Spin Motion Research Articles

Oblique bouncing of a droplet from a non-slip boundary: computational realization and application of self-spin droplets

Recent studies have demonstrated the significance of droplet spinning motion on droplet collision outcomes and raised the question of generating spinning droplets in an experiment. The present work proposed the idea of making an initially non-spinning droplet self-spin by its oblique impact on a non-slip boundary. We computationally demonstrated the feasibility of the idea and exploited its application in the binary collision of spinning droplets. Specifically, a parametric study was conducted to investigate the effects of impact velocity, impact angle, and liquid viscosity on the spinning droplet. The results showed that a larger impact velocity or a larger liquid viscosity causes an increased angular speed of the spinning droplet, however increasing impact angle leads to a nonmonotonic variation of the angular speed. The oblique-impact-induced droplet spin is attributed to the asymmetric gas film flow, asymmetric lubrication pressure, and shear stress within the gas film region, leading to the earlier bouncing motion on one side to rotate the droplet. In addition, the synergetic effects of the droplet stretching length and the intensity of asymmetric lubrication pressure account for the variation of droplet spin angular speed and droplet separation. As a preliminary application of the proposed idea, the feasible technical approach of realizing the binary collision of spinning droplets was computationally realized.

Robust measurement of angular velocity based on rotational Doppler effect in misaligned illumination

The discovery of the rotational Doppler effect (RDE) provides opportunities to achieve fast, accurate, and contactless measurement of angular velocity. As the RDE signal is sensitive to the illumination light and its scattering characteristic, how to effectively extract the angular velocity of a detected target becomes one of the key difficulties in this measurement technique. Here, we put forward a scattering modified RDE model to fully understand the frequency-domain behavior of an RDE signal in misaligned illumination. A robust strategy based on the peak-to-peak intervals in a wideband spectrum of the RDE signal is demonstrated to acquire accurate angular velocity, without strict requirement to the alignment of illumination and roughness of the detected surface. Furthermore, the feasibility to extract the spin angular velocity from a composite motion of spin and translation is discussed.

The synchrotron photons from the wave solution of the Dirac equation

The goal of this article is to show the derivation of the power spectrum of the synchrotron radiation from the Volkov solution of the Dirac equation and from S-matrix. We also generalize the Bargmann-Michel-Telegdi equation for the spin motion in case it involves the radiation term. This equation plays the crucial role in spin motion of protons in LHC and FERMILAB. The axion production in the magnetic field described by the Volkov solution is discussed.

Transfers of the OAM and SAM densities of Bessel beam into the orbital and spin torques upon gold nanorod

The optomechanical interplay of a Bessel beam (an optical vortex) with a gold nanorod (GNR) in water is theoretically studied. The optical force and torque exerted on the GNR are analyzed using Maxwell's stress tensor to investigate the transfer rates of the orbital angular momentum (OAM) and the spin angular momentum (SAM) densities of Bessel beam into the orbital and spin torques upon GNR, respectively. From the numerical analysis, we find that the ratio of the orbital torque to the spin torque on the GNR matches the ratio of OAM to SAM of Bessel beam, corresponding to the order (topological charge) of the Bessel beam. This is to say that the transfer rate of the OAM density of Bessel beam into the orbital torque exerted on GNR nearly equals that of the SAM density into the spin torque. It implies that the spin-orbit interaction (SOI) of the optical vortex beam via a GNR is very weak. In addition, the spin torque is nearly equal to the corresponding zilch torque, the surface integral of Lipkin's zilch stress tensor. This could be because a GNR can be regarded as a perfect electric dipole particularly when the wavelength of the incident light is close to and little longer than the longitudinal surface plasmon resonance of GNR; the dipole's orientation is along the long axis of GNR. In contrast, the SOI via a spherical gold nanoparticle (GNP) is strong due to no preferred orientation for the equivalent dipole; a part of SAM of the optical vortex is transferred to the orbital torque upon GNP. Our results demonstrate that GNR, functioning as an ideal electric dipole, is a highly effective probe for detecting the OAM and SAM densities of an optical vortex individually through its distinct orbital and spin motions, without any crosstalk.

