Exchange Of Linear Momentum Research Articles

An experimental evaluation of particle impact dampers applied on the tool for milling of hardened steel complex surface

In the manufacturing of dies and molds, vibrations may represent serious problems, since the finishing tool used is usually slender (high length/diameter ratio) in order to machine deep cavities with complex geometries, typical of these products. Vibration is an undesirable phenomenon in any machining operation as it can lead to poor surface finish, low material removal rate, and high tool wear rate. Impact dampers have been put into the tools as a method for reducing vibration in machining processes. Damping occurs through energy dissipation and linear momentum exchange during intermittent collisions between the main structure (in this case the milling tool) and a free mass (spheres or cylinders placed within a tool cavity). Although efficient, these types of dampers are highly nonlinear. Thus, the aim of this work is to analyze the effect of different materials and geometries (steel spheres, tungsten spheres and steel cylinders) acting as impact damper elements inside a ball nose end milling tool. To perform this task, a comparison of commercial tool holders and dampened tool holders was done in the milling of a convex D6 steel surface, comparing commercial tool holders with dampened ones. The results showed that the tools with impact dampers generated lower values of roughness in the workpiece (around 30% of the value observed in the conventional steel tool holder for the case of steel cylinders and around 40% for both spheres) and presented lower levels of vibration when compared to the same tool without the impact damper, mainly in the machining of workpiece regions where radial and tangential forces are predominant. The tool which used tungsten spheres as damper elements generated roughness surfaces similar to those obtained with steel spheres, while the tool which used steel cylinders only generated lower roughness in the regions where the axial force component is not predominant, which shows that its performance is highly dependent on the resulting force direction.

Chiral exchange drag and chirality oscillations in synthetic antiferromagnets

Long-range interactions between quasiparticles give rise to a ‘drag’ that affects the fundamental properties of many systems in condensed matter physics1–11. Drag typically involves the exchange of linear momentum between quasiparticles and strongly influences their transport properties. Here, we describe a kind of drag that involves the exchange of angular momentum between two current-driven magnetic domain walls. The motions of the domain walls are correlated and determined by the strength of the drag. When the drag is below a threshold value, the domain walls move together at a constant intermediate velocity with a steady leakage of angular momentum from the faster to the slower wall. However, we find that when the drag exceeds a threshold value, a different dynamic can take place in which the faster domain wall’s magnetization oscillates synchronously with a precessional motion of the slower domain wall’s magnetization, and angular momentum is continuously transferred between them. Our findings demonstrate a method for delivering spin angular momentum remotely to magnetic entities that otherwise could not be manipulated directly by current, for example, by coupling domain walls or other non-collinear spin textures in metallic and insulating media. Drag effects between interacting particles in nearby layers can impact their motion. Here, this idea is extended to angular momentum in domain walls in a synthetic antiferromagnet and synchronization is observed.

Mechanism of kinetic energy transfer in homogeneous bidisperse gas-solid flow and its implications for segregation

