and volume fraction of MNPs. The change in shape and size of the diffraction fringes is observed with the variation of both, size of dispersed MNPs and the Dilution of the FFs which is used to calculate the refractive index coefficient. The number of diffraction rings increases with an increase in the size of dispersed MNPs and its diameter decrease with an increase in particle volume fraction. The effect of thermal lens and nonlinear optical (NLO) behavior gives the validation of field-induced refractive index coefficient appearing in FFs commutated by the fringe diffraction method. Further, the variation of intensity of transmitted light has been corroborated with the forward scattering resulting from the Brownian motion of the nanoparticle. The experiment validates the chain formation in FFs, as MNPs are working as mesoscopic cylinders resulting in specified scattering patterns. This experimental approach can lead to a new pathway for the development of potential applications such as tunable optical filters, optical limiters, and various photonics fields based on the tunable MO properties of FFs.

Ground state of magnetocrystals.

Neodyme spherical magnets are inexpensive objects that demonstrate how dipolar particles self-assemble into various structures ranging from 1D chains to 3D crystals. The dipole-dipole interactions confer the stability to these particular architectures. In the present paper, we explore ordered structures only, and we evidence that hybrid magnetocrystals, alternating hexagonal planes of antiparallel dipoles, have the lowest magnetic energy. This cohesion is the magnetic counterpart of the Madelung lattice energy found for ionic solids.

Dielectric continuum model examination of real-space electrostatic treatments.

Electrostatic interaction is long ranged; thus, the accurate calculation is not an easy task in molecular dynamics or Monte Carlo simulations. Though the rigorous Ewald method based on the reciprocal space has been established, real-space treatments have recently become an attractive alternative because of the efficient calculation. However, the construction is not yet completed and is now a challenging subject. In an earlier theoretical study, Neumann and Steinhauser employed the Onsager dielectric continuum model to explain how simple real-space cutoff produces artificial dipolar orientation. In the present study, we employ this continuum model to explore the fundamental properties of the recently developed real-space treatments of three shifting schemes. The result of the distance-dependent Kirkwood function GK(R) showed that the simple bare cutoff produces a well-known hole-shaped artifact, whereas the shift treatments do not. Two-dimensional mapping of electric field well explained how these shift treatments remove the hole-shaped artifact. Still, the shift treatments are not sufficient because they do not produce a flat GK(R) profile unlike ideal no-cutoff treatment. To test the continuum model results, we also performed Monte Carlo simulations of dipolar particles. The results found that the continuum model could predict the qualitative tendency as to whether each electrostatic treatment produces the hole-shaped artifact of GK(R) or not. We expect that the present study using the continuum model offers a stringent criterion to judge whether the primitive electrostatic behavior is correctly described or not, which will be useful for future construction of electrostatic treatments.

Numerical investigation on the optical properties of TiO2 photocatalyst suspension by light scattering model of particulate aggregates

The optical properties of suspensions of P25 TiO2 photocatalyst are calculated with particulate-aggregates-based light scattering model. The effects of structure, number of monomer, and particle radius of cluster on the absorption and scattering coefficients are analyzed. It is found that the radius of particle in clusters significantly affects their scattering capacity, and this effect could be attributed to the dielectric resonance of single particle and the cooperative scattering caused by coupling of electric dipoles of adjacent particles. Our work may promote the understanding of interaction between light and photocatalysts.

A Compound Volume Scattering Model With Emphasis on the Morphological Diversity of Vegetation Canopy Scatterers

Existing volume scattering models play an important role in interpreting the scattering characteristics of vegetation canopies. A common assumption of these models is that a vegetation canopy consists of one type of particle. However, since a vegetation canopy comprises boughs, twigs, and leaves (or needles), they tend to form scatterers with various shapes, resulting in differing scattering processes. In this paper, we propose a new volume scattering model. The proposed scattering model uses an isotropic plate, isotropic dihedral, and dipole particles to model vegetation scatterers. Different preferred orientation angles characterize the latter two types of particles. The proposed method is validated with AIRSAR and ALOS2/PALSAR2 data sets. The results show that the proposed model provides us an opportunity to extract refined scattering processes of vegetation canopies. In addition, it can be found that the proposed model shows potential in vegetation classification and biomass estimation, resulting from modeling a vegetation canopy as various types of particles.

Electrical Excitation Decay Time in Chains of Nanoscale Non-Point Dipoles.