Numerical Study of Aerodynamic Characteristics of No-Swinging and Free-to-Swing Cross Parachute

During wind-tunnel experiments on parachutes, it is common to either fix the apex of the canopy to maintain a specific angle of attack or only fix the confluence point of the suspension lines to allow the canopy to move freely. These no-swinging and free-to-swing conditions affect considerably the aerodynamic characteristics of the parachute. This paper investigates the influence of these conditions on the aerodynamic characteristics and dynamic motion characteristics of both symmetric cross parachutes with eight equal suspension lines, and asymmetric cross parachutes with two (of eight) shortened suspension lines. The research reveals that the canopy of a no-swinging cross parachute exhibits a spinning motion around its axis. The free-to-swing parachute undergoes a conical swing motion around the suspension-line confluence point. The frequency of spin and conical swing increases in proportion to . The magnitude and direction of the lift force varies periodically owing to the conical swing of the free-to-swing parachute. Conversely, the no-swinging parachute, at a fixed angle of attack, maintains a comparatively stable lift force. The free-to-swing asymmetric cross parachute produces a stronger lift force because of the larger swing angle compared to the symmetric parachute. The resultant asymmetry in the aerodynamic shape caused by the shortening of suspension lines has minimal effect on aerodynamic characteristics.

The combined effect of contact interface size and spin on lubricated traction in rolling-sliding point contacts

Traction in lubricated rolling and sliding contacts plays an important role in the performance of several machine elements. Many of such elements are subject to spin motion, which can affect their performance. Recent studies have also highlighted the effects of the contact scale on the traction behaviour. Nevertheless, their combined effect was not investigated to date. The present study, therefore, analyses the effect of both on the traction in heavily-loaded lubricated contacts. To this end, a novel semi-analytical traction prediction model was developed considering spin motion. The results show that increasing the contact size and/or spinning speed leads to a reduction in traction coefficient and contact efficiency due to thermal effects. These conclusions have important practical applications designing machine elements.

Acoustic tweezers for high-throughput single-cell analysis.

Acoustic tweezers provide an effective means for manipulating single cells and particles in a high-throughput, precise, selective and contact-free manner. The adoption of acoustic tweezers in next-generation cellular assays may advance our understanding of biological systems. Here we present a comprehensive set of instructions that guide users through device fabrication, instrumentation setup and data acquisition to study single cells with an experimental throughput that surpasses traditional methods, such as atomic force microscopy and micropipette aspiration, by several orders of magnitude. With acoustic tweezers, users can conduct versatile experiments that require the trapping, patterning, pairing and separation of single cells in a myriad of applications ranging across the biological and biomedical sciences. This procedure is widely generalizable and adaptable for investigations in materials and physical sciences, such as the spinning motion of colloids or the development of acoustic-based quantum simulations. Overall, the device fabrication requires ~12 h, the experimental setup of the acoustic tweezers requires 1-2 h and the cell manipulation experiment requires ~30 min to complete. Our protocol is suitable for use by interdisciplinary researchers in biology, medicine, engineering and physics.

Aeroelastic Response of Spinning Projectiles with Large Slenderness Ratio at Supersonic Speed

Obvious aeroelastic deformation occurs in spinning projectiles with large slenderness ratio, which seriously affects flight stability and maneuverability. This paper investigates the aeroelastic response of spinning projectiles with large slenderness ratio under supersonic speed. Based on a dynamic mesh method, an unsteady numerical simulation method is developed to study the aeroelasticity of spinning projectiles by coupling aerodynamics and structural dynamics. The numerical simulation method is well validated by the experimental results of AGARD 445.6 wing flutter. Then, the aeroelastic response of spinning projectiles with large slenderness ratio is numerically explored under different flight conditions. The aeroelastic response is obtained, revealing the presence of beat vibrations and variations in response frequency. Furthermore, the influence mechanism of flight conditions on the aeroelastic response is analyzed. The results suggest that the coupling of the first two modes of the projectile caused by the spinning motion leads to the occurrence of beat vibrations in the aeroelastic response; the coupling degree of the first two modes decreases as the angle of attack increases and it increases with the increase in spinning speed; and the time−averaged deformation caused by the time−averaged aerodynamic force is beneficial to the convergence of the aeroelastic response of spinning projectiles, while the rotation−induced Magnus effect is counterproductive.

Electrophoresis of a dielectric droplet with constant surface charge density.

Electrophoresis of a dielectric fluid droplet with constant surface charge density is investigated theoretically in this study. A pseudo-spectral method based on Chebyshev polynomials is adopted to solve the governing electrokinetic equations. It is found, among other things, that the larger the electrolyte strength in the ambient solution is, the slower the droplet moves in general. This is due to the strong screening effect of the large amount of indifferent counterions in the neighborhood of the droplet, with no reinforcement of potential-determining ions adsorbing to the droplet surface. The droplet comes to a complete halt eventually. Critical points are discovered for highly charged droplets, at which the droplet surface becomes immobile and the interior fluid stops recirculating. The droplet moves like a rigid particle with constant mobility regardless of its viscosity, a situation referred to as the "solidification phenomenon." The deadlock between the spinning motions on the charged droplet surface induced by the electric driving force and the hydrodynamic driving force respectively is responsible for this peculiar phenomenon. This is also observed for a dielectric droplet with constant surface electric potential. We demonstrate here that it occurs in the constant surface charge density situation as well.

