Dipole Force Research Articles

Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions

Tracer particles immersed in suspensions of biological microswimmers such as E. coli or C. reinhardtii display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, 3-dimensional suspensions, these phenomena have typically been explained by the hydrodynamic advection of point tracers by isolated microswimmers, while, at higher concentrations, correlations between pusher microswimmers such as E. coli can increase the effective diffusivity even further. Anisotropic tracers in active suspensions can be expected to exhibit even more complex behaviour than spherical ones, due to the presence of a nontrivial translation-rotation coupling. Using large-scale lattice Boltzmann simulations of model microswimmers described by extended force dipoles, we study the motion of ellipsoidal point tracers immersed in 3-dimensional microswimmer suspensions. We find that the rotational diffusivity of tracers is much less affected by swimmer-swimmer correlations than the translational diffusivity. We furthermore study the anisotropic translational diffusion in the particle frame and find that, in pusher suspensions, the diffusivity along the ellipsoid major axis is higher than in the direction perpendicular to it, albeit with a smaller ratio than for Brownian diffusion. Thus, we find that far field hydrodynamics cannot account for the anomalous coupling between translation and rotation observed in experiments, as was recently proposed. Finally, we study the probability distributions (PDFs) of translational and rotational displacements. In accordance with experimental observations, for short observation times we observe strongly non-Gaussian PDFs that collapse when rescaled with their variance, which we attribute to the ballistic nature of tracer motion at short times.

Manipulation and control of droplets on surfaces in a homogeneous electric field

A method to manipulate and control droplets on a surface is presented. The method is based on inducing electric dipoles inside the droplets using a homogeneous external electric field. It is shown that the repulsive dipole force efficiently suppresses the coalescence of droplets moving on a liquid-infused surface (LIS). Using a combination of experiments, numerical computations and semi-analytical models, the dependence of the repulsion force on the droplet volumes, the distance between the droplets and the electric field strength is revealed. The method allows to suppress coalescence in complex multi-droplet flows and is real-time adaptive. When the electric field strength exceeds a critical value, tip streaming from the droplets sets in. Based on that, it becomes possible to withdraw minute samples from an array of droplets in a parallel process.

Inclined magnetic dipole and nanoscale energy exchange with infinite shear rate viscosity of 3D radiative cross nanofluid

AbstractWorking nanoliquids are used in bionomical concerns and to enhance the energy in the refrigeration system. The unique thermophysical characteristics of nanoliquids made nanoscience interesting and beneficial for fields of engineering, medical, and industry with real applications, like, food science, employing nanoparticles to deliver drugs, food microbiology, detection of foodborne pathogens, detection of foodborne pathogens, electronic device cooling, controlling fusion, magnetic cell separation, cancer therapeutics, nanocryosurgery, and so forth. The aim of this study is to deliver a broad analysis on nanoscale energy exchange and inclined magnetic dipole impact with the mathematical model of cross nanofluid. Scrutiny of movement of fluid is made by placing the magnetic dipole and buoyancy force. Transport of energy is investigated by thermal radiation, nonuniform heat sink–source while checking of mass transfer facts of activation energy and the chemical process is taken into consideration. Moreover, the nanostructured concepts of thermophoresis and Brownian movement along with zero‐mass flux constraint are also utilized for the comprehensiveness of this study. Mathematical relations of cross nanofluid are processed with similarity transformation, shooting methodology, and bvp4c Matlab procedure is rehearsed to get the numerical results of this attempt.

On a Change of Variables in Lagrange’s Equations

In addition, some methodical examples interesting in theory and applications are considered. In some of them (the equilibrium of a polytropic gas sphere between whose particles the forces of gravitational attraction act and the problem of the planar motion of a charged particle in the dipole force field) Lagrange’s equations are not only covariant, but also possess the invariance property.

