arXiv subjects

Zhao Pan

Publications and source records attributed to Zhao Pan.

At least 19 recordsLinked to original sources

arXiv2026

$d$-wave altermagnetism revealed by resonant inelastic X-ray scattering

Altermagnetism defines a third fundamental class of collinear magnetic order, featuring compensated magnetic moments with antiparallel spin alignment, yet lifted Kramers degeneracy without the need for relativistic spin-orbit coupling. Its ability to host spin-polarized electronic bands and unconventional chiral magnons makes it a promising platform for functional materials. However, experimental verification has proven challenging; while circular dichroism in resonant inelastic X-ray scattering (RIXS) has been suggested as a signature of chiral magnons, it remains controversial whether this effect is an intrinsic property of altermagnetism or an artifact of experimental geometry. In this work, we resolve this debate and provide unambiguous experimental evidence of $d$-wave altermagnetism in the strongly correlated Lieb-lattice magnet La$_2$O$_3$Mn$_2$Se$_2$. The RIXS spectra exhibit a $d$-wave-symmetry circular dichroism in the magnetic excitations that vanishes in the paramagnetic phase. Through RIXS-operator symmetry analysis and exact-diagonalization calculations, we prove that the observed dichroism is a direct consequence of altermagnetic symmetry constraints, independent of magnon branch splitting. Our results provide definitive evidence for the experimental realization of $d$-wave altermagnetism in La$_2$O$_3$Mn$_2$Se$_2$ and establish circularly polarized RIXS as a highly symmetry-sensitive spectroscopic framework for detecting magnetic phases that evade conventional probes.

cond-mat.str-el↗

arXiv2026

Hydrodynamically engineered Indigenous arrows skip on water for waterfowl hunting

Across the Northern Hemisphere, Indigenous hunters developed arrows capable of skipping across the water surface to strike waterfowl. Archaeological and ethnographic records reveal remarkably similar projectile designs spanning millennia and geographically distant cultures, suggesting a convergent technological solution. Despite extensive study of water-entry dynamics, the physical principles underlying this behaviour remain poorly understood. Here we show that successful water-skipping arises from a small set of coupled geometric and dynamical parameters that define a bounded operational regime separating rebound, plunging, and overshoot. Using a combination of controlled experiments, hydrodynamic modeling, and historical reconstruction, we demonstrate that reconstructed arrow designs from independent cultures consistently fall within this predicted regime. These results demonstrate that Indigenous technologies were effectively tuned to satisfy the hydrodynamic constraints governing controlled skipping, providing evidence of convergent optimization in human-engineered systems. More broadly, our results suggest that material culture encodes physical knowledge that formal science is only beginning to articulate, and that the archaeological record and Indigenous culture may be an underexplored archive of empirical discovery.

physics.flu-dyn↗

arXiv2026

Long-range magnetic order with disordered spin orientations in a high-entropy antiferromagnet

Disorder in magnetic systems typically suppresses long-range order, promoting short-range states such as spin glasses and magnetic clusters. This is particularly prominent in high-entropy materials, characterized by the random distributions of local magnetic entities and exchange interactions. However, in rare exceptions, long-range magnetic order can persist in high-entropy systems, while the microscopic characters and underlying mechanisms remain elusive, especially the magnetic behaviors of individual elements. Here, combining neutron diffraction and resonant soft x-ray scattering, we have conducted an element-specific investigation into the magnetic order of a high-entropy honeycomb-lattice van der Waals material (Mn1/4Fe1/4Co1/4Ni1/4)PS3. Despite significant atomic disorder, long-range zigzag antiferromagnetic order is observed below 72 K, with all four transition-metal elements participating in a unified phase transition. However, the spin orientations of various elements are distinct, attributed to the competition between single-ion anisotropies and exchange interactions. Our findings showcase a novel form of long-range magnetic order with disordered spin orientations, which is synergically stabilized by distinct magnetic elements in a high entropy magnet, offering a new paradigm for understanding complex magnetic systems.

cond-mat.str-el↗

arXiv2026

BLISSNet: Deep Operator Learning for Fast and Accurate Flow Reconstruction from Sparse Sensor Measurements

