Fluid Dynamics · Cross-listed from math.AP
Mixing and enhanced dissipation in a time-translating shear flow
Johannes Benthaus, Giuseppe Maria Coclite and Camilla Nobili
Motivated in part by the work of Vanneste and Byatt-Smith, we study mixing and enhanced dissipation for the advection-
diffusion equation with velocity field $\mathbf{u}(x,y,t)=(\sin(y-ct),0)$, a shear flow whose profile translates rigidly with speed $c$. This is a prototypical example of a flow whose critical points move in time. We quantify how the decay properties depend on the relation between translation speed $c$ and diffusivity $\nu$. We first analyse the inviscid transport problem and establish time-averaged $H^{-1}$ mixing estimates for $t\lesssim c^{-1}$, yielding decay rates faster than stationary estimates. Building on these estimates, we prove enhanced dissipation for moderate translation speeds $c=c_0\nu^\ell$ with $\ell\in(1/3,3/4)$. In this regime we obtain decay at rate $\nu^{(1+2\ell)/5}$, which interpolates continuously between the sharp rates $\nu^{1/2}$ for stationary shear flows with simple critical points and $\nu^{1/3}$ for monotone flows. This quantifies how increasing translation speed progressively weakens the influence of the critical points. Comparing the inviscid mixing and enhanced dissipation timescales heuristically explains the lower endpoint $\ell=1/3$. For $c\gg 1$, we show that solutions remain close to those of the heat equation on fixed time intervals, such that the rapid translation averages out advection and weakens mixing. The mixing estimate relies on a refined stationary phase analysis exploiting cancellations generated by the motion of the critical points. The enhanced dissipation result requires an adaptation of the hypocoercivity framework for stationary shear flows to the non-autonomous setting. The translating flow prevents the commutator hierarchy from closing in the standard way, which we overcome by constructing an extended energy functional. The large-$c$ analysis exploits the averaging effect of rapid translations in this regime.
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Optics
Incoherent Fourier transform spectroscopy with room-temperature coverage from NIR to THz
Jakub Mnich, Grzegorz Gomółka, Marco Schossig et al. (+2 more)
Despite the broadband nature of thermal light sources, optical spectroscopy over multiple spectral bands simultaneously remains challenging. Here, we demonstrate a practical
Fourier transform infrared spectrometer (FTIR) that achieves room-temperature spectral coverage from 1 to 50 $\mu$m (300–6 THz) in seconds using a single set of optics, with the long wave cutoff extendable to 90 $\mu$m (3.3 THz) and the short wave to the ultraviolet (0.39 $\mu$m). The interferometer employs a diamond plate beam splitter and windowless lithium tantalate (LTO) detector to probe the spectrum of combined incoherent sources operating at different temperatures. Applications of the instrument in modern chemometry, material science, and medicine are envisioned.
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All authors Jakub Mnich, Grzegorz Gomółka, Marco Schossig, Jarosław Sotor, Łukasz A. Sterczewski
Plasma Physics
Long lasting plasma density structures utilizing tailored density profiles
Mufei Luo, Caterina Riconda, Anna Grassi et al. (+4 more)
Using fully kinetic Particle In
Cell simulations, we investigate the stability and performance of autoresonant
plasma beat wave excitation in
plasmas with tailored density profiles. We show that a prescribed spatial variation of the background density sustains continuous phase locking between the driving
laser beat and the excited
plasma mode, thereby enabling precise control of the
plasma wave packet shape and group velocity and providing an alternative to frequency chirping of the drive
lasers. The density gradient scale is found to govern the nonlinear autoresonant growth, and the attainable saturation amplitude can exceed the classical Rosenbluth Liu prediction and, for appropriate
laser intensities, approach the nonrelativistic wave breaking limit. We show that a four
laser configuration in a steep
parabolic density profile can generate a specially confined two phase quasiperiodic
plasma lattice. The generation of such structures may lead to novel applications in
plasma photonics.
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All authors Mufei Luo, Caterina Riconda, Anna Grassi, Ning Wang, Jonathan Wurtele, Istvan Pusztai, Tünde Fülöp
Physics and Society
Distance Backbones Optimize Spreading Dynamics and Centrality Ranks in the Sparsification of Complex Networks
Bernardo Pereira, Felipe Xavier Costa and Luís M. Rocha
Detailed network models of social, biological and other complex systems are often dense, which increases their computational complexity in simulations and analysis. To address this challenge, graph sparsification is used to remove edges while preserving desired network properties. Distance backbones of weighted graphs, which remove edges that break a generalized triangle inequality for any given path-length measure, preserve all shortest paths of weighted graphs. They have been shown to typically sparsify graphs more, as well as preserve community structure and spreading dynamics better than alternative state-of-the-art methods. Here, We show that they significantly best preserve node centrality ranks, as well as local and global dynamics in spreading phenomena. This is done by introducing the distance backbone synthesis (DBS) to progressively sparsify weighted graphs according to a general family of nested distance backbones, whereby each edge is associated with the smallest distance backbone in which it appears. DBS provides a principled and natural method to sweep all degrees of sparsification possible while preserving connectivity, allowing us to precisely study (directed and undirected) weighted graph sparsification under multi-objective criteria. It provides an algebraically-principled explanation of edge importance by revealing the precise topological space associated with each edge. The theory is demonstrated with a battery of social contact networks obtained from real-world social activity in different scenarios. Our study also shows that the optimal preservation of node centrality and spreading dynamics happens for the distance backbone obeying the generalized triangle inequality for the path-length measure $g(x, y) = (\sqrt[3]{x}+\sqrt[3]{y})^3$, which removes more than half of the edges from the empirical networks studied.
