Momentum Transfer Research Articles (Page 1)

Insulating antiferromagnets are anticipated as the main protagonists of ultrafast spintronics, with their intrinsic terahertz dynamics and their ability to transport spin information over long distances. However, ultrafast transfer of spin angular momentum to an antiferromagnetic insulator remains to be demonstrated. Here, studying the picosecond and subpicosecond dynamics of ferromagnetic metal/antiferromagnetic insulator bilayers, we evidence the generation of coherent terahertz excitations in the antiferromagnet combined with a modulation of the demagnetization behavior in the ferromagnet. We thus demonstrate that magnetic information can indeed be propagated into antiferromagnetic spin waves at picosecond timescales, thereby opening an avenue toward ultrafast manipulation of magnetic information.

Graphene and graphene oxide (GO) sheets have different surface characters with respect to their hydration properties, hence can influence the kinetics of evaporation of thin water layers on them in a different manner. In the current study, we have explored the evaporation dynamics of water layers from graphene and graphene oxide surfaces by means of molecular dynamics simulations and also performed the associated free energy calculations using an enhanced sampling technique. In particular, we have focused on the effects of surface hydrophobicity/hydrophilicity and the thickness of water layers on the evaporation behavior, and our findings are further corroborated by vibrational sum frequency generation spectroscopic calculations. The current study shows that graphene promotes higher evaporation rates than GO, especially for thinner water layers. This is likely due to the strengthening of hydrogen bonds as evident from the relatively increased intensity and red-shifted O-H peak at ∼3350 cm-1 in the VSFG spectrum of the water layer on GO compared to that on graphene surfaces. The potential of mean force (PMF) profiles reveal a lower free energy barrier for water evaporation for graphene than for GO, suggesting that the hydrophobic nature of graphene enhances evaporation of water. Kinetic energy analysis shows significant momentum transfer between interacting molecules, while hydrogen bond analysis further emphasizes the role of molecular interactions in evaporation dynamics. These results provide key insights into the evaporation kinetics of water layers on hydrophobic and hydrophilic surfaces.

We extend our recent analytic study of the strong coupling α eff in the nonperturbative and near-perturbative regimes [] by imposing rigorous renormalization-group constraints from asymptotically free gauge theories at Q 2 → ∞ . The asymptotic boundary conditions modify the scaling properties of α eff at large values of the momentum transfer Q 2 and lead to a scale-dependent confinement strength κ ( Q 2 ) . This requires that both κ ( Q 2 ) and α eff ( Q 2 , κ ( Q 2 ) ) remain holomorphic in the complex Q 2 plane, except at the physical cuts associated with the heavy-quark thresholds and the singularity flow trajectory studied in our previous Letter. For color SU(3), a precise connection is found between the scaling exponent of κ ( Q 2 ) in the ultraviolet, the value of the infrared fixed point of the strong coupling, and the number of flavors in agreement with observations. The nonperturbative analytic model gives an accurate description of the strong coupling across all scales, up to the highest available data.

We study for the first time the transverse momentum transfer distributions d σ / d t in coherent production of charmonia in nuclear ultraperipheral and electron-ion collisions within the QCD color dipole approach based on a rigorous Green function formalism. This allows us to treat properly the color transparency effects, as well as the higher and leading-twist shadowing corrections associated with the | Q Q ¯ ⟩ and | Q Q ¯ n G ⟩ Fock components of the photon. While the multigluon photon fluctuations represent the dominant source of nuclear shadowing at kinematic regions related to the recent LHC and its future upgrade to LHeC, the upcoming electron-ion collider at RHIC will additionally require the proper incorporation of reduced quark shadowing. The latter effect leads to a significant decrease in the differential cross sections d σ / d t compared to standard calculations based on the eikonal form for the dipole-nucleus amplitude. The leading-twist shadowing corrections, corresponding to a nonlinear QCD evolution of a partial dipole-nucleus amplitude, reduce substantially charmonium t -distributions in the LHeC energy range. We predict a nonmonotonic energy dependence of d σ / d t suggesting so possible gluon saturation effects with increased onset at larger t -values. In addition to shadowing corrections, we study how the color transparency effects affect the shape of t -dependent nuclear modification factor.