Reconstructing fluid flows from sparse sensor measurements is a fundamental challenge in science and engineering. Widely separated measurements and complex, multiscale dynamics make accurate recovery of fine-scale structures difficult. In addition, existing methods face a persistent tradeoff: high-accuracy models are often computationally expensive, whereas faster approaches typically compromise fidelity. In this work, we introduce BLISSNet, a model that strikes a strong balance between reconstruction accuracy and computational efficiency for both flow reconstruction and nudging-based data assimilation. The model follows a DeepONet-like architecture, enabling zero-shot inference on domains of arbitrary size. After the first model call on a given domain, certain network components can be precomputed, leading to low inference cost for subsequent evaluations on large domains. Consequently, the model can achieve faster inference than classical interpolation methods such as radial basis function or bicubic interpolation. This combination of high accuracy, low cost, and zero-shot generalization makes BLISSNet well-suited for large-scale real-time flow reconstruction and data assimilation tasks.

physics.flu-dyn↗

arXiv2025

Maximum droplet volume on cylindrical surfaces

The maximum volume ($\Omega$) of a droplet that can remain attached to a horizontal fiber defines the stability limit of droplet-fiber interactions, phenomena common in nature and critical to diverse engineering applications. Existing predictive models for $\Omega$ show limitations in accurately capturing the dependence of $\Omega$ on fiber size and wettability. To address this gap, we systematically investigate $\Omega$ on a horizontal fiber through numerical simulations and experiments. A comprehensive semi-empirical model for $\Omega$ is developed and validated against both experimental measurements and reference simulations. This model establishes a new scaling under which the normalized maximum droplet volume depends solely on the contact angle and remains valid across a wide spectrum from a sub-millimeter thin fiber to the flat-surface limit, regardless of the diverse morphologies that droplets exhibit.

physics.flu-dyn↗

arXiv2025

Droplet at the Corner of a V-Shaped Fiber

A fundamental question in the physics of droplet--fiber interactions is: What is the maximum droplet volume a fiber can retain? While this problem has been studied for horizontal fibers and at the apex $\Lambda$-shaped bent fibers, it remains less explored for V-shaped bent fibers, despite their demonstrated advantages in engineering applications such as fog harvesting. This work investigates the capability of V-shaped fibers in retaining droplets against gravity. An analytical model to predict the maximum droplet volume on V-shaped fibers is developed based on free energy analysis, and validated against experimental data from five liquid--fiber pairs. The dependence of the maximum droplet volume on $\alpha$ can be reasonably captured by the function $\cos\beta/\cos\left(\beta-\alpha/2\right)$, where $\beta$ denotes the droplet's off-axis angle. As $\alpha$ increases from $0^\circ$ to $180^\circ$, the maximum droplet volume slightly decreases before entering a broad transition region around $\alpha \approx 40^\circ$--$100^\circ$, and then increases at larger $\alpha$.

physics.flu-dyn↗

arXiv2025

Droplet on a sugar fiber

The motion of a water droplet on a single vertical sugar fiber is analyzed. The fiber is positioned vertically, with the droplet placed at its pending end. If the capillary force exceeds the weight of the droplet, the droplet remains suspended at the fiber's extremity. As water dissolves, the fiber eventually breaks. Subsequently, the droplet may fall down carrying a portion of the undissolved fiber or, more interestingly, the droplet may be propelled upwards, remaining attached to the fiber. The process can then restart. A phase diagram is constructed based on the droplet's volume and the fiber's diameter. A model has been built to assess the critical droplet volume for a given fiber diameter, above which the droplet falls. Below this volume, the droplet can move upwards along the fiber.