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Medical Physics
A mathematical model of tumor growth using fractional derivatives
Karen Escutia, Carlos Islas and Pablo Padilla
In this work, we investigate a fractional-order tumor growth model aimed at capturing
memory effects and nonlocal temporal dynamics inherent to tumor
evolution. The model is formulated using Caputo fractional derivatives and incorporates key biological mechanisms related to tumor growth, vascular interaction, and
cell clearance. To numerically solve the resulting fractional differential equations, a second-order fractional Runge-Kutta scheme is derived based on a truncated fractional Taylor expansion, providing an accurate and stable computational framework. The proposed model is calibrated using experimental tumor volume data from five patients, and its performance is evaluated through the Root Mean Square Deviation (RMSD) between numerical simulations and experimental observations. The results show that, for all patients considered, the fractional-order model significantly improves the agreement with experimental data compared to the classical integer-order formulation. An optimal fractional order {\alpha} < 1 is identified in each case, highlighting the relevance of
memory effects in tumor growth dynamics and their patient-specific nature. Further insight is obtained through phase-space and projection analyses, which reveal substantial
geometric differences between integer-order and fractional-order dynamics. Although the present study is based on a limited number of patient
datasets, the results demonstrate the potential of fractional-order modeling as a flexible and powerful framework for describing individualized tumor growth behavior. The proposed approach provides a solid basis for future extensions involving larger
datasets, uncertainty quantification, and the incorporation of treatment effects and control strategies.
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Medical Physics
Retrieval of multiple fibre orientations using X-ray dark-field signal modelling
Lorenzo Massimi, Michela Fratini, Shashidhara Marathe et al. (+4 more)
Dark-field imaging is widely used to infer fibre orientation from signal modulation as a function of sample orientation. However, current X-ray dark-field
retrieval methods are restricted to single orientations and require
tomography to resolve overlapping structures. This approach is time-consuming and not suitable for thin materials, which are common in materials science. Here we present a dark-field model capable of retrieving multiple fibre orientations within a single pixel. The model, based on a geometrical description of fibre scattering, was validated through Monte Carlo simulations and experiments using beam-tracking setups with 1D and 2D masks. Results demonstrate reliable orientation
retrieval for up to two fibres per pixel, with the 2D mask providing multi-directional sensitivity in a single acquisition and enabling faster and simplified data collection.
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All authors Lorenzo Massimi, Michela Fratini, Shashidhara Marathe, Christoph Rau, Giuseppe Gigli, Alessandro Olivo, Alessia Cedola
General Physics
Quantum geometry from commutators: a Heisenberg-picture framework and a toy application to early structure
Vahid Kamali
We develop a Heisenberg-picture kinematical framework in which (i) time is treated as a
quantum observable, admitting both a relational POVM construction for semibounded spectra and a fully self-adjoint realization on an enlarged (conjugate-energy)
Hilbert space enabled by a gravitational conjugation
symmetry $\mathcal{C}_g$, and (ii) the generators of spacetime translations need not commute in curved backgrounds. The central postulate, $[\,\hat{x}_\mu,\hat{P}_\nu\,]=\mathrm{i}\hbar\,\hat g_{\mu\nu}(\hat{x})$, makes the spacetime metric a metric operator defined by the symmetrized commutator. Jacobi identities close the
algebra and imply an operator form of metric compatibility; in a worked FRW example we obtain $[\,\hat{P}_0,\hat{P}_i\,]=2\mathrm{i}\hbar\,N^2(t)\,H(t)\,\hat{P}_i$, which reduces to $2\mathrm{i}\hbar\,H\,\hat{P}_i$ in
cosmic-time gauge $N=1$, exhibiting Hubble–controlled non-commuting “translations.” A key structural ingredient is the
symmetry $\mathcal{C}_g$: an antiunitary map that flips all translation generators, $\hat P_\mu\!\to\!-\Theta \hat P_\mu \Theta^{-1}$, while covariantly transforming the metric and Lorentz sectors, leaving the canonical commutators and the $[P,P]$
algebra invariant. We discuss uncertainty relations and show how metric-operator fluctuations can rescale
primordial amplitudes; an explicitly labeled toy propagation of such a rescaling to high-$z$
halo abundances is given in Appendix~$D$.
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Classical Physics · Cross-listed from cond-mat.soft
Air Drag Controls the Finite-Time Singularity of Euler's Disk
Benjamin G. Thorne, Ahmad Zareei, Kausalya Mahadevan et al. (+2 more)
The motion of a disk spinning to rest after being tipped on its side is a classic example of a finite-time singularity, yet the dominant dissipation mechanism governing this process remains debated. Using stereoscopic high-speed imaging, we study the dynamics of disks with varying mass and radius on different surfaces. We show that the late-time motion near the singularity is governed by viscous air-drag arising from shear in the boundary layer beneath the disk, as evidenced by the mass dependence of the dynamics, measurements in a partial
vacuum, and a
geometric control using a steel ring. At earlier times, dissipation is dominated by rolling friction, which on glass exhibits an unexpected sublinear scaling with disk mass, suggesting an adhesion-based rolling resistance. These results clarify the dissipation mechanisms underlying the singularity of Euler's disk and have broader implications for rolling-contact systems operating under low loads on smooth surfaces.