Most gas–solid flows encountered in nature and industrial applications are polydisperse, and the segregation or mixing of particle classes in polydisperse gas–solid flows is a phenomenon of great practical importance. A statistically homogeneous gas–solid flow with a bidisperse distribution (in size or density) of particles is a canonical representation of polydisperse flows. A key feature that distinguishes the bidisperse flow from its monodisperse counterpart is the exchange of momentum and kinetic energy between the particle classes due to collisions, which are important for applications outside the very dilute regime. The average exchange of linear momentum between particle classes due to collisions occurs through the particle–particle drag term. The conservation equations for average momentum corresponding to each particle class can be used to deduce the average slip velocity between the particle size and density classes, which is the signature of particle segregation. In this canonical problem, the steady value of particle mean slip velocity results from a balance between three terms, each in turn involving the body force or the mean fluid pressure gradient, the gas–particle drag, and the particle–particle drag. The particle–particle drag depends on the particle velocity fluctuations in each class [Louge, M. Y. et al., “The role of particle collisions in pneumatic transport,” J. Fluid Mech. 231, 345–359 (1991)], thereby coupling the mean and second–moment equations. For monodisperse gas-solid flows the transfer of kinetic energy from the mean to second-moment equations was explained by Subramaniam and co-workers who proposed the conservation of interphase turbulent kinetic energy transfer principle [Xu, Y. and Subramaniam, S., “Consistent modeling of interphase turbulent kinetic energy transfer in particle-laden turbulent flows,” Phys. Fluids 19(8), 085101 (2007)], and this was subsequently verified by particle–resolved direct numerical simulation [Mehrabadi, M. et al., “Pseudo-turbulent gas-phase velocity fluctuations in homogeneous gas-solid flow: Fixed particle assemblies and freely evolving suspensions,” J. Fluid Mech. 770, 210–246 (2015)]. The principle shows that the power supplied to the mean flow to maintain the mean slip velocity between solid and gas phases partitions into transfer terms that supply power to maintain fluid and particle velocity fluctuations. One question this paper seeks to answer is what the role of particle–particle drag is in this transfer process in bidisperse flows, given that the particle–particle drag does not appear in the mixture mean momentum conservation equation for the solid phase. The conservation equations for mean momentum and kinetic energy in each particle class are coupled through interphase and interclass exchange terms. This coupling between the mean momentum and kinetic energy equations due to interphase and interclass interactions is explained by extending the conservation of interphase turbulent kinetic energy transfer principle originally proposed for monodisperse gas-solid flows to bidisperse suspensions. This explains the role of particle–particle drag in the partitioning of kinetic energy in velocity fluctuations between particle classes and provides insight into segregation and mixing of particle classes in industrial devices.

Magnon-driven domain-wall motion with the Dzyaloshinskii-Moriya interaction.

We study domain-wall (DW) motion induced by spin waves (magnons) in the presence of the Dzyaloshinskii-Moriya interaction (DMI). The DMI exerts a torque on the DW when spin waves pass through the DW, and this torque represents a linear momentum exchange between the spin wave and the DW. Unlike angular momentum exchange between the DW and spin waves, linear momentum exchange leads to a rotation of the DW plane rather than a linear motion. In the presence of an effective easy plane anisotropy, this DMI induced linear momentum transfer mechanism is significantly more efficient than angular momentum transfer in moving the DW.

Helicity-dependent three-dimensional optical trapping of chiral microparticles.

The rule of thumb of tailored optical forces consists in the control of linear momentum exchange between light and matter. This may be done by appropriate selection of the interaction geometry, optical modes or environmental characteristics. Here we reveal that the interplay of the helicity of light and the chirality of matter turns the photon spin angular momentum into an efficient tool for selective trapping of chiral particles. This is demonstrated, both experimentally and theoretically, by exploring the three-dimensional optical trapping of chiral liquid crystal microspheres with circularly polarized Gaussian or Laguerre-Gaussian beams. These results suggest the development of novel optomechanical strategies that rely on the photon helicity towards selective trapping and manipulation of chiral objects by chiral light.

Source mechanism of Vulcanian eruption at Tungurahua Volcano, Ecuador, derived from seismic moment tensor inversions

Source mechanisms of explosive volcanic eruptions are critical for understanding magmatic plumbing systems and magma transport. Tungurahua is a large andesitic stratovolcano where seismoacoustic data has been recorded over several years. In May 2010, an energetic eruption cycle began with a midsize Vulcanian explosion followed by swarms of explosive eruptions. The five-station seismoacoustic network recorded significant seismic and infrasonic signals from the explosions. Source mechanisms of 50 explosion earthquakes associated with Vulcanian explosions during this eruptive period are investigated here. The source centroid locations and geometries of explosive signals in the 10–2 s band were determined by full-waveform moment tensor inversion. The observed waveforms are well explained by a combination of volumetric moment tensor components and a single, vertical, downward force component. The source centroids are positioned about 1.5 km below and about 320 m north of the active crater. Eigenvalue and eigenvector analysis indicates that the source geometries can be described by a subhorizontal, thin ellipsoid representing a sill-like magma accumulation. Resultant source time histories show a repetitive sequence of inflation and deflation from event to event, indicating identical source processes frequently occurred over the period. The inflation/deflation in the deep source region may be the result of crack opening. Volatile bubble growth at depth opens a pathway for gases to escape and triggers shallow explosions at the summit crater. The associated downward single force is interpreted as an exchange of linear momentum between the source and the surrounding region during the escaping gas flow.