Управление динамикой спиновой поляризации электронов проводимости электрическим и механическим воздействием

A quantum model for the interaction of a collectivized conduction electron with a crystal field in a homogeneous and isotropic deformed polycrystalline sample, taking into account the spin-orbit interaction, is proposed. A dynamic equation of motion of the conduction electron spin in a stress-strained metal is obtained. It is shown that under conditions of inhomogeneous torsion in the stationary case, the average spin of conduction electrons is oriented predominantly along the current density vector. The maximum value of the spin-orbit interaction is observed when the torsion axis is orthogonal to the current density vector.

Transport barrier and spinning blob dynamics in the tokamak edge

In this work, we investigate the dynamics of plasma blobs in the edge of magnetic confinement devices using a full-f gyrokinetic particle-in-cell code with X-point geometry. In simulations, the evolution of a seeded blob is followed as it approaches a naturally-forming zonal shear layer near the separatrix, where the blob is stabilized by a large spin induced by the self-consistent adiabatic electron response, and blob bifurcation and trapping are observed during the cross-field propagation of blobs. A new theoretical explanation in both the zonal free and zonal shear layer is constructed, where the dominant spin motion is included. A theoretical condition for a transport barrier induced by the interaction between spinning blobs and the zonal shear layer is obtained, and its scaling is verified with simulations. The new theoretical framework, especially the transport barrier, is applicable to explain and predict various experimental phenomena. In particular, the transport barrier condition calculated with experimental parameters demonstrates that the blob radial transport for H mode is smaller than L mode in experiments.

Effects of the Dzyaloshinskii-Moriya interaction on the Fermi-Pasta-Ulam behavior in a magnetic system

We study the behavior of nonlinear spin motion in one-dimensional magnetic chain models. We explore how the various parameters affect the phase transitions between linear and nonlinear behaviors both with and without the Dzyaloshinskii-Moriya interaction (DMI). A Fourier analysis method is employed to investigate the modes in the structure with DMI. In all cases, we find that uniaxial anisotropy is necessary for the Fermi-Pasta-Ulam effect to appear. We investigate the DMI influence through the use of phase diagrams that examine the transitions between the three fundamental states: linear, Fermi-Pasta-Ulam, and ergodic. The DMI does make significant changes in these phase diagrams because it allows for additional routes for the distribution of energy.

Nonlinear low-velocity impact of graphene platelets reinforced metal foams cylindrical shell: Effect of spinning motion and initial geometric imperfections

A nonlinear analysis is presented for low-velocity impact behavior of a graphene platelets-reinforced metal foams (GPLRMF) cylindrical shell with spinning motion in thermal environment, in which the initial geometric imperfection is incorporated. Taking three different graphene platelets (GPLs) distribution patterns into account, the GPLs reinforcement is either uniformly-distributed or functionally-graded along the shell thickness direction. The temperature-dependent micromechanical model is applied to estimate the material properties of GPLs reinforced composites. The nonlinear Donnell thin shell theory is utilized to derive the motion equations, in which von Kármán-type of kinematic nonlinearity is included. The influences of geometrical imperfections, spinning velocity, different boundary conditions, GPLs distribution patterns, foam distribution types, foam coefficient, GPLs weight fraction, temperature changes, the impactor' radius and initial velocity, prestressing force and damping coefficient on the low-velocity impact problems are discussed using the Runge-Kutta method in detail.

Stability of interlocked self-propelled dumbbell clusters.

Combining experimental observations of Quincke roller clusters with computer simulations and a stability analysis, we explore the formation and stability of two interlocked self-propelled dumbbells. For large self-propulsion and significant geometric interlocking, there is a stable joint spinning motion of two dumbbells. The spinning frequency can be tuned by the self-propulsion speed of a single dumbbell, which is controlled by an external electric field for the experiments. For typical experimental parameters the rotating pair is stable with respect to thermal fluctuations but hydrodynamic interactions due to the rolling motion of neighboring dumbbells leads to a breakup of the pair. Our results provide a general insight into the stability of spinning active colloidal molecules, which are geometrically locked.

Dynamics and interactions of Quincke roller clusters: From orbits and flips to excited states.

Active matter systems may be characterized by the conversion of energy into active motion, e.g., the self-propulsion of microorganisms. Artificial active colloids form models that exhibit essential properties of more complex biological systems but are amenable to laboratory experiments. While most experimental models consist of spheres, active particles of different shapes are less understood. Furthermore, interactions between these anisotropic active colloids are even less explored. Here, we investigate the motion of active colloidal clusters and the interactions between them. We focus on self-assembled dumbbells and trimers powered by an external dc electric field. For dumbbells, we observe an activity-dependent behavior of spinning, circular, and orbital motions. Moreover, collisions between dumbbells lead to the hierarchical self-assembly of tetramers and hexamers, both of which form rotational excited states. On the other hand, trimers exhibit flipping motion that leads to trajectories reminiscent of a honeycomb lattice.