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All authors Benjamin G. Thorne, Ahmad Zareei, Kausalya Mahadevan, Shmuel M. Rubinstein, Ariel Amir
Chemical Physics · Cross-listed from cs.LG
Excited Pfaffians: Generalized Neural Wave Functions Across Structure and State
Nicholas Gao, Till Grutschus, Frank Noé et al. (+1 more)
Neural-network wave functions in Variational Monte Carlo (VMC) have achieved great success in accurately representing both ground and excited states. However, achieving sufficient numerical accuracy in state overlaps requires increasing the number of Monte Carlo samples, and consequently the computational cost, with the number of states. We present a nearly constant sample-size approach, Multi-State Importance Sampling (MSIS), that leverages samples from all states to estimate pairwise overlap. To efficiently evaluate all states for all samples, we introduce Excited Pfaffians. Inspired by Hartree-Fock, this architecture represents many states within a single neural network. Excited Pfaffians also serve as generalized wave functions, allowing a single model to represent multi-state potential energy surfaces. On the carbon dimer, we match the $O(N_s^4)$-scaling natural excited states while training $>200\times$ faster and modeling 50% more states. Our favorable scaling enables us to be the first to use neural networks to find all distinct energy levels of the beryllium atom. Finally, we demonstrate that a single wave function can represent excited states across various molecules.
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All authors Nicholas Gao, Till Grutschus, Frank Noé, Stephan Günnemann
Data Analysis and Statistics · Cross-listed from cond-mat.soft
Directed Polymer Transfer Matrices as a Unified Generator of Distinct One-Point Fluctuation Laws
Sen Mu, Abbas Ali Saberi, Roderich Moessner et al. (+1 more)
We numerically revisit the transfer-
matrix formulation of directed
polymers in random media and show that a common finite-dimensional framework organizes the canonical one-point fluctuation laws in $(1+1)$ dimensions. For a fixed realization of the bulk disorder, full-space partition functions are obtained from the same time-ordered product $W(t)$ through endpoint contractions or a Brownian-weighted initial vector, while the half-space construction modifies only the transfer rule at the absorbing boundary. These choices yield distributions consistent with the standard KPZ subclasses: Tracy–Widom GUE for point-to-point geometry, Tracy–Widom GOE for point-to-line geometry, Tracy–Widom GSE for half-space point-to-point geometry, and Baik–Rains for the stationary line-to-point construction. In all four cases, the free-energy fluctuations grow as $t^{1/3}$, and the low-order cumulants approach the corresponding universal
benchmarks. The
matrix-product formulation also provides access to intrinsic spectral observables. For the leading
eigenvalue $\lambda_1(t)$, the fluctuations of $\ln\lambda_1(t)$ exhibit an intermediate $t^{1/3}$ regime, while the standardized distribution remains distinct from the canonical
benchmark laws over the studied time range.
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All authors Sen Mu, Abbas Ali Saberi, Roderich Moessner, Mehran Kardar
Chemical Physics
Explicit, Machine-Learned Two-Body Potentials for Molecular Simulations
Kham Lek Chaton, Eric D. Boittier, Mike Devereux et al. (+1 more)
A new pairwise hybrid machine-learning/molecular mechanics (ML/MM) potential is introduced that is conceived for application to large, heterogeneous condensed-phase systems. The PhysNet ML method describes monomers and short-range dimer interactions, while a classical MM force field describes pairwise interactions beyond a defined switching distance. Models are fitted to MP2 dimer and pairwise cluster energies, and the quality of each model is assessed at different switching distances and using MM approaches with and without detailed distributed charge electrostatics. The applicability of the approach to molecular dynamics simulations is demonstrated for a basic implementation applied to a small model system. Dichloromethane and acetone are used as test systems to demonstrate the accuracy of the approach in describing pairwise reference data, and also to highlight the limitations of the pairwise approach for systems that exhibit significant many-body effects in condensed phase, paving the way for the addition of a general many-body correction in future work.
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All authors Kham Lek Chaton, Eric D. Boittier, Mike Devereux, Markus Meuwly
Plasma Physics
Reduced One-Fluid GENERIC Closure from Relativistic Moment Kinetics
Madison J. Newell and Salman A. Nejad
In this work we derive a reduced one-fluid
plasma model from the relativistic Vlasov–Boltzmann–Maxwell system using a moment hierarchy reduction combined with strong-guide-field anisotropic ordering. The unresolved higher-moment sector of the hierarchy is projected onto its dominant slow thermodynamic mode, producing a scalar regulator variable that represents a coarse-grained combination of charge imbalance, pressure anisotropy, and irreversible kinetic production channels. The resulting reduced state vector admits a GENERIC (General Equation for Non-
Equilibrium Reversible–Irreversible Coupling) representation in which the reversible sector reproduces reduced electromagnetic field-line dynamics while the irreversible sector governs slow thermodynamic relaxation. Linearization yields a pair of electromagnetic eigenmodes together with an additional real thermodynamic
eigenvalue. The fast modes recover the standard gyrotropic cold-
plasma response, including familiar limits such as whistler dispersion, while the slow mode drives gradual drift of the effective electromagnetic spectrum and provides a reduced mechanism for variability in relativistic magnetized
plasmas with slowly evolving macroscopic
equilibrium. The previous 3-field model captured only the thermodynamic slow-mode sector, whereas the fully closed extended GENERIC model also contains the explicit reversible nonneutral charge degree of freedom, whose frozen-thermodynamic limit recovers the nonneutral whistler-Alfven equations as a strict subset. The model is formulated within the GENERIC framework, ensuring consistency with first-principles nonequilibrium thermodynamics.