Simulations on the rheology of dry magneto-rheological fluid under various working modes

This work studied the rheological properties and magnetorheological (MR) mechanism of dry magnetorheological fluid (MRF) under various working modes. A novel simulation method combining the discrete element method and computational fluid dynamics was developed, in which the bilateral coupling between particles and the flow field of the matrix (air) was considered. The microstructures and mechanical properties in the redispersion process, shear mode, and valve mode were systematically simulated for the first time. The results indicated that dry MRF presented superior redispersion property and response time (several μs) than liquid-based MRFs. In shear mode, the magnetic dipolar force and friction force dominated the evolution of microstructures. In valve mode, the magnetic dipolar force and viscous drag force of air became the main interactions. Magnetic particles aggregated into sturdy chain structures and hindered the airflow. The MR effect in valve mode was the pressure gradient of the matrix, which increased up to 1.08 × 105 Pa m−1 with the increasing particle volume fractions and decreased under a large inflow velocity. The best MR effect in valve mode was achieved under a magnetic field of B = 63 mT. Simulations revealed the influence of dimensionless Mn and Re number on the MR effect. The pressure gradient of the matrix was controlled by the external field and can be utilized to design a dry MRF valve for precious and transient vibration control. Simulated dimensionless shear stress in shear mode agreed well with experiments. This work will promote the development and applications of novel high-performance MRFs.

Reaction microscope for investigating ionization dynamics of weakly bound alkali dimers.

We report on the implementation of a far-off-resonant, optical dipole force trap in a reaction microscope combined with a magneto-optical trap. Kinematically complete multi-photon ionization experiments were performed on optically trapped 6Li atoms and photo-associated 6Li2 molecules in their highest vibrational state. The apparatus allows us to distinguish different ionization mechanisms related to the presence of the IR field of the optical dipole trap that can occur during ionization of 6Li and 6Li2 in strong fields. In a series of proof-of-principle experiments, we detect weakly bound dimers via three-photon ionization with femtosecond pulses (τ = 30 fs) at a central wavelength of 780 nm and measure directly the momenta of the photoelectrons in coincidence with recoil ions.

Molecular dynamics study of droplet electrocoalescence in the oil phase and the gas phase

• Dispersive attraction and steric effect of oil molecules affect droplet coalescence. • Solvent accessible surface area could determine when droplets contact and coalesce. • The migration of molecules causes uneven distribution of charges in water droplets. • The electro-coalescence behavior of droplets in oil and gas are highly similar. Under a uniform electric field, water droplets in a water-in-oil (W/O) emulsion approach and coalesce by dipole–dipole forces, which fundamentally affect the efficiency of electric dehydration. In order to unravel the molecular mechanism of droplet electrocoalescence, we explored the coalescence behavior of droplets in the oil phase and the simplified oil phase (nitrogen) under an external electric field using molecular dynamics (MD) simulations. The results indicated that the coalescence process of droplets can be divided into three stages based on the solvent accessible surface area (SASA), before coalescence, coalescing and after coalescence. The two droplets completely coalesced when the center-of-mass distance between droplets was 2.73 nm. Weak interaction and interaction energy analysis revealed that the electrostatic attraction between water molecules dominated the coalescence process of water droplets due to the significant characteristics of electrostatic potential (ESP). The dispersion attraction of C 6 H 14 H 2 O and C 6 H 14 C 6 H 14 was very pronounced while the interactions related to nitrogen molecules were negligible, which was the underlying reason for the differences between droplet coalescence behaviors in the two media. Nevertheless, the coalescence behaviors of a droplet pair in oil and gas had a high level of similarity, which implied that the MD simulation results of the electric coalescence based on nitrogen is acceptable to some extent. The field-induced hydrogen and oxygen atoms migrated toward both ends of the droplet, causing the redistribution of charges and changing the overall dipole moment of the droplets. This study made it possible to use oil-phase molecules for MD simulation study on the electrostatic coalescence behavior of droplets in the W/O emulsion, and confirmed the feasibility of nitrogen replacing with the oil phase.

Active Instability and Nonlinear Dynamics of Cell-Cell Junctions.