physics.flu-dyn↗

arXiv2025

Droplets Suspended Beneath a Fiber Hub

Droplet-fiber interactions, prevalent in nature and widely applied across various engineering fields, have garnered significant research interest. Many works have focused on the interactions between droplets and single or two fibers. However, the wetting behavior of droplets, especially the maximum droplets that can be retained, on fiber hubs formed by many fibers is rarely studied. The current work explores the capability of fiber hubs to retain liquid droplets. We develop analytical and semi-empirical models to predict the maximum droplet volume on a fiber hub, validating them against experimental data. The variation of maximum volume follows two distinct regimes as the fiber count increases, with a critical fiber number ($n^* = 32$) marking the transition between them. In Regime I ($n\le n^*$), the volume increases with fiber number, and the stability of a droplet is dictated by the pinning of three-phase contact lines. In Regime II ($n>n^*$), the volume plateaus, with droplets under a fiber hub behaving similarly to those on a flat surface, where the stability is governed by Rayleigh-Taylor instability.

physics.flu-dyn↗

arXiv2025

Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3

The rare physical property of negative thermal expansion (NTE) is intriguing because materials with large NTE over a wide temperature range can serve as high-performance thermal expansion compensators. However, applications of NTE are hindered by the fact that most of the available NTE materials show small magnitudes of NTE, and/or NTE occurs only in a narrow temperature range. Herein, for the first time, we investigated the effect of anion substitution instead of general Pb/Ti-site substitutions on the thermal expansion properties of a typical ferroelectric NTE material, PbTiO3. Intriguingly, the substitution of S for O in PbTiO3 further increases the tetragonality of PbTiO3. Consequently, an unusually enhanced NTE with an average volumetric coefficient of thermal expansion $\bar{\alpha}_V$ = -2.50 $\times$ 10$^{-5}$/K was achieved over a wide temperature range (300 -- 790 K), which is contrasted to that of pristine PbTiO3 ($\bar{\alpha}_V$ = -1.99 $\times$ 10$^{-5}$/K RT -- 763 K). The intensified NTE is attributed to the enhanced hybridization between Pb/Ti and O/S atoms by the substitution of S, as evidenced by our theoretical investigations. We therefore demonstrate a new technique for introducing mixed anions to achieve large NTE over a wide temperature range in PbTiO3-based ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2024

Breaking The Ice: Video Segmentation for Close-Range Ice-Covered Waters

Rapid ice recession in the Arctic Ocean, with predictions of ice-free summers by 2060, opens new maritime routes but requires reliable navigation solutions. Current approaches rely heavily on subjective expert judgment, underscoring the need for automated, data-driven solutions. This study leverages machine learning to assess ice conditions using ship-borne optical data, introducing a finely annotated dataset of 946 images, and a semi-manual, region-based annotation technique. The proposed video segmentation model, UPerFlow, advances the SegFlow architecture by incorporating a six-channel ResNet encoder, two UPerNet-based segmentation decoders for each image, PWCNet as the optical flow encoder, and cross-connections that integrate bi-directional flow features without loss of latent information. The proposed architecture outperforms baseline image segmentation networks by an average 38% in occluded regions, demonstrating the robustness of video segmentation in addressing challenging Arctic conditions.

cs.CV↗

arXiv2024

Revisiting Liu and Katz (2006) and Zigunov and Charonko (2024b): on the Equivalence of the Omnidirectional Integration and the Pressure Poisson Equation

In this work, we demonstrate the equivalency of the Rotating Parallel Ray Omnidirectional Integration (RPR-ODI) and the Pressure Poisson Equation (PPE) for pressure field reconstruction from corrupted image velocimetry data (dubbed 'ODI $\equiv$ PPE'). Building on the work by Zigunov and Charonko (2024b), we show that performing the ODI is equivalent to pursuing the minimum norm least squares (MNLS) or minimum norm (MN) solution to a Poisson equation with all Neumann boundary conditions. By looking through the lens of linear algebra, regression, optimization, and the well-posedness of the Poisson equation, we provide a comprehensive and integrated framework to analyze ODI/PPE-based pressure field reconstruction methods. The new comprehensions on ODI $\equiv$ PPE provides theoretical and computational insights valuable to experimentalists beyond reducing the high computational cost of ODI to that of PPE. More importantly, we i) provide a comprehensive guideline for robust pressure reconstruction, and ii) unveil the shared strengths and limitations of ODI and PPE, which are elaborated in remarks and notes throughout this work. Some remarks suggest simple regularization strategies that serve as 'minimal reproducible examples' and provide a foundation for further refinement. This work paves the way for further improvements in ODI/PPE-based pressure field reconstruction by utilizing the extensive literature on fast and robust elliptic solvers as well as their associated regularization methods. Numerical experiments are presented to support and illustrate these arguments.