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Geophysics
Multi-instrument constraints on a hemispherically asymmetric positive ionospheric storm in the 60-180 deg E sector during the 12-13 November 2025 geomagnetic storm
Pan Xiong, Jianghe Chen, Xuhui Shen et al. (+3 more)
Geomagnetic storms drive complex ionospheric responses through coupled electrodynamic and thermospheric processes, yet attributing storm-time TEC perturbations to specific mechanisms remains challenging. We investigate the ionospheric response to the 12-13 November 2025 intense geomagnetic storm (Dst minimum = -214 nT) in the 60-180 deg E sector using a coordinated multi-instrument
dataset comprising JPL GIM TEC, dense regional GNSS networks, continuous BeiDou GEO links,
COSMIC-2
radio occultation, ground ionosondes, Swarm in-situ electron density, HF Doppler soundings, and TIMED/GUVI thermospheric composition observations. The observations reveal a dayside-dominant positive TEC storm with pronounced hemispheric asymmetry, where Northern Hemisphere mid-to-low latitudes exhibit stronger and longer-lasting enhancement than the Southern Hemisphere. Joint analysis of
radio occultation, ionosonde, and Swarm data indicates that the enhancement is density-dominated with NmF2 and foF2 increases but with no coherent, sector-scale peak-height uplift in hmF2 or h'F2, posing challenges for uplift-only electrodynamic interpretations. Coherent large-scale traveling ionospheric disturbances propagate across the equator during UT 1-6, while HF Doppler oscillations maximize later during UT 6-24, revealing a timing offset between integrated TEC responses and reflection-height dynamics. Southern Hemisphere O/N2 ratio depletion observed by TIMED/GUVI provides compositional context consistent with the faster positive-phase decay there, although concurrent Northern Hemisphere GUVI coverage is limited during this interval. These findings highlight the value of multi-observable diagnostics for developing testable constraints on storm-time mechanisms and improving sector-specific space weather nowcasting capabilities.
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All authors Pan Xiong, Jianghe Chen, Xuhui Shen, Tong Liu, Angelo De Santis, Sergey Pulinets
Plasma Physics
Inertial-Range Suppression and Ponderomotive Density Cavitation in Broadband Sub-Alfvénic Turbulence under Plasma Sheet Boundary Layer Conditions
Mani K Chettri, Vivek Shrivastav, Britan Singh et al. (+2 more)
Kinetic Alfvén waves (KAWs) are among the most pervasive electromagnetic fluctuations in magnetized astrophysical
plasmas, from Earth's magnetosphere to
galaxy clusters. Their ponderomotive coupling to compressive density fluctuations is poorly understood in the broadband turbulent regime. We present two-dimensional pseudospectral simulations of the modified nonlinear Schrödinger–magnetosonic (MNLS–MS) system governing KAW envelopes, initialized with a broadband power-law spectrum ($|\psi(\mathbf{k})|^2\propto k^{-5/6}$) spanning many interacting modes, at $\beta \sim 0.1$–$0.3$ representative of
plasma sheet boundary layer (PSBL) conditions. A fourth-order Runge–Kutta scheme on a $256\times 256$ grid integrates the system to $t = 40$ (normalized), with total energy conserved to within $0.085\%$ in the undamped run; a damped run with dissipation loses $\sim 4\%$ of the
magnetic energy over the same interval. The nonlinearity parameter $\chi_\mathrm{NL} \approx 0.25$ confirms broadband sub-Alfvénic
turbulence throughout.
Magnetic field intensity and
plasma density develop spatially intermittent, filamentary structures within the first few wave periods, consistent with ponderomotive density cavitation and
plasma expulsion from wave-intense regions. The
magnetic energy spectra show inertial-range suppression, with a rapid transition from injection ($k < 0.3$) to dissipation without an extended power-law cascade, in agreement with the moderate
magnetic Reynolds number ($\mathrm{R_m} \sim 250$–$370$) of the simulation and the observationally constrained range for PSBL
turbulence. These results provide numerical evidence that broadband KAW
turbulence self-organizes into coherent density structures at kinetic scales, and that the spectral character of such
turbulence is governed primarily by moderate-Reynolds-number constraints rather than by the wave physics alone.