Active cell-junction remodeling is important for tissue morphogenesis, yet its underlying physics is not understood. We study a mechanical model that describes junctions as dynamic active force dipoles. Their instability can trigger cell intercalations by a critical collapse. Nonlinearities in tissue's elastic response can stabilize the collapse either by a limit cycle or condensation of junction lengths at cusps of the energy landscape. Furthermore, active junction networks undergo collective instability to drive active in-plane ordering or develop a limit cycle of collective oscillations, which extends over regions of the energy landscape corresponding to distinct network topologies.

Do we need a non-perturbative theory of Bose-Einstein condensation?

We recall the experimental data of one-dimensional axial propagation of sound near the center of the Bose-Einstein condensate cloud, which used the optical dipole force method of a focused laser beam and rapid sequencing of nondestructive phase-contrast images. We reanalyze these data within the general quantum fluid framework but without model-specific theoretical assumptions; using the standard best fit techniques. We demonstrate that some of their features cannot be explained by means of the perturbative two-body approximation and Gross-Pitaevskii model, and conjecture possible solutions.

Surface Characterization of Lipid Biomimetic Systems.

Zeta potential and dipole potential measures are direct operational methodologies to determine the adsorption, insertion and penetration of ions, amphipathic and neutral compounds into the membranes of cells and model systems. From these results, the contribution of charged and dipole groups can be deduced. However, although each method may give apparent affinity or binding constants, care should be taken to interpret them in terms of physical meaning because they are not independent properties. On the base of a recent model in which the lipid bilayer is considered as composed by two interphase regions at each side of the hydrocarbon core, this review describes how dipole potential and zeta potential are correlated due to water reorganization. From this analysis, considering that in a cell the interphase region the membrane extends to the cell interior or overlaps with the interphase region of another supramolecular structure, the correlation of dipole and electrostatic forces can be taken as responsible of the propagation of perturbations between membrane and cytoplasm and vice versa. Thus, this picture gives the membrane a responsive character in addition to that of a selective permeability barrier when integrated to a complex system.

Inter-droplet force between magnetically polarizable Pickering oil-in-water nanoemulsions stabilized with γ-Al2O3 nanoparticles: Role of electrostatic and electric dipolar interactions

HypothesisThe presence of nanoparticles at oil-water interface influences the interaction forces between Pickering emulsions. When charged nanoparticles are at the oil-water interface of an electrostatically stabilized emulsion, in addition to the screened Coulombic interaction, electric dipolar force also influences the total inter-droplet force profiles. An in-depth understanding of the effects of such electric dipolar forces is essential for designing colloidally stable Pickering nanoemulsions for various applications. ExperimentsInter-droplet forces between γ-Al2O3 nanoparticle stabilized oil-in-water nanoemulsion, containing superparamagnetic nanoparticles (magnetically polarizable) in the oil phase, are measured using the magnetic-chaining technique at different pH and salt concentrations. The role of mono-, di- and tri-valent salts on the inter-droplet force profiles are assessed. FindingsForce measurement studies reveal a lowering of inter-droplet spacing, within the linear chains, for higher salt concentrations due to an increased screening. Strong interfacial attachment of the charged nanoparticles results in the formation of an asymmetric charge cloud leading to an electric dipolar interaction. Incorporating the contributions of electric dipolar and screened Coulombic interactions, the theoretically estimated total repulsive force magnitudes are in good agreement with the experimental data. The obtained results offer better insights into the nature of colloidal force between charged particle stabilized nanoemulsions.

Does mesoscopic elasticity control viscous slowing down in glassforming liquids?