physics.flu-dyn↗

arXiv2024

Error propagation of direct pressure gradient integration and a Helmholtz-Hodge decomposition based pressure field reconstruction method for image velocimetry

Recovering pressure fields from image velocimetry measurements has two general strategies: i) directly integrating the pressure gradients from the momentum equation and ii) solving or enforcing the pressure Poisson equation (divergence of the pressure gradients). In this work, we analyze the error propagation of the former strategy and provide some practical insights. For example, we establish the error scaling laws for the Pressure Gradient Integration (PGI) and the Pressure Poisson Equation (PPE). We explain why applying the Helmholtz-Hodge Decomposition (HHD) could significantly reduce the error propagation for the PGI. We also propose to use a novel HHD-based pressure field reconstruction strategy that offers the following advantages or features: i) effective processing of noisy scattered or structured image velocimetry data on a complex domain; ii) using Radial Basis Functions (RBFs) with divergence/curl-free kernels to provide divergence-free correction to the velocity fields for incompressible flows and curl-free correction for pressure gradients; and iii) enforcing divergence/curl-free constraints without using Lagrangian multipliers. Complete elimination of divergence-free bias in measured pressure gradient and curl-free bias in the measured velocity field results in superior accuracy. Synthetic velocimetry data based on exact solutions and high-fidelity simulations are used to validate the analysis as well as demonstrate the flexibility and effectiveness of the RBF-HHD solver.

physics.flu-dyn↗

arXiv2024

A Simple Boundary Condition Regularization Strategy for Image Velocimetry Based Pressure Field Reconstruction

We propose a simple boundary condition regularization strategy to reduce error propagation in pressure field reconstruction from corrupted image velocimetry data. The core idea is to replace the canonical Neumann boundary conditions with Dirichlet ones obtained by integrating the tangential part of the pressure gradient along the boundaries. Rigorous analysis and numerical experiments justify the effectiveness of this regularization.

physics.flu-dyn↗

arXiv2023

Three-Dimensional Time Resolved Lagrangian Flow Field Reconstruction Based on Constrained Least Squares and Stable Radial Basis Function

The three-dimensional Time-Resolved Lagrangian Particle Tracking (3D TR-LPT) technique has recently advanced flow diagnostics by providing high spatiotemporal resolution measurements under the Lagrangian framework. To fully exploit its potential, accurate and robust data processing algorithms are needed. These algorithms are responsible for reconstructing particle trajectories, velocities, and differential quantities (e.g., pressure gradients, strain- and rotation-rate tensors, and coherent structures) from raw LPT data. In this paper, we propose a three-dimensional (3D) divergence-free Lagrangian reconstruction method, where three foundation algorithms -- Constrained Least Squares (CLS), stable Radial Basis Function (RBF-QR), and Partition-of-Unity Method (PUM) -- are integrated into one comprehensive reconstruction strategy. Our method, named CLS-RBF PUM, is able to (i) directly reconstruct flow fields at scattered data points, avoiding Lagrangian-to-Eulerian data conversions; (ii) assimilate the flow diagnostics in Lagrangian and Eulerian descriptions to achieve high-accuracy flow reconstruction; (iii) process large-scale LPT data sets with more than hundreds of thousand particles in two dimensions (2D) or 3D; (iv) enable spatiotemporal super-resolution while imposing physical constraints (e.g., divergence-free for incompressible flows) at arbitrary time and location. Validation based on synthetic and experimental LPT data confirmed that our method can consistently achieve the above advantages with accuracy and robustness.