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All authors Mani K Chettri, Vivek Shrivastav, Britan Singh, Rupak Mukherjee, Hemam D. Singh
Fluid Dynamics
A minimal model of pump foil dynamics
Eunok Yim and Francois Gallaire
Pump foiling enables a hydrofoil surfboard to sustain forward motion on flat water using only periodic leg pumping, converting vertical oscillations into hydrodynamic lift and thrust. We present a minimal mechanical model of pump foil propulsion, formulated as a coupled second-order system for horizontal and vertical translation and pitch in an inertial frame. The rider-board system is modeled in reduced form, with the rider mass concentrated at the body center and the foil mass assigned to the mast-linker pivot, about which the front and rear wing dynamics are written. Hydrodynamic loading includes quasi-steady lift and drag, buoyancy, and rotational effects, including rotational lift and nonlinear rotational drag. Under simplified control assumptions in which the pumping frequency is fixed and the net rider pumping input is represented by a single effective force-amplitude parameter, the model predicts sustained stable forward-propulsion regimes at modest forcing amplitude and small positive pitch angle. In this regime, the front wing acts primarily as the main lifting surface, while the rear wing, although contributing less to vertical support, is essential for pitch stability because of its longer moment arm. These results provide a mechanistic interpretation of pump foil propulsion and identify measurable quantities and parameter sensitivities that can guide targeted field and laboratory experiments and help refine assumptions on rider control inputs and hydrodynamic loading.
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Plasma Physics
Strong gradient neoclassical transport in the plateau regime
Silvia Trinczek, Felix I. Parra, Peter J. Catto et al. (+1 more)
Strong gradient regions in tokamaks such as the pedestal or internal transport barriers are regions of reduced
turbulence where neoclassical transport can play a dominant role. In pedestals, gradient lengths comparable to the ion poloidal gyroradius have been measured. Standard neoclassical theory can miss important strong gradient effects in these regions because it assumes that the gradient length scales of density, temperature and potential are larger than the ion poloidal gyroradius. We extend plateau regime neoclassical theory into regions of gradients of the order of the ion poloidal gyroradius to capture strong gradient effects on transport processes in the pedestal and internal transport barriers. The fundamental idea behind our new framework is to keep a scale separation between the orbit widths and the gradient length scales by performing a large aspect ratio expansion. In the plateau regime, strong gradients cause poloidal variation that is in-out as well as up-down asymmetric. We study two different test cases assuming either radial force balance or the absence of
turbulence and show that strong gradient effects can enhance or reduce standard neoclassical theory predictions in the plateau regime in strong gradient regions.
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All authors Silvia Trinczek, Felix I. Parra, Peter J. Catto, Iván Calvo
Classical Physics
Self-Force of a Dirac String: An Explicit Calculation
Alberto G. Rojo
A Dirac string can be modeled as a semi-infinite solenoid carrying a fixed
magnetic flux. Dirac pointed out that such a string should experience a nonvanishing and divergent self-force, but explicit calculations are rarely shown. Motivated by a recent comment by McDonald, we present a direct and elementary derivation of this self-force. Treating the string as a stack of current loops, we compute the axial force produced by the radial
magnetic field generated by the rest of the solenoid. The resulting force, $F=\Phi^2/(2\pi\mu_0 a^2)$, diverges as the solenoid radius $a\to0$ with flux $\Phi$ fixed, making explicit the singular nature of the Dirac string.
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Chemical Physics
Auto-WHATMD : Automated Wasserstein-based High-dimensional feature extraction Analysis of Trajectories from Molecular Dynamics
Sosuke Asano, Ikki Yasuda, Katsuhiro Endo et al. (+2 more)
Comparing multiple
protein systems with variation such as different binding ligands or mutations, and understanding their effects is one of the objectives in molecular dynamics simulations. Representation of these systems by a few features enables quantitative comparison. However, because molecular dynamics simulation trajectories are high-dimensional spatiotemporal data, selection of key features relies on domain expertise, sometimes introducing arbitrary assumptions. Here, we present an approach that uses the optimal transport distance to compare high-dimensional trajectory data, and employs simulated annealing to identify the residues that best distinguish multiple systems. We term this
algorithm auto-WHATMD (automated Wasserstein-based High-dimensional feature extraction Analysis for Trajectories of Molecular Dynamics). We applied auto-WHATMD to multiple
protein-ligand systems of bromodomain 4 with different ligands, identifying the most discriminative residues in the loop region. Moreover, even a few selected residues were sufficient to capture the correlation with ligand-binding affinities, indicating that auto-WHATMD effectively prioritizes the most informative residues. Our approach can be used to efficiently determine key residues and design features for multiple analogous systems.
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All authors Sosuke Asano, Ikki Yasuda, Katsuhiro Endo, Yoshinori Hirano, Kenji Yasuoka
Optics
4D reconstruction of alumina laser melt pools at 25 kHz via operando X-ray multi-projection imaging
Lars Witte, Eliot Jermann, Zhe Hu et al. (+9 more)
Advancing additive manufacturing, e.g.,
laser powder-bed fusion (LPBF), requires resolving rapid processes such as melt-pool dynamics and keyhole
evolution in 4D (3D + time). Operando X-ray
tomography is a state-of-the-art approach for 4D characterization, but its temporal resolution is fundamentally constrained by the sample rotation speed, limiting achievable 4D imaging rates and preventing the resolution of these fast phenomena. Here we present rotation-enabled X-ray Multi-Projection Imaging (rotation-XMPI), which captures three angularly resolved projections per time step and thereby decouples temporal resolution from the sample rotation speed. Combined with a self-supervised deep-learning reconstruction framework for multi-angle inputs, rotation-XMPI enables high-fidelity 4D imaging at unprecedented speed. We demonstrate the approach in an operando alumina
laser-remelting experiment at MAX IV using three beamlets combined with 25 Hz sample rotation. Rotation-XMPI resolves melt-pool morphology and keyhole
evolution; in contrast, conventional and limited-angle
tomography remain rotation-limited, and motion blur prevents resolving these dynamics. Overall, rotation-XMPI delivers a 250-fold increase relative to state-of-the-art melt-pool imaging, effectively achieving 25,000 reconstructed volumes per second. This method establishes a practical route to scalable ultrafast 4D imaging for additive manufacturing and other materials processes.