Optical Nonreciprocity in Optomechanical Structures

We demonstrate that optomechanical devices can exhibit nonreciprocal behavior when the dominant light-matter interaction takes place via a linear momentum exchange between light and the mechanical structure. As an example, we propose a microscale optomechanical device that can exhibit a nonreciprocal behavior in a microphotonic platform operating at room temperature. We show that, depending on the direction of the incident light, the device switches between a high and low transparency state with more than a 20 dB extinction ratio.

Entanglement of a Laguerre-Gaussian cavity mode with a rotating mirror

It has previously been shown theoretically that the exchange of linear momentum between the light field in an optical cavity and a vibrating end mirror can entangle the electromagnetic field with the vibrational motion of that mirror. In this paper we consider the rotational analog of this situation and show that radiation torque can similarly entangle a Laguerre-Gaussian cavity mode with a rotating end mirror. We examine the mirror-field entanglement as a function of ambient temperature, radiation detuning, and orbital angular momentum carried by the cavity mode.

Shallow-conduit dynamics at Stromboli Volcano, Italy, imaged from waveform inversions

Abstract Modelling of Very-Long-Period (VLP) seismic data recorded during explosive activity at Stromboli in 1997 provides an image of the uppermost 1 km of its volcanic plumbing system. Two distinct dyke-like conduit structures are identified, each representative of explosive eruptions from two different vents located near the northern and southern perimeters of the summit crater. Observed volumetric changes in the dykes are viewed as the result of a piston-like action of the magma associated with the disruption of a gas slug transiting through discontinuities in the dyke apertures. Accompanying these volumetric source components are single vertical forces resulting from an exchange of linear momentum between the source and the Earth. In the dyke system underlying the northern vent, a primary disruption site is observed at an elevation near 440 m where a bifurcation in the conduit occurs. At a depth of 80 m below sea level, a sharp corner in the conduit marks another location where the elastic response of the solid to the action of the upper source induces pressure and momentum changes in the magma. In the conduit underlying the southern vent, the junction of two inclined dykes with a sub-vertical dyke at 520 m elevation is a primary site of gas slug disruption, and another conduit corner 280 m below sea level represents a coupling location between the elastic response of the solid and fluid motion.

Shoaling waves: A discussion

Surface gravity waves are commonly observed to slow down and to stop at a beach without any noticeable reflection taking place. We assume that as a consequence the waves are continuously giving up their linear and angular momenta, which they carry with them, along with energy, as they propagate into gradually decreasing mean depths of water. It takes a force to cause a time rate of decrease in the linear momentum and a torque to produce a time rate of decrease in the angular momentum. Both a force and a torque operate on the shoaling waves, due to the presence of the sloping bottom, to cause the diminution of their linear and angular momenta. By Newton’s third law, action equals reaction, an equal but opposite force and torque are exerted on the bottom. No other mechanisms for transferring linear and angular momenta are included in the model. Since the force on the waves acts over a horizontal distance during shoaling, work is done on the waves and energy flux is not conserved. Bottom friction, wave interaction with a mean flow, scattering from small-scale bottom irregularities and set-up are neglected. Mass flux is conserved, which leads to a shoreward monotonic decrease in amplitude consistent with available swell data. The formula for the time-independent force on the bottom agrees qualitatively with observations in seven different ways: four for swell attenuation and three for sediment transport on beaches. Ardhuin (2006) argues against a mean force on the bottom that is not hydrostatic, mainly by using conservation of energy flux. He also applies the action balance equation to shoaling waves. Action is a difficult concept to grasp for motion in a continuum; it cannot be easily visualized, and it is not really necessary for solving the shoaling wave problem. We prefer angular momentum because it is clearly related to the observed orbital motion of the fluid particles in progressive surface waves. The physical significance of wave action for surface waves has been described recently by showing that in deep water action is equivalent to the magnitude of the wave’s orbital angular momentum (Kenyon and Sheres, 1996). Finally, Ardhuin requires that there be a significant exchange of linear momentum between shoaling waves and an unspecified mean flow, although the magnitude and direction of the exchange are not predicted. No mention is made of what happens to the orbital angular momentum during shoaling. Mass flux conservation is not stated.