This study investigates droplet impact on elastic plates using a two-phase lattice Boltzmann method in both two-dimensional (2-D) and three-dimensional (3-D) configurations, with a focus on rebound dynamics and contact time. The 2-D simulations reveal three distinct rebound modes – conventional bounce, early bounce and rim rising – driven by fluid–structure interaction. Among them, the early bounce mode uniquely achieves a significant reduction in contact time, occurring only at moderate plate oscillation frequency. Momentum analysis shows a non-monotonic relationship between vertical momentum transfer and rebound efficiency: increased momentum does not necessarily promote rebound if it concentrates in a central jet, which contributes minimally to lift-off. This introduces a novel rebound mechanism governed by momentum distribution morphology rather than total magnitude. A theoretical model treating the droplet–plate system as coupled oscillators is developed to predict contact time in the early bounce regime, showing good agreement with numerical results. The mechanism and model are further validated through fully 3-D simulations, confirming the robustness of the findings.

Abstract The Beaufort Gyre (BG) is a critical reservoir of Arctic freshwater, its stability governed by interactions between atmospheric forcing, sea ice motion, and ocean surface stress. Over the past 2 decades, surface stress and stress curl patterns have undergone marked changes shaped by shifting atmospheric modes and thinning sea ice. Although both atmospheric forcing and ice stress contribute to Ekman transport, neither alone fully captures the changing momentum transfer dynamics. Although atmospheric winds remained relatively stable, enhanced momentum transfer across the ice‐ocean interface, driven by faster ice drift and reduced ice thickness, led to increased surface stress and amplified variability. This strengthened the influence of the Arctic Oscillation and elevated the role of the Arctic Dipole, which now contributes more predominantly to Beaufort Sea downwelling. These shifts produced a spatially asymmetric stress curl pattern with stronger anticyclonic forcing in the southeast and sustained cyclonic anomalies in the northwest. The resulting increase in Ekman downwelling is linked to deepening winter mixed layers potentially weakening the gyre's ability to store freshwater. As the Arctic transitions toward thinner more mobile ice, the evolving balance between atmospheric variability, ice‐ocean coupling, and Ekman dynamics will be critical for future understanding of BG circulation and freshwater retention.

Multi-method modeling of nonlinear heat and momentum transfer in Riga plate flow: bvp5c and MLP-ANN approaches

Abstract We examine an instability mechanism for generating very low frequency (VLF) vortical motions in the surf zone that oscillate with periods of O(10) min and associated alongshore wavelengths of O(100) m. The instability arises due to the mutual, dynamic interaction of wave and current. The mathematical formulation of the instability analysis follows the previous studies for generating rip current circulations. The unstable modes of the instability are examined for two classes of beach profiles, each of which is generalized from the realistic bathymetry of its kind: a fringing reef beach and a sandy beach with a well-defined alongshore bar. In each case, we find the unstable modes with temporal periods and alongshore wavelengths in the ranges of scales of the observed VLF surf zone eddies. Since we consider the stability of a basic state that is steady and alongshore uniform, there are no prescribed temporal or spatial scales in the system. Therefore, the wavenumber–frequency relationship of the unstable VLF modes is intrinsic to the coupling dynamics of wave and current. Since the central physics is the transfer of wave momentum to the current as the wave breaks and wave refraction by the current, which similarly exists regardless of beach and wave conditions, we expect that the intrinsic dynamics to generate VLF surf zone eddies applies more generally. We do not imply that the instability mechanism is necessary for surf zone VLF eddies but call attention to the intrinsic physics that manifest in the mutual, dynamic wave–current coupling and can interact and/or resonate with other excitation forces to influence VLF generation.

Abstract The Arctic Ocean double estuary is a “three‐legged” overturning system in which inflowing waters are converted into both lighter and denser waters before being exported equatorwards. As the northern terminus of the Atlantic Meridional Overturning Circulation (MOC), it thus both affects, and is affected by, the Atlantic MOC. Here we quantify the magnitudes of the two overturning cells in density space, and then decompose the water mass transformation rates into net pan‐Arctic contributions from surface forcing and diapycnal mixing. We use a high‐resolution, quasi‐synoptic ice and ocean hydrographic data set spanning the four main Arctic Ocean gateways—Fram, Davis and Bering Straits, and the Barents Sea Opening. Two surface flux reanalyses and a hydrographic climatology are used to generate estimates of surface water mass transformation rates by density class. A box model then determines the profiles of turbulent mixing transformation rates, and associated turbulent diffusivities. We show that turbulent mixing and surface forcing drive transformations of similar magnitudes, while mixing dominates in the upper cell and surface fluxes in the lower cell. Consideration of uncertainties and timescales leads to the tentative suggestion that our results might be representative of recent decades. We discuss the possible significance of tides and sea ice brine rejection as energy sources driving turbulent mixing. Finally, we speculate as to whether water mass transformation rates may change in future as ocean heat transport into the Arctic increases. As sea ice declines and the efficiency of atmosphere‐to‐ocean momentum transfer increases, the Arctic Ocean is expected to “spin up,” causing more intense turbulent mixing, with uncertain consequences.