On the basis of a previously developed model of disperse systems containing non-point dipole particles self-assembled into chains inside a liquid substrate, the decay time of electrical excitations induced in dipoles by an external field is investigated. It was shown that when the external field is completely turned off (from V / m to V / m levels) at biologically significant low frequencies (for example, 13 Hz), the decay time of the excitations of nanoscale dipoles nonlinearly depends on the chain length. It was found that the decay time of excitations increases sharply (by four to five orders of magnitude), with an increase in the chain length more than 19–20 dipoles.

Resonant radiative heat transfer and many-body effects between nanoparticles and a multilayered slab

We derive the formula for the near-field radiative heat transfer between dipole particles and a multilayered slab by combing the transmission Green's function and transfer matrix method. In this framework we study the radiative heat transfer (RHT) from a multilayered slab to a particle and the many-body effect of particles. By adjusting the multilayered structure, we optimize heat exchange and achieve a significant enhancement (as high as 7.6 times) in heat transfer from slab to particle. Enhancement occurs at the particle resonance, owing to the surface resonance modulated to couple with the localized resonance of the particle. The roles of the number of layers and particle-slab distance on the enhancement are discussed. For a system of multiple particles horizontally placed above a multilayered slab, we demonstrate that the RHT from slab to particle is significantly inhibited due to strong particle-particle interaction. Analysis of the particle-particle interaction shows that the match of surface resonance and localized resonance of a particle intensifies the interactions among particles, inhibiting heat transfer from slab to particle. This inhibitory effect plays an important role in the tuning of near-field radiation and near-field thermal microscopy.

Wigner function and kinetic theory for massive spin-1/2 particles

We calculate the Wigner function for massive spin-1/2 particles in an inhomogeneous electromagnetic field to leading order in the Planck constant ħ. Going beyond leading order in ħ we then derive a generalized Boltzmann equation in which the force exerted by an inhomogeneous electromagnetic field on the particle dipole moment arises naturally. Furthermore, a kinetic equation for this dipole moment is derived. Carefully taking the massless limit we find agreement with previous results. The case of global equilibrium with rotation is also studied. Our framework can be used to study polarization effects induced by vorticity and magnetic field in relativistic heavy-ion collisions.

Superfluid transition in disordered dipolar Fermi gases

We consider a weakly interacting two-component Fermi gas of dipolar particles (magnetic atoms or polar molecules) in two-dimensional geometry. The dipole-dipole interaction (together with the short-range interaction at Feshbach resonances) for dipoles perpendicular to the plane of translational motion may provide a superfluid transition. The dipole-dipole scattering amplitude is momentum dependent, which violates the Anderson theorem that claims the independence of the transition temperature on the presence of weak disorder. We have shown that the disorder can strongly increase the critical temperature (up to $10\phantom{\rule{0.16em}{0ex}}\mathrm{nK}$ at realistic densities). This opens wide possibilities for the studies of the superfluid regime in weakly interacting Fermi gases, which has not been observed so far.

Numerical modeling of the magnetic response of interacting superparamagnetic particles to an ac field with arbitrary amplitude

The dynamic magnetic response of immobilized superparamagnetic nanoparticles to an ac field with arbitrary amplitude is studied using numerical simulations. The nanoparticles are considered to be distributed randomly within an implicit solid matrix, but the easy axes of the particles are aligned parallel to the ac magnetic field. Modeling of dynamic response is based on the Fokker–Planck–Brown equation in which the interparticle dipole–dipole interactions are included within the framework of the modified mean-field theory. The effects of the magnetic crystallographic anisotropy barrier, the ac field amplitude, and the interparticle interactions on the dynamic magnetization, susceptibility, and relaxation time are analyzed. It is shown that an increase in the amplitude of the ac field significantly accelerates relaxation processes in the system under consideration whereas an increase in magnetic anisotropy of a particle and interparticle dipole–dipole interactions slows them down. The numerical results for dynamic susceptibility and relaxation time are compared against theories reliable in the weak ac field, and an excellent agreement is obtained.

Analytical Model for Particle Capture in Nanopores Elucidates Competition among Electrophoresis, Electroosmosis, and Dielectrophoresis.

The interaction between nanoparticles dispersed in a fluid and nanopores is governed by the interplay of hydrodynamical, electrical, and chemical effects. We developed a theory for particle capture in nanopores and derived analytical expressions for the capture rate under the concurrent action of electrical forces, fluid advection, and Brownian motion. Our approach naturally splits the average capture time in two terms, an approaching time due to the migration of particles from the bulk to the pore mouth and an entrance time associated with a free-energy barrier at the pore entrance. Within this theoretical framework, we described the standard experimental condition where a particle concentration is driven into the pore by an applied voltage, with specific focus on different capture mechanisms: under pure electrophoretic force, in the presence of a competition between electrophoresis and electroosmosis, and finally under dielectrophoretic reorientation of dipolar particles. Our theory predicts that dielectrophoresis is able to induce capture for both positive and negative voltages. We performed a dedicated experiment involving a biological nanopore (α-hemolysin) and a rigid dipolar dumbbell (realized with a β-hairpin peptide) that confirms the theoretically proposed capture mechanism.