physics.flu-dyn↗

arXiv2023

On the Jets Induced by a Cavitation Bubble Near a Cylinder

The dynamics of cavitation bubbles in the vicinity of a solid cylinder or fibre are seen in water treatment, demolition and/or cleaning of composite materials, as well as bio-medical scenarios such as ultrasound-induced bubbles near the tubular structures in the body. When the bubble collapses near the surface, violent fluid jets may be generated. Understanding whether these jets occur and predicting their directions -- departing or approaching the solid surface -- is crucial for assessing their potential impact on the solid phase. However, the criteria for classifying the onset and directions of the jets created by cavitation near a curved surface of a cylinder have not been established. In this research, we present models to predict the occurrence and directions of the jet in such scenarios. The onset criteria and the direction(s) of the jets are dictated by the bubble stand-off distance and the cylinder diameter. Our models are validated by comprehensive experiments. The results not only predict the jetting behaviour but can serve as guidelines for designing and controlling the jets when a cavitation bubble collapses near a cylinder, whether for protective or destructive purposes.

physics.flu-dyn↗

arXiv2022

Large cavitation bubbles in the tube with a conical-frustum shaped closed end during a transient process

The transient process accompanied by extreme acceleration in the conical sections of hydraulic systems (e.g., draft tube, diffuser) can induce large cavitation bubbles both at the closed ends and in the bulk liquid. The collapses of the large cavitation bubbles can cause severe damage to the solid walls. We conduct experiments in the tubes with different conical-frustum shaped closed ends with the `tube-arrest' method and observe bubbles generated at these two locations. For the bubbles generated at the close end of the tube, we propose the onset criteria, consisting of two universal non-dimensional parameters $Ca_1$ and $Ca_2$, of large cavitation bubbles separating the water column. We investigate their dynamics including the collapse time and speed. The results indicate that the larger the conical angle, the faster the bubbles collapse. For the bubbles generated in the bulk liquid, we numerically study the collapse time, the jet characteristics and the pressure pulse at bubble collapse. We observe a much stronger jet and pressure pulse of bubbles in tubes, comparing with a bubble near an infinite plate. Our results can provide guidance in the design and safe operation of hydraulic machinery with complex geometries, considering the cavitation during the transient process.

physics.flu-dyn↗

arXiv2020

Targeted particle delivery via vortex ring reconnection

A conceptual model for targeted particle delivery is proposed using controlled vortex ring reconnection. Entrained particles can be efficiently transported within the core of the vortex ring which is propelled via self-induction. A pair of these particle-transporting vortices travelling in the streamwise direction along parallel trajectories will mutually interact resulting in vortex reconnection. The reconnection causes a topological change to the vortex rings which is accompanied by a rapid repulsion in a perpendicular plane to the direction of travel; effectively, transporting the particles to the desired delivery site on the sidewalls. This work explains the physics of the process and the considerations for targeted delivery.

physics.flu-dyn↗

arXiv2020

On the criteria of large cavitation bubbles in a tube during a transient process

Extreme cavitation scenarios such as water column separations in hydraulic systems during transient processes caused by large cavitation bubbles can lead to catastrophic destruction. In the present paper, we study the onset criteria and dynamics of large cavitation bubbles in a tube. A new cavitation number $Ca_2 = {l^*}^{-1} Ca_0$ is proposed to describe the maximum length $L_{\max}$ of the cavitation bubble, where $l^*$ is a non-dimensional length of the water column indicating its slenderness, and $Ca_0$ is the classic cavitation number. Combined with the onset criteria for acceleration-induced cavitation ($Ca_1<1$, Pan et al. (2017)), we show that the occurrence of large cylindrical cavitation bubbles requires both $Ca_2<1$ and $Ca_1<1$ simultaneously. We also establish a Rayleigh-type model for the dynamics of large cavitation bubbles in a tube. The bubbles collapse at a finite end speed, and the time from the maximum bubble size to collapse is $T_c=\sqrt{2}\sqrt{lL_{\max}}\sqrt{\frac{\rho}{p_\infty}}$, where $l$ is the length of the water column, $L_{\max}$ is the maximum bubble length, $\rho$ is the liquid density, and $p_{\infty}$ is the reference pressure in the far field. The analytical results are validated against systematic experiments using a modified 'tube-arrest' apparatus, which can decouple acceleration and velocity. The results in the current work can guide design and operation of hydraulic systems encountering transient processes.

physics.flu-dyn↗