The dramatic slowing down of relaxation dynamics of liquids approaching the glass transition remains a highly debated problem, where the crux of the puzzle resides in the elusive increase in the activation barrier ΔE(T) with decreasing temperature T. A class of theoretical frameworks-known as elastic models-attribute this temperature dependence to the variations of the liquid's macroscopic elasticity, quantified by the high-frequency shear modulus G∞(T). While elastic models find some support in a number of experimental studies, these models do not take into account the spatial structures, length scales, and heterogeneity associated with structural relaxation in supercooled liquids. Here, we propose and test the possibility that viscous slowing down is controlled by a mesoscopic elastic stiffness κ(T), defined as the characteristic stiffness of response fields to local dipole forces in the liquid's underlying inherent structures. First, we show that κ(T)-which is intimately related to the energy and length scales characterizing quasilocalized, nonphononic excitations in glasses-increases more strongly with decreasing T than the macroscopic inherent structure shear modulus G(T) [the glass counterpart of liquids' G∞(T)] in several computer liquids. Second, we show that the simple relation ΔE(T) ∝ κ(T) holds remarkably well for some computer liquids, suggesting a direct connection between the liquid's underlying mesoscopic elasticity and enthalpic energy barriers. On the other hand, we show that for other computer liquids, the above relation fails. Finally, we provide strong evidence that what distinguishes computer liquids in which the ΔE(T) ∝ κ(T) relation holds from those in which it does not is that the latter feature highly fragmented/granular potential energy landscapes, where many sub-basins separated by low activation barriers exist. Under such conditions, it appears that the sub-basins do not properly represent the landscape properties relevant for structural relaxation.

Impact of Ni alloying on Fe-C martensite ageing: an atomistic investigation

We propose a computational investigation of Ni-alloying impact on Fe-C martensite ageing, at the atomic scale. Using the Climbing Image Nudged Elastic Band technique, we showed the repulsive nature of Ni-C pairwise interactions in α-iron, and demonstrated that Ni alloying lowers the migration energies and force dipole tensor components associated to an interstitial carbon in α-iron. Computed values of pairwise interactions, migration energies and dipole components were used to implement a Kinetic Monte-Carlo approach. We found that Ni alloying: (i) has negligible impact on martensite thermodynamical stability, (ii) accelerates ageing kinetics, by increasing carbon diffusivity.

Long-range correlations in pinned athermal networks

We derive exact results for displacement fields that develop as a response to external pinning forces in two-dimensional athermal networks. For a triangular lattice arrangement of particles interacting through soft potentials, we develop a Green's function formalism which we use to derive exact results for displacement fields produced by localized external forces. We show that in the continuum limit the displacement fields decay as 1/r at large distances r away from a force dipole. Finally, we extend our formulation to study correlations in the displacement fields produced by the external pinning forces. We show that uncorrelated pinned forces at each vertex give rise to long-range correlations in displacements in athermal systems, with a nontrivial system size dependence. We verify our predictions with numerical simulations of athermal networks in two dimensions.

Branched alkylated polynorbornene and 3D flower-like MoS2 nanospheres reinforced phase change composites with high thermal energy storage capacity and photothermal conversion efficiency

Form-stable phase change materials (FSPCMs) have a great attraction for relieving the contradiction of energy consumption and environmental pollution. However, the practical effects of FSPCMs are severely compromised by their intrinsic drawbacks including liquid leakage, inferior cycling durability and lacking light to heat conversion ability, etc. In this paper, we have constructed a group of novel composite FSPCMs with high latent heat storage density, enhanced thermal stability and excellent cycling life, using 1-octadecanol (OCC) as the latent heat storage units and the branched alkylated polynorbornene as well as 3D flower-like MoS2 nanospheres as the supporting scaffolds for the composite FSPCMs. The branched alkylated polynorbornene enhances the toughness of the composites under the induced dipole force among their alkyl chains so that the liquid leakage during the phase transition can be minimized or completely circumvented. Furthermore, 3D flower-like MoS2 nanospheres, as a photothermic absorbent, impart the composite FSPCMs with enhanced light to thermal conversion efficiency (85.5%), promising for a potential application in conversion and storage of solar energy.