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All authors Lars Witte, Eliot Jermann, Zhe Hu, Zisheng Yao, Eleni Myrto Asimakopoulou, Julia Katharina Rogalinski, Yuhe Zhang, Kim Nygård, Malgorzata G. Makowska, Markus Bambach, Mohamadreza Afrasiabi, Pablo Villanueva-Perez
Fluid Dynamics
Nonlocal flow sampling enables vortex trapping of heavy particles
Sachin Kulkarni, Sumithra R. Yerasi, Vishwanath Kadaba Puttanna et al. (+3 more)
Most analyses of inertial particle motion in vortical flows rely on the point-particle approximation, in which the fluid velocity is assumed to be linear at the scale of the particle, and for heavy particles inertia typically leads to centrifugal expulsion from
vortex cores. Here, we show that a spatially extended particle, modeled as a rigid symmetric dumbbell of two identical inertial point particles connected by a massless rod that samples the flow at two points, can converge to a
vortex-centered spinning state. We study the dynamics of this inertial dumbbell in a steady two-dimensional Lamb-Oseen
vortex and identify three qualitatively distinct long-time behaviors controlled by the Stokes number. In the weak-inertia limit, the motion remains bounded and traces spirographic-like trajectories around the
vortex center, while at sufficiently large inertia centrifugal effects dominate and trajectories spiral outward, approaching inertial point-particle behavior. Between these limits, the dumbbell can reach a trapped spinning state in which the center-of-mass converges to the
vortex center and
spins steadily, with accessibility determined by the initial conditions. Basin-of-attraction maps and ensemble statistics reveal a non-monotonic dependence of the accessibility of the spinning state on inertia, with basins of finite measure occurring only over an intermediate range of Stokes numbers. Linear stability is governed by the logarithmic slope of the
vortex angular-velocity profile, and for the Lamb-Oseen
vortex the spinning state is stable for all Stokes numbers. These results highlight how nonlocal flow sampling by spatially extended inertial particles can fundamentally alter transport and long-time behavior in vortical flows.
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All authors Sachin Kulkarni, Sumithra R. Yerasi, Vishwanath Kadaba Puttanna, Dario Vincenzi, S. Ravichandran, KVS Chaithanya
Optics
Robust and Active Visible-Light Integrated Photonics on Thin-Film Lithium Tantalate for Underwater Optical Wireless Communications
Changjian Guo, Xingjie Li, Xiaofeng Wu et al. (+7 more)
Visible-light integrated photonics enables compact platforms for sensing, precision metrology, and free-space data links at visible wavelengths. However, many applications remain limited by the lack of high-speed and robust modulators in the blue-green band. Here we report, both operating at 532 nm, thin-film lithium tantalate waveguides of propagation losses of dB/cm scale and modulators with a flat frequency response to ~50 GHz. The modulator remains stable when delivering 5 dBm modulated optical power for an hour, which cannot be achieved by thin-film lithium niobate based counterparts under similar conditions and structures. System-level
underwater optical wireless communication (UWOC) is validated with 112-Gb/s transmission over 3-m and 64-Gb/s transmission over 9-m
underwater links. This represents the first integrated external modulator based UWOC system, overcoming the bandwidth-power-chirp trade-offs of traditional directly modulated
laser-based systems. We further demonstrate dual-drive modulators for optical single-sideband and electro-optic frequency-comb generations in the green-wavelength band. These results provide a foundation for complex, robust, and active visible-light photonic integrated
circuits for
underwater optical applications.
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All authors Changjian Guo, Xingjie Li, Xiaofeng Wu, Jiajie Deng, Wenchang Yang, Weilong Ma, Ziliang Ruan, Kaixuan Chen, Sailing He, Liu Liu
Physics Education
Predictive Modeling for High Impact Active Learning Classrooms
Olive Ross, Meagan Sundstrom and N.G. Holmes
Over the past several decades, a large body of research has shown that undergraduate science students learn more and more equitably in active learning classrooms; however, the term "active learning" lacks definition and little research has examined which types and combinations of active learning strategies are most effective. In this study, we use a
dataset representing over 10,000 students and 24 institutions to create a predictive model that maps classroom time spent on different activities to student conceptual learning. We find that four variables – classroom time spent on lecture, group worksheets, clicker questions, and student questions – are sufficient to reliably predict student learning, as measured by concept inventory scores. We identify one type of class that consistently demonstrates exceptional student learning gains (effect sizes greater than 2): those that spend 10-20% of class time on group worksheets, 20-40% of class time on group clicker questions, and average two or more student questions per hour of class time. We also find that classes which do not utilize group worksheets consistently have learning outcomes comparable to fully lecture classes. These results provide testable recommendations for future controlled studies to investigate effective active learning implementation in undergraduate physics courses.