Polaron stability under collision with different defects in conjugated polymers

AbstractPolarons are charge carriers generated by doping or photo‐excitation in conjugated polymers. The dynamics of polaron–soliton and polaron–bipolaron collisions in conjugated polymers is numerically studied using the Su–Schrieffer–Heeger model extended to include an external electric field, Coulomb interactions, and impurities interactions. The time‐dependent unrestricted Hartree–Fock approximation is considered. The stability of polarons determines whether the charge transport by these structural defects are modified under these collisions. An energetic analysis determines whether the energy levels associated with solitons, polarons, and bipolarons change with collisions. We show that these structural defects have great stability in collisions. The bipolaron mobility does not change under polaron–bipolaron collisions. In contrast, we find that there is a great exchange of linear momentum in the collisions between solitons and polarons. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

On Porous Soil Materials Saturated with a Compressible Pore‐fluid Mixture

AbstractGeomaterials can be understood as consisting of a porous solid skeleton, with the pores saturated by a fluid. The fluid may be water, air or, a water‐air mixture. The mechanical behaviour of the whole soil body, consisting of the soil skeleton and the pore‐fluid, is governed by the properties of its constituents. Since the exact structure of the pore‐space is not known, a homogenization over a representative elementary volume is required in order to obtain a macroscopic approach. The description of the motion of a multiphase‐mixture of immiscible constituents can be performed by the Theory of Porous Media (TPM). Based on the classical mixture theory, the TPM deals with superimposed continua. Additionally, the information of the structure is included via the concept of volume fractions [1–3].Here, the soil skeleton material is assumed to be incompressible, whereas the pore‐fluid mixture is compressible according to its composition of water and air by variable volume fractions. The pressure‐density‐function of the pore‐fluid is derived within the framework of the TPM. Furthermore, the viscous pore‐fluid allows for a linear momentum exchange (interaction force) between the constituents, which, under special assumptions, leads to the well‐known Darcy law.

Angular momentum photon drag in a mesoscopic ring

It is predicted that an incoming electromagnetic wave possessing a net angular momentum may generate a steady-state current in a mesoscopic metallic (or semiconducting) ring by means of its nonlinear electrodynamic interaction with the conduction electrons of the ring. In a sense this phenomenon is analogous to the classical photon-drag effect which originates in a nonlinear exchange of linear momentum between the photon and electron fields. In the wake of the establishment of the basic theory, a numerical calculation and an analysis of the angular-momentum photon drag in a mesoscopic Au ring are presented. Particular attention is paid to a study of the photon-drag current as a function of the photon energy, and it is shown that the drag current provides a detailed fingerprint of the allowed angular electronic transitions of the ring if the collision frequency of the conduction electrons is sufficiently low.

New Mechanisms for Laser Cooling

When an atom or a molecule interacts with a light beam, the light emitted or absorbed carries valuable information about the atomic or molecular structure. This phenomenon underlies the whole field of spectroscopy. But the interaction of a photon with an atom can be used to manipulate the atom as well as to probe its structure. For example, in an approach called optical pumping, invented by Alfred Kastler, one can use the resonant exchange of angular momentum between atoms and polarized photons to align or orient the spins of atoms or to put them in nonequilibrium situations. In his original 1950 paper Kastler also proposed using optical pumping to cool and to heat the internal degrees of freedom, calling the phenomena the “effet luminofrigorique” and the “effet luminocalorique.” Another famous example of the use of photon-atom interaction to control atoms is laser cooling. This technique relies on resonant exchange of linear momentum between photons and atoms to control their external degrees of freedom and thus to reduce their kinetic energy. Laser cooling was suggested independently by Theodor Hänsch and Arthur Schawlow for neutral atoms and by David Wineland and Hans Dehmelt for trapped ions. In an article written three years ago for PHYSICS TODAY (June 1987, page 34), Wineland and Wayne Itano presented the principle of laser cooling and the potential applications of cold atoms to fields of physics such as ultrahigh resolution spectroscopy, atomic clocks, collisions, surface physics and collective quantum effects. At that time laser cooling had brought temperatures down to a few hundred microkelvin, but unexpected improvements during the last three years have dramatically lowered those temperatures to only a few microkelvin. We now feel we understand the new physical mechanisms responsible for these very low temperatures.