Introduction. Emergency situations may lead to explosions accompanied by the release of heat and pressure waves that destroy structures in their path and cause fires.Problem Statement. Modeling heat transfer in solids of complex geometry remains a critical task, as predicting the distribution of temperature fields is essential in the design of protective structures. Therefore, the development of a new mathematical model that adequately describes transient thermal processes in solids, as well as thecreation of an efficient numerical method and its implementation as a modern information system for engineering analysis and prediction, is highly relevant.Purpose. To perform mathematical modeling of unsteady temperature fields in solids within regions of significant temperature gradients arising from accidental explosions of gas mixtures.Materials and Methods. Numerical modeling of transient heat transfer processes in multiply connected solids of complex geometry, surrounded by a thermally conductive gaseous medium, has been carried out using a unifiedfinite-diff erence algorithm. Results. A coupled direct problem involving the flow of a continuous gaseous medium, heat transfer between the gas and solid, and heat conduction within the solid has been considered. The mathematical model accounts for the spatial transfer of mass, momentum, and energy, as well as the complex geometry of streamlined solids. The model has been verified through comparison with analytical solutions to benchmark problems involving an infi nite steel plate. Three-dimensional temperature fields in spatially complex solids have been obtained for individual geometric primitives and their combinations. Heat transfer simulationshave been performed for a turbine blade with a continuous cross-section and internal cooling channels.Conclusions. The newly developed mathematical model has demonstrated suitability for engineering applications in thermal analysis and predictive modeling. The resulting three-dimensional temperature fields can be used to assess the thermal stress state and strength characteristics of structural elements located within the impact zone of high excess pressure caused by accidental explosions of gas mixtures at industrial sites.

We investigate the nucleon’s chiral-odd generalized parton distribution functions (GPDs) in the large- N c limit of quantum chromodynamics (QCD). Extending previous work [] on the leading-order contribution in the 1 / N c expansion, we focus on the next-to-leading-order contributions and provide a complete set of flavor-singlet and flavor-non-singlet chiral-odd GPDs. This study includes the derivation of the spin-flavor structure of the baryon matrix element of the chiral-odd operator, the proof of the polynomiality property and associated sum rules, and numerical estimates based on the gradient expansion. The spin-flavor structure of the nucleon matrix element is interpreted through a multipole expansion in the transverse momentum transfer, leading to the definition of multipole mean-field GPDs. Using these GPDs as the basis of our analysis, we take their m th moments and demonstrate the polynomiality property within the mean-field framework, making use of discrete symmetries in the proof. In particular, the first moments ( m = 1 ) of the GPDs are related to the nucleon tensor form factors, as generally required. By performing a gradient expansion, we compute the chiral-odd GPDs and present numerical estimates. Our results show reasonable agreement with lattice QCD predictions.

The majority of massive stars are born with a close binary companion. How this affects their evolution and fate is still largely uncertain, especially at low metallicity. We derive synthetic populations of massive post-interaction binary products and compare them with corresponding observed populations in the Small Magellanic Cloud (SMC). We analyse 53298 detailed binary evolutionary models computed with MESA. Our models include the physics of rotation, mass and angular momentum transfer, magnetic internal angular momentum transport, and tidal spin-orbit coupling. They cover initial primary masses of $5--100 initial mass ratios of 0.3--0.95, and all initial periods for which interaction is expected, 1--3162 d. They are evolved through the first mass transfer and the donor star death, and a a possible ensuing Be X-ray binary phase, and they end when the mass gainer leaves the main sequence. In our fiducial synthetic population, 8% of the OB stars in the SMC are post-mass-transfer systems, and 7% are merger products. In many of our models, the mass gainers are spun up and expected to form Oe/Be stars. While our model underpredicts the number of Be X-ray binaries in the SMC, it reproduces the main features of their orbital period distribution and the observed number of SMC binary WR stars. We further expect ∼50 OB+BH binaries below and ∼170 above the 20,d orbital period. The long-period OB+BH binaries might produce merging double black holes. However, their progenitors, the predicted long-period WR+OB binaries, are not observed. While the comparison with the observed SMC stars supports many physics assumptions in our high-mass binary models, a better match for the large number of observed OBe stars and Be X-ray binaries likely requires a lower merger rate and/or a higher mass transfer efficiency during the first mass transfer. The fate of the initially wide O, star binaries remains particularly uncertain.