A geometric motion planning for a spin-rolling sphere on a plane

The paper deals with motion planning for a spin-rolling sphere when the sphere follows an optimal straight path on a plane. Since the straight line constrains the sphere’s motion, controlling the sphere’s spin motion is essential to converge to a desired full configuration of the sphere. In this paper, we show a new geometric-based planning approach that is based on a full-state description of this nonlinear system. First, the problem statement of the motion planning is posed. Next, we develop a geometric controller implemented as a virtual surface by using the Darboux frame kinematics. This virtual surface generates arc-length-based inputs for controlling the trajectories of the sphere. Then, an iterative algorithm is designed to tune these inputs for the desired configurations. Finally, the feasibility of the proposed approach is verified by simulations.

Heavy symmetric tops and the Hannay angle

The dynamics of a heavy symmetric top are studied in connection with the Hannay angle. When the top undergoes a steady precession due to gravity without nutation, the Hannay angle has a geometric nature such that it is identical to the solid angle subtended by the loop swept out by the symmetry axis of the top. Here, we show that the Hannay angle can also be described by the angle between two radial vectors on the disk of the top corresponding to the pure spinning motion and the coupled motion of spin and precession for one period of the precession. The geometric nature of the angle between the two radial vectors is verified by demonstrating, via parallel transport, that the magnitude of the angle is the same as that of the solid angle. In the presence of nutation, the path constructed by the symmetry axis is not closed, and the steady precession appears in the limit of infinite initial spin angular velocity. As a consequence, in an ideal situation of no friction, the Hannay angle as a pure geometric effect does not exist in the superposed motion of precession and nutation.

Theory of optical generation and detection of propagating magnons in an antiferromagnet

We report a theory of optical generation and detection of the propagating spin waves in antiferromagnetic materials relevant for ultrafast pump-probe experiments. We derive and solve the equations of motion for antiferromagnetic spins in response to the light-induced effective magnetic field in the linear regime. Different forms of the excitation and the properties of the generated spin waves are analyzed. We theoretically show the selective detection of the spin waves by the magneto-optical Kerr effect. The developed formalism is readily applicable to inform future experiments on antiferromagnetic optomagnonics.

Structure Design of Multi-DOF Motor with Stator Permanent Magnet based on Flux Switching

This paper aims at the problems of no core in the stator coil of a compact multi-degree-of-freedom (M-DOF) motor, i.e., small output torque and low power density. In order to increase the output torque, a stator permanent magnet spherical motor based on flux switching is proposed. Firstly, the principle of flux switching is introduced. The basic structure of the new motor is preliminarily designed, and the motion mechanism is analysed for spin motion and tilt motion respectively. Then the minimum model of a novel motor is established in ANSYS by using the virtual displacement method. The key parameters such as the ratio of stator pole tooth to permanent magnet size, different stator pole tooth end structures, different pole tooth surface shapes, different pole sizes, permanent magnet thickness, different magnetisation methods and different ampere-turn numbers are optimised within a reasonable parameter range. The simulation results are analysed and discussed, and the optimal results for each key parameter are thus given. Finally, considering the arrangement of the stator and rotor poles, the rationality of the three-DOF motion was verified by means of two energising strategies, spin motion and tilt motion, verified the reasonableness of the above optimal parameters and thus determined the structural parameters of the new spherical motor to accomplish the three-DOF motion and the maximum torque range. The simulation results show that the electromagnetic torque varies approximately linearly over a range of ampere-turns for the same position of the rotor pole teeth. It provides an opportunity for the M-DOF motor design of a high torque output and power density.

Self-stabilizing three-dimensional particle manipulation via a single-transducer acoustic tweezer

This paper investigates the mechanism of self-stabilizing, three-dimensional Mie particle manipulation in water via an acoustic tweezer with a single transducer. A carefully designed acoustic lens is attached to the transducer to form an acoustic vortex, which provides angular momentum on the trapped polymer sphere and leads to a fast-spinning motion. The sphere can find equilibrium positions spontaneously during the manipulation by slightly adjusting its relative position, angular velocity, and spinning axis. The spinning motion greatly enhances the low-pressure recirculation region around the sphere, resulting in a larger pressure induced drag. Simultaneously, the Magnus effect is induced to generate an additional lateral force. The spinning motion of the trapped sphere links the acoustic radiation force and hydrodynamic forces together, so that the sphere can spontaneously achieve new force balance and follow the translational motion of the acoustic tweezer. Non-spherical objects can also be manipulated by this acoustic tweezer.