Synchrotron X-Ray Multi-Projection Imaging (XMPI) for High-Resolution 4D Characterization of Multiphase Flows

Multiphase flows where particles, bubbles, or droplets are suspended in a fluid govern critical processes in biology, medicine, materials processing, and geophysics. However, observing their microscale dynamics in opaque systems has remained a fundamental challenge. We present Synchrotron X-ray Multi-Projection Imaging (XMPI), a novel approach enabling four-dimensional (3D+time) tracking of microparticles in dense suspension flows without requiring sample rotation. By capturing simultaneous projections from multiple angles using beam-split X-rays at synchrotron facilities, we resolve instantaneous particle positions and trajectories in opaque fluids such as blood. We demonstrate the potential of XMPI through individual particle tracking velocimetry (3D PTV) in dilute conditions, as well as multi-projection optical flow analysis in dense suspensions. The methodology provides otherwise inaccessible experimental validation for particle-resolved computational fluid dynamics models and allows, e.g., observation of inertial focusing effects and microstructural dynamics relevant to suspension rheology and biomedical flows. This work paves the way for high-resolution, time-resolved 4D imaging of complex multiphase flows across a range of scientific and industrial applications. Combining XMPI with recent AI-supported 4D reconstruction algorithms opens a new spatiotemporal frontier for high-speed, rotation-free microtomography.

💡 Research Summary

This paper introduces Synchrotron X‑ray Multi‑Projection Imaging (XMPI), a novel technique that captures simultaneous X‑ray projections from multiple angles without rotating the sample, thereby enabling true four‑dimensional (3D + time) imaging of opaque multiphase flows. The authors implemented XMPI at the MAX IV For MAX beamline using a 16.55 keV synchrotron beam split by silicon (Si‑111) and germanium (Ge‑400) crystals into two beamlets separated by roughly 48°. Each beamlet illuminates the same region of a flowing suspension, and two high‑resolution X‑ray microscopes (effective pixel size 1.3 µm) record the projections at 40 Hz. By triangulating particle positions from the two views, the system achieves micrometer‑scale spatial accuracy and sub‑millisecond temporal resolution.

The experimental system was tested with silver‑coated hollow glass spheres (10 µm diameter) suspended in glycerol (μ = 1.4 Pa·s) and in human whole blood. Flow was driven by a syringe pump at 0.1 mL h⁻¹, yielding a theoretical maximum axial velocity of 0.134 mm s⁻¹ and a Reynolds number of ~5 × 10⁻⁵. For dilute suspensions, conventional 3‑D particle tracking velocimetry (PTV) was performed by detecting particle centroids in each projection and solving the triangulation problem. For dense suspensions, a multi‑angle optical‑flow algorithm extracted velocity fields from the paired projections, revealing collective dynamics such as inertial focusing and particle clustering.

A key limitation of traditional 4‑D X‑ray computed tomography is the need for sample rotation, which restricts temporal resolution and can introduce secondary flows. XMPI removes this constraint, but the number of projections is inherently limited by the number of beamlets. To overcome the sparsity of angular information, the authors incorporated recent AI‑driven 4‑D reconstruction methods, including neural radiance fields, which exploit phase‑contrast effects and prior physical models of X‑ray propagation to recover volumetric information from only two views.

The authors demonstrate that, with the current setup, particle blur becomes noticeable when particles move more than ~2 pixels during an exposure (≈0.1 mm s⁻¹). By reducing exposure time or employing high‑speed cameras (e.g., Photron Nova S‑16 at 16 kHz), the measurable velocity range can be extended up to ~8 m s⁻¹, enabling studies at Reynolds numbers of order 4 × 10³ in water‑based suspensions.

Results show accurate reconstruction of Poiseuille velocity profiles, observation of wall‑induced inertial migration, and, notably, real‑time visualization of particle‑particle interactions within flowing blood. The technique provides a high‑fidelity experimental benchmark for particle‑resolved CFD/DEM simulations, which have previously lacked direct validation data for dense, opaque flows.

In summary, XMPI offers a rotation‑free, multi‑angle X‑ray imaging platform that combines synchrotron brilliance, precise crystal beam‑splitting, high‑resolution detection, and AI‑enhanced reconstruction to deliver sub‑micrometer spatial and kilohertz temporal resolution of multiphase flows. The method opens new avenues for investigating micro‑scale turbulence, suspension rheology, biomedical flow phenomena, and industrial processes where opacity has previously precluded direct observation. Future work will focus on increasing the number of simultaneous beamlets, integrating even faster detectors, and extending the approach to larger fields of view, thereby broadening its applicability across scientific and engineering domains.