Abstract We study the local balance of momentum for weak solutions of incompressible Euler equations obtained from the zero-viscosity limit in the presence of solid boundaries, taking as an example external flow around a finite, smooth body. We show that both viscous skin friction and wall pressure exist in the inviscid limit as distributions on the body surface. We define a nonlinear spatial flux of momentum toward the wall for the Euler solution, and show that wall friction and pressure are obtained from this momentum flux in the limit of vanishing distance to the wall, for the wall-parallel and wall-normal components, respectively. We show furthermore that the skin friction describing anomalous momentum transfer to the wall will vanish if velocity and pressure are bounded in a neighbourhood of the wall and if also the essential supremum of wall-normal velocity within a small distance of the wall vanishes with this distance (a form of continuity at the wall of the normal velocity). In the latter case, all of the limiting drag arises from pressure forces acting on the body and the pressure at the body surface can be obtained as the limit approaching the wall of the interior pressure for the Euler solution. As one application of this result, we show that Lighthill’s theory of vorticity generation at the wall is valid for the Euler solutions obtained in the inviscid limit. This work furthermore provides the foundation for a rigorous study of the inviscid limit of the Josephson–Anderson relation and for proof of conditional weak-strong uniqueness of the d’Alembert potential Euler solution.

This work constructs a one-dimensional optimal system of Lie subalgebras for analyzing heat and mass transfer in a laminar liquid film over a stretching surface with a variable magnetic field. For each element of the optimal system, corresponding system invariants lead to similarity transformations. These transformations are used to reduce the governing equations for momentum, heat, and mass transfer, mapping them to systems of ordinary differential equations (ODEs). By applying the derived Lie similarity transformations, we identify multiple new classes of nonlinear ODEs associated with the flow being studied. These classes are categorized into two groups based on their admitted Lie point symmetries: linearly space-dependent and linearly time-dependent symmetries. The resulting systems of ODEs are solved using the Homotopy Analysis Method (HAM), which yields all invariant solutions of the governing equations. Through these solutions, we present profiles for velocity, temperature, and concentration, highlighting how the Lie symmetry approach can effectively control flow, heat transfer, and concentration within the system.

Abstract The weak mixing angle $$\theta _W$$ θ W is a fundamental parameter in the electroweak theory with a value running according to the energy scale, and its precision measurement in the low-energy regime is still ongoing. We propose a method to measure the low-energy $$\sin {^2\theta _W}$$ sin 2 θ W by taking advantage of Argo, a future ton-scale liquid argon dark matter detector, and the neutrino flux from a nearby core-collapse supernova (CCSN). We evaluate the expected precision of this measurement through the coherent elastic neutrino-nucleus scattering (CE $$\nu $$ ν NS) channel. We show that Argo is potentially capable of achieving a few percent determination of $$\sin {^2\theta _W}$$ sin 2 θ W , at the momentum transfer of $$q \sim 20{\,\text {MeV}}$$ q ∼ 20 MeV , in the observation of a CCSN within $$\sim 3{\,\text {kpc}}$$ ∼ 3 kpc from the Earth. Such a measurement is valuable for both the precision test of the electroweak theory and searching for new physics beyond the standard model in the neutrino sector.

We present the first results of a comprehensive microscopic approach to describe nucleus-nucleus elastic collisions by means of an optical potential derived at first order in multiple-scattering theory and computed by folding the projectile and target nuclear densities with the nucleon-nucleon t matrix, which describes the interaction between each nucleon of the projectile and each nucleon of the target. Chiral interactions are consistently used in the calculation of the t matrix and of the nonlocal nuclear densities, which are computed within the abinitio no-core shell model. Cross sections calculated for α collisions on ^{12}C and ^{16}O at projectile energies in the range 100-300MeV are presented and compared with available data. For momentum transfer q up to about 1.0 fm^{-1} our results are in good agreement with the experimental data, whereas for higher momenta a reduction of the imaginary contributions is needed.