Kinetics of Crystallization and Orientational Ordering in Dipolar Particle Systems

The kinetic mechanisms underlying the bottom-up assembly of colloidal particles have been widely investigated in efforts to control crystallization pathways and to direct growth into targeted super...

Strong recoil optical forces on dipoles via high-k plasmons excitation in thin metallic films

The recoil optical force that acts on emitters near a surface or waveguide relies on near-field directionality and conservation of momentum. It features desirable properties uncommon in optical forces, such as the ability to produce it via wide-area illumination of vast numbers of particles without the need for focusing or being dynamically switchable via the polarization of light. Unfortunately, these recoil forces are usually very weak and have not been experimentally observed in small dipolar particles. Some works theoretically demonstrate orders-of-magnitude enhancement of these forces via complex nano-structuring involving hyperbolic surfaces or metamaterials, complicating the fabrication and experimental demonstration. In this work, we theoretically and numerically show enhancement of the lateral recoil force by simply using thin metallic films, which support ultra-high-momentum plasmonic modes. The high-momentum carried by these modes imparts a correspondingly large recoil force on the dipole, enhancing the force by several orders of magnitude in a remarkably simple geometry, bringing it closer to practical applications.

Optical forces in coupled chiral particles

Structural chirality can induce counter-intuitive optical forces due to inherent symmetry properties. While optical forces on a single chiral particle in the Rayleigh regime have been well studied, optical forces in coupled chiral particles remain less explored. By using full-wave numerical simulations and analytical methods of source representation and coupled mode theory, we investigated the optical forces induced by a plane wave on two chiral particles coupling with each other via the evanescent near fields. We found that the induced electric and magnetic dipoles of the chiral particles have complicated couplings that give rise to dark and bright modes. The interaction force between the particles can be either attractive or repulsive, and its magnitude can be significantly enhanced by the resonance modes. The attractive force is much stronger if two particles are of opposite handedness compared with the case of same handedness. The electric dipole force and the magnetic dipole force have the same sign for two particles with the same handedness, while they are of different signs for two particles with opposite handedness. The results can lead to a better understanding of chirality-induced optical forces with potential applications in optical manipulations and chiral light-matter interactions.

Effects of interactions on magnetization relaxation dynamics in ferrofluids.

The dynamics of magnetization relaxation in ferrofluids are studied with statistical-mechanical theory and Brownian dynamics simulations. The particle dipole moments are initially perfectly aligned, and the magnetization is equal to its saturation value. The magnetization is then allowed to decay under zero-field conditions toward its equilibrium value of zero. The time dependence is predicted by solving the Fokker-Planck equation for the one-particle orientational distribution function. Interactions between particles are included by introducing an effective magnetic field acting on a given particle and arising from all of the other particles. Two different approximations are proposed and tested against simulations: a first-order modified mean-field theory and a modified Weiss model. The theory predicts that the short-time decay is characterized by the Brownian rotation time τ_{B}, independent of the interaction strength. At times much longer than τ_{B}, the asymptotic decay time is predicted to grow with increasing interaction strength. These predictions are borne out by the simulations. The modified Weiss model gives the best agreement with simulation, and its range of validity is limited to moderate, but realistic, values of the dipolar coupling constant.

Swimming Suppresses Correlations in Dilute Suspensions of Pusher Microorganisms

Active matter exhibits various forms of non-equilibrium states in the absence of external forcing, including macroscopic steady-state currents. Such states are often too complex to be modelled from first principles and our understanding of their physics relies heavily on minimal models. These have mostly been studied in the case of "dry" active matter, where particle dynamics are dominated by friction with their surroundings. Significantly less is known about systems with long-range hydrodynamic interactions that belong to "wet" active matter. Dilute suspensions of motile bacteria, modelled as self-propelled dipolar particles interacting solely through long-ranged hydrodynamic fields, are arguably the most studied example from this class of active systems. Their phenomenology is well-established: at sufficiently high density of bacteria, there appear large-scale vortices and jets comprising many individual organisms, forming a chaotic state commonly known as bacterial turbulence. As revealed by computer simulations, below the onset of collective motion, the suspension exhibits very strong correlations between individual microswimmers stemming from the long-ranged nature of dipolar fields. Here we demonstrate that this phenomenology is captured by the minimal model of microswimmers. We develop a kinetic theory that goes beyond the commonly used mean-field assumption, and explicitly takes into account such correlations. Notably, these can be computed exactly within our theory. We calculate the fluid velocity variance, spatial and temporal correlation functions, the fluid velocity spectrum, and the enhanced diffusivity of tracer particles. We find that correlations are suppressed by particle self-propulsion, although the mean-field behaviour is not restored even in the limit of very fast swimming.