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Optics
Thermal Noise Reduction in Ternary Optical Coatings: From Ti::GeO$_2$-Based Ternary Systems to High Index Materials
Vincenzo Pierro, Guerino Avallone, Jessica Steinlechner et al. (+11 more)
Minimizing coating thermal noise is crucial for enhancing gravitational wave detector sensitivity, with a target Amplitude Spectral Density Reduction Factor (ASD RF) of $0.5$ relative to standard coatings. This study investigates the design of low-noise dielectric stacks using the 'Double Stack of Doublet' strategy, explored via ad-hoc optimization heuristics specifically developed for efficient parametric analysis of coating performance. We analyze the performance limits of ternary coatings based on SiO$_2$, Ti::SiO$_2$, and Ti::GeO$_2$, considering material property uncertainties and absorption constraints. Optimization results show that this system, even with relaxed absorbance constraint (1 ppm), falls short of the target, achieving a best ASD RF of $\sim 0.69$. Consequently, we explore alternative ternary 'Double Stack of Doublet' designs incorporating higher-refractive-index materials. Simulations demonstrate that incorporating alternative high-index materials offers a promising pathway, potentially enabling the achievement of the project target. We discuss the optimization strategies, performance trade-offs, design robustness, and implications of using high-index, potentially higher-loss materials for next-generation optical coatings.
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All authors Vincenzo Pierro, Guerino Avallone, Jessica Steinlechner, Marco Bazzan, Francesco Chiadini, Roberta De Simone, Marianna Fazio, Massimo Granata, Veronica Granata, Gerardo Iannone, Graeme McGhee, Carmen S Menoni, Christophe Michel, Vincenzo Fiumara
Chemical Physics
Carbon black and hydrogen production from methane pyrolysis: measured and modeled insights from integrated gas and particle diagnostics in shock tubes
Gibson Clark, Mohammad Adib, Chengze Li et al. (+5 more)
Methane (CH4) pyrolysis is a promising route to co-produce hydrogen (H2) and carbon black (CB) while avoiding emissions associated with steam-methane reforming and furnace black processes. Model development of pyrolytic CB synthesis requires experimental observations of concurrent gas chemistry, particulate formation, and morphology. This work presents a combined experimental and modeling study of CH4 pyrolysis behind reflected shock waves in 5% CH4/Argon mixtures at post-reflected shock temperatures (T5) of 1850-2450 K and P5 around 4.5 atm.
Laser absorption diagnostics quantified CH4, C2H4, and C2H2 mole fractions, while multiwavelength extinction (633 and 1064 nm) resolved time-dependent particle formation and the temperature-dependent
evolution of optical maturity. Simulations reproduce small-molecule speciation well, but large variations in predicted polycyclic aromatic hydrocarbons (PAHs) persist among models. Coupled gas-particle simulations capture accurate volume fraction (fv) trends and the influence of gas dynamics but underpredict induction times at high T5. Samples collected at the shock tube endwall were analyzed by transmission electron microscopy (TEM) to quantify primary particle size distributions and nanostructure arrangement. Image
segmentation and manual measurements showed reduced primary particle size growth (dp) with increasing T5, while graphitic nanostructure generally increased. This study provides an integrated
benchmark for improving models of CB and H2 production from CH4 pyrolysis by constraining gas-phase kinetics, PAH-driven inception, particle dynamics, and particle maturity. The results highlight that accurate partitioning of mass between particle number and particle size is an important constraint for further model development.
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All authors Gibson Clark, Mohammad Adib, Chengze Li, Taylor M. Rault, Jesse W. Streicher, Enoch Dames, M. Reza Kholghy, Ronald K. Hanson
Instrumentation and Detectors
Towards a Reflective PICOSEC detector?
A. Breskin
PICOSEC is an ultrafast particle-detector concept, combining a photocathode-coated Cherenkov radiator coupled to a gas-avalanche multiplier. Particle-induced Cherenkov
photons create photoelectrons emitted from an ultrathin semitransparent photocathode; they are multiplied and detected in fast gas-avalanche mode. In parallel to the constant progress made in the PICOSEC technique, we propose different detector configurations and operation modes with the aim of enhancing robustness and performance. They incorporate thick reflective photocathodes deposited on the readout electrodes of various types of avalanche multipliers. Some of these Reflective-PICOSEC detectors operate at mbar gas pressures.
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Atmospheric and Oceanic Physics
A framework for modeling aerosol-cloud-lightning interactions: Validation of charge structure and aerosol effects
Weishan Wang, Guoxing Chen, Yijun Zhang et al. (+3 more)
This study develops a novel framework within the Weather Research and Forecast Model for modeling aerosol-cloud-lightning interactions. The framework explicitly represents aerosol-cloud interactions by prescribing aerosols with two configurations: an idealized setup, where both cloud condensation
nuclei (CCN) and ice nucleating particles (IN) are assumed to have a single chemical composition and spatially uniform distributions; and a quasi-realistic configuration, with multi-species aerosols assigned spatially varying distributions, where hygroscopic components act as CCN,
dust particles act as IN, and all aerosol species influence radiative transfer. Cloud microphysics is coupled with detailed charge separation and discharge processes to enable the lightning simulation. The framework is evaluated using two thunderstorms in Guangdong, China. For an isolated storm, the model successfully reproduces the observed tripolar charge structure (positive-negative-positive), demonstrating its capability in simulating cloud electrification. For a frontal storm, it captures well the observed precipitation and lightning, and shows that increasing CCN suppresses the rainfall while enhancing the lightning. Higher CCN concentrations produce more numerous but smaller cloud droplets, which suppresses the coalescence into rain droplets, allows a greater number of droplets to loft into the upper troposphere, and forms more but smaller cloud ice particles. This boosts graupel-ice collisions, intensifies non-inductive charging, strengthens the upper positive charge and the vertical electric-field gradient, ultimately increasing the lightning frequency. In contrast, no significant aerosol-induced invigoration of updrafts is observed. These results highlight the dominant role of aerosol microphysical effects over dynamical invigoration in modulating thunderstorm electrification and lightning activity.