We investigate the semileptonic decays of baryons containing double charm or double bottom quarks, focusing on their transitions to single heavy baryons through three-point QCD sum rule framework. In our calculations, we take into account nonperturbative operators with mass dimensions up to five. We calculate the form factors associated with these decays, emphasizing the vector and axial-vector transition currents in the corresponding amplitude. By applying fitting functions for the form factors based on the squared momentum transfer, we derive predictions for decay widths and branching ratios in their possible lepton channels. These findings offer valuable insights for experimentalists exploring semileptonic decays of doubly charmed or bottom baryons. Perhaps they can be validated in upcoming experiments like LHCb. These investigations contribute to a deeper understanding of the decay mechanisms in these baryonic channels.

We have studied the viscous properties as well as the Bjorken expansion of a rotating QGP medium. In the noncentral events of heavy-ion collisions, the produced medium can carry a finite angular momentum with a finite range of angular velocity. This rotation can significantly affect various properties, including viscous properties and the expansion of the QGP medium. Using a novel relaxation time approximation for the collision integral in the relativistic Boltzmann transport equation at finite angular velocity, we have calculated the shear and bulk viscosities and compared them with their counterparts in the standard relaxation time approximation within the kinetic theory approach. Our results show that the angular velocity increases both shear and bulk viscosities, suggesting an enhanced momentum transfer within the medium and greater fluctuations in local pressure. This rotational effect on viscosities is more evident at lower temperatures than at higher temperatures. Our analysis also shows that, compared to the standard relaxation time approximation, the shear viscosity is lower while the bulk viscosity is higher in the novel relaxation time approximation for all temperatures. Additionally, some observables related to the flow characteristic, fluid behavior, and conformal symmetry of the medium are markedly impacted due to rotation. We have also studied the hydrodynamic evolution of matter within the Bjorken boost-invariant scenario and have found that the energy density evolves faster in the presence of finite rotation than in the nonrotating case. Consequently, rapid rotation accelerates the cooling process of the QGP medium.

Many properties of polymers are well-described by a few simple concepts, such as a coarse-grained chain of N beads. Here, we study the motion of the two end-blocks. To observe end-block relaxation, we chose a molecular weight high enough to avoid effects by chemical end-groups but still avoid entanglement effects. We take advantage of molecular interactions, molecular weight, isotopic sensitivity, and length-scale information encoded in the momentum transfer of neutron spin echo spectroscopy to arrive at a more holistic picture of the contributions of the end-blocks to the polymer dynamics of unentangled chains. Our model polymer melt, poly(dimethyl siloxane), PDMS, has weak intermolecular interactions, leading to a lower sub-diffusion contribution. The experimental results are in excellent agreement with the theory and indicate the appearance of two regions that show terminal bead relaxation in addition to the well-known sub-diffusion of the center of mass, the center of mass diffusion, and (Rouse) segmental relaxation. The quantitative analysis shows terminal beads with a length equivalent to 1-2 Kuhn segments that relax twice as fast as beads in the middle of the polymer chain. The time-dependence of the mean square displacement is in quantitative agreement with simulations with terminal bead segmental relaxation (t0.65) and sub-Rouse relaxation of end-beads (t0.57) in the intermediate range between sub-diffusion (t0.8) and center of mass diffusion (t1). For the specific example of a polymer with 143 segments, 11 end-segments define one end-block with faster dynamics, whereas the remaining 121 belong to the middle block. Segments in the end block exhibit faster dynamics compared to those in the middle block. Both the end block and the middle block consist of multiple segments. Therefore, analyzing the data requires considering both concepts to effectively describe the dynamics of polymer chains.

The relaxation dynamics of liquid and glass can be studied by inelastic x-ray or neutron scattering through the intermediate scattering function F(Q, t), where Q is the momentum transfer of scattering. Because of the time-consuming nature of these measurements, F(Q, t) is usually measured only at the first peak of the structure function S(Q), and its principal decay time is referred to as the α-relaxation time τα. τα is generally considered to describe the relaxation time of the bulk, which is related to viscosity and is controlled by the atomic cage around an atom. Here, through simulations on metallic liquids, we show that the α-relaxation time determined by scattering experiments does not purely reflect viscosity but is influenced by changes in spatial cooperativity. We also demonstrate that atomic caging is not exerted by the nearest neighbors but involves more cooperative atomic dynamics of the atomic medium-range order.