On the magnetization of a dilute suspension in a uniform magnetic field: Influence of dipolar and hydrodynamic particle interactions

In this work, the magnetization of a dilute magnetic suspension undergoing a shear flow and a uniform field in the presence of dipolar and hydrodynamic interactions is investigated. Firstly, the problem of a single magnetic particle in a simple shear flow in the presence of an external uniform magnetic filed is examined. From this solution, we study the influence of non-equilibrium effects on the magnetization at condition of strong flows, including the spinning behavior of the particles. Next, we use a non-renormalized cluster expansion in order to derive the magnetization O(ϕ2) in a sheared suspension of non-Brownian magnetic from a solution of a creeping flow problem of two magnetic spherical particles interacting magnetically and hydrodynamically in the presence of a uniform applied magnetic field, where ϕ is the particle volume fraction. The numerical results suggest that under condition of strong flows the aggregative nature of the dipolar interactions may lead to a decrease in the magnetization component in the field direction. In contrast, the dispersive character of the viscous hydrodynamic interactions produces a substantial increase in the same component of magnetization even under the condition of strong flow. This result is attributed to the misalignment produced by the hydrodynamic disturbances on the particle dipoles orientated with the flow direction.

Dipolar interactions among magnetic dipoles of iron oxide particles dispersed in mili-size hydrogel beads

The recently published Mean Field Interacting Superparamagnet Model (MFISP model), which introduces the effective demagnetizing factor NE, is tested in specimens having a random-like spatial distribution of magnetic nanoparticles, where different hierarchies of clustering are present. These specimens are ferrogel PVA/iron oxide beads synthesized by a one-pot route, having spheroidal shapes and sizes of about 1 mm, and chain and disk-like arrays (superstructures) of beads. Raman analyses indicated that magnetic nanoparticles are composed by a mixture of magnetite and maghemite. Beads swell 208% by hydration in about 40 min.The increase of the ac susceptibility as a function of hydration time closely reflects the effect of bead swelling, in agreement with the expected diminution of dipole–dipole interactions. Measured susceptibility is analyzed in terms of the susceptibility χ of non-interacting particles and the effective demagnetizing factor NE of the specimen, which depends on swelling. The Specific Absorption Rate of electromagnetic power by the beads grows with the hydration time in agreement with ac susceptibility behavior.For long hydration times susceptibility and high field magnetization decrease. This is explained by the occurrence of oxidation of magnetite/maghemite to hematite.Isothermal magnetization experiments are performed on each superstructure in two perpendicular principal directions each. Results are consistently described with the MFISP model by considering two hierarchies of clustering: beads themselves and clusters within the beads.From the whole set of experiments, it is possible to estimate values for the volume fractions of particles in clusters and clusters in beads, given by xpc=0.46(15) and xcb=0.16(5). The susceptibility of non-interacting particles, χ=13(4), is also obtained, which results about five times larger than the measured (apparent) one.The MFISP model proves to be a convenient and efficient tool for the analysis of magnetization studies of complex 3d dispersions of magnetic nanoparticles, allowing an experimental determination of relevant physical information, otherwise not accessible by magnetic measurements.

Clustering and Phase Separation in Mixtures of Dipolar and Active Particles in an External Field.

Directing the assembly of colloidal particles through the use of external electric or magnetic fields shows promise for the creation of reconfigurable materials. Self-propelled particles can also be used to dynamically drive colloidal systems to nonequilibrium steady states. We investigate colloidal systems that combine both of these methods of directed assembly, simulating mixtures of passive dipolar colloids and active soft spheres in an external magnetic field using Brownian dynamics in two dimensions. In these systems, the dipolar particles align in the direction of the external field, but the active particles are unaffected by the field. The phase behaviors exhibited included a percolated dipolar network, dipolar string-fluid, isotropic fluid, and phase-separated state. We find that the external field allows the dipolar particles to form a percolated network more easily compared to when no external field is present. Additionally, the mixture phase separates at lower active particle velocity in an external field than when no field is present. Our results suggest that combining multiple methods of directing colloidal assembly could lead to new pathways to fabricate reconfigurable materials.