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All authors Weishan Wang, Guoxing Chen, Yijun Zhang, Jen-Ping Chen, Dong Zheng, Liangtao Xu
Fluid Dynamics
Shock-induced tipping in a thermoacoustic system
Bhadra Sreelatha, Rohit Radhakrishnan and R. I. Sujith
Tipping refers to the transition of a system from one state to another. In this study, we focus on shock-induced tipping, which occurs due to a sudden and large disturbance in a control parameter, which is referred to as the shock. This shock drives the system from one dynamical state to another. We present the first experimental demonstration of shock-induced tipping using a prototypical thermoacoustic system, the horizontal Rijke tube. In a thermoacoustic system, unsteady heat release and sound waves interact through positive feedback, leading to self-sustained, high-amplitude oscillations known as limit cycles. The system transitions from a quiescent state to a state of self-sustained oscillations when a shock is introduced in the power supplied to the heat source (an electrically heated grid). This shock is created by abruptly increasing the voltage supplied to the grid, which takes the system into a bistable region. To explain the underlying mechanism linking the shock in the supplied power to the observed tipping behaviour, we model the system by modifying the governing equations of the Rijke tube to incorporate the heat transfer properties of the grid. We demonstrate that the shock in the supplied power manifests as a shock in the grid temperature, causing the system to fall into the basin of attraction of an alternate stable state. The tipping event depends on the magnitude of the shock and the temperature of the grid. Understanding the mechanisms underlying shock-induced tipping is crucial for developing systems with improved safety and reliability.
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Classical Physics · Cross-listed from math.DS
Data-driven Experimental Modal Analysis by Dynamic Mode Decomposition
Akira Saito and Tomohiro Kuno
This paper discusses the application of Dynamic Mode Decomposition (DMD) to the extraction of modal properties of linear mechanical systems, i.e., experimental modal analysis (EMA). First, theoretical background of the DMD is briefly reviewed and its relevance to the Ibrahim time-domain method is discussed. Second, DMD is applied to a single DOF system and multi-DOF discrete system to discuss the applicability and interpretation of the DMD as a method of EMA. Furthermore, the effects of measurement errors on the results of DMD are discussed. It is shown that with relatively small measurement errors, DMD can capture modal parameters accurately. However, with relatively large measurement errors, DMD fails to capture modal parameters. Finally, DMD is applied to experimentally-obtained displacement field of a cantilevered beam, and its modal parameters are extracted. It is shown that the modal parameters extracted by DMD are as accurate as the ones obtained by the existing modal parameter extraction method.
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General Physics
CETOmega: The Causal-Informational Completion of Gravity
Christian Balfagon
We present CETOmega, a unified framework that completes
gravity through a causal-informational principle. The theory reconciles general relativity and
quantum mechanics within a strictly four-dimensional, nonlocal, and causal formulation. At its core lies an analytic and retarded kernel K^-1(Box_R), derived from a discrete causal network, which governs the propagation of the gravitational and scalar sectors. A scalar field, the texon, emerges as the effective excitation of causal connectivity and accounts simultaneously for dark matter and dark energy without introducing extra degrees of freedom or breaking locality. The formalism ensures analyticity, spectral positivity, and
holographic completeness. The kernel admits a Stieltjes representation with positive spectral density rho(mu) greater than or equal to zero, guaranteeing unitarity and causal propagation. At cosmological scales, CETOmega predicts stable inflationary dynamics consistent with Planck observations. Black hole ringdown frequencies acquire perturbative corrections controlled by the causal scale and remain subleading for astrophysical black holes within the fiducial window l* between 10^-5 and 10^-4 meters, where l* defines the mean causal correlation length of the texonic field. CETOmega thus provides a complete, causal, and informational foundation for spacetime dynamics, recovering Einstein
gravity in the infrared while extending its validity to the
quantum and cosmological domains.
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Fluid Dynamics
Non-Reciprocal Capillary Waves
Holly du Plessis, Pedro Cosme, Hugo França et al. (+1 more)
Capillary waves are a classical free-surface phenomenon in fluid mechanics, yet their behavior in
chiral fluids remains largely unexplored. We show that odd viscosity breaks the reciprocity of capillary waves. Using linear theory together with fully nonlinear direct numerical simulations, we find that surface tension creates two inequivalent branches of odd capillary waves: a dispersive branch and a quasi-acoustic branch absent in the capillarity-free limit. Their unequal propagation and attenuation transform standing waves into traveling waves and produce an anomalously deep vortical boundary layer. Above a threshold odd viscosity, nonlinear accumulation of vorticity near the surface reverses the induced shear current and drives bulk particles opposite to the wave motion, giving rise to an anti-Stokes drift with no counterpart in conventional fluids. Our results show how combining capillarity with broken parity can be used to control wave propagation and transport at fluid interfaces, opening a route toward one-way fluidic waveguiding and chirality-programmed interfacial flows.
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All authors Holly du Plessis, Pedro Cosme, Hugo França, Maziyar Jalaal