Self‐Healable Fluorinated Copolymers Governed by Dipolar Interactions

Although dipolar forces between copolymer chains are relatively weak, they result in ubiquitous inter‐ and/or intramolecular interactions which are particularly critical in achieving the mechanical integrity of polymeric materials. In this study, a route is developed to obtain self‐healable properties in thermoplastic copolymers that rely on noncovalent dipolar interactions present in essentially all macromolecules and particularly fluorine‐containing copolymers. The combination of dipolar interactions between C─F and C═O bonds as well as CH2/CH3 entities facilitates self‐healing without external intervention. The presence of dipole‐dipole, dipole‐induced dipole, and induced‐dipole induced dipole interactions leads to a viscoelastic response that controls macroscopic autonomous multicycle self‐healing of fluorinated copolymers under ambient conditions. Energetically favorable dipolar forces attributed to monomer sequence and monomer molar ratios induces desirable copolymer tacticities, enabling entropic energy recovery stored during mechanical damage. The use of dipolar forces instead of chemical or physical modifications not only eliminates additional alternations enabling multiple damage‐repair cycles but also provides further opportunity for designing self‐healable commodity thermoplastics. These materials may offer numerous applications, ranging from the use in electronics, ion batteries, H2 fuel dispense hoses to self‐healable pet toys, packaging, paints and coatings, and many others.

Investigation of Coating Adhesion to Wax Added Film Laminated Steel Sheets by Surface Free Energy

Wax-added films are used as exterior films of laminated steel sheets for severe can-forming use. However, one difficulty in using wax-added film laminated steel sheets is the decreased coating adhesion which accompanies wax addition. Therefore, wax addition effects on coating adhesion were investigated using the surface free energy. Results demonstrated that wax on the polyethylene terephthalate（PET）film surface decreases the dipole force component（γp）of the surface free energy. The decrease in γp of the laminated film decreases the work of adhesion between the film and the coating. X-Ray photoelectron spectroscopy analysis revealed that the decrease in γp is attributable to a decrease in the ester bonds of PET that occurs along with increase of the wax ratio on the film surface. Furthermore, the laminated steel sheet coating adhesion was found to be related to the work of adhesion between the film and the coating, suggesting that the decreased coating adhesion caused by wax addition is explainable by the decrease in γp of the laminated film caused by the increase of the wax ratio on the film surface.

On the formation of antihydrogen beams using travelling optical lattices

The production of beams of antihydrogen atoms using the dipole force provided by a travelling optical lattice to accelerate a sample of the anti-atoms held in a magnetic gradient atom trap is investigated. By considering current and near-future antihydrogen trapping capabilities we find that useful fluxes of the anti-atoms can be achieved with directional properties that can be manipulated using laser parameters such as pulse duration and frequency chirp rate. Applications of the beams are briefly discussed.

Crystallization of Polymers under the Influence of an External Force Field.

We simulated the crystallization and melting behavior of entangled polymer melts using molecular dynamics where each chain is subject to a force dipole acting on its ends. This mimics the deformation of chains in a flow field but represents a well-defined equilibrium system in the melt state. Under weak extension within the linear response of the chains, the mechanical work done on the system is about two orders of magnitude smaller as compared with the heat of fusion. As a consequence, thermodynamic and simple arguments following the secondary nucleation model predict only small changes of the crystalline phase. By contrast, an increase of the stem length up to a factor of two is observed in our simulations. On the other hand, the lamellar thickening induced by the external force is proportional to the increase of the entanglement length in the melt prior to crystallization as measured by the primitive path method. While the mechanical work done on the system is only a small perturbation for thermodynamics of polymer crystallization, the change of the primitive path is large. This suggests that a strong increase in the lamellar thickness induced, by external deformation, a topological rather than a thermodynamic origin.

Interfacial Flow around Brownian Colloids.

Understanding the flow created by particle motion at interfaces is a critical step toward understanding hydrodynamic interactions and colloidal self organization. We have developed correlated displacement velocimetry to measure flow fields around interfacially trapped Brownian particles. These flow fields can be decomposed into interfacial hydrodynamic multipoles, including force monopole and dipole flows. These structures provide key insights essential to understanding the interface's mechanical response. Importantly, the flow structure shows that the interface is incompressible for scant surfactant near the ideal gaseous state and contains information about interfacial properties and hydrodynamic coupling with the bulk fluid. The same dataset can be used to predict the response of the interface to applied, complex forces, enabling virtual experiments that produce higher order interfacial multipoles.