Radiography

Radiography using X-rays and neutron radiation

Optically opaque fluids and systems

Multiphase and interfacial flows are often optically opaque. Therefore, optical measurements are mainly limited to a two-dimensional field of view, e.g. the surface flow at the free surface or the near-wall flow observed through a transparent wall. Using X-rays and neutrons instead of visible light, radiographic measurements are able to provide unique insights into three-dimensional configurations. Such flow experiments include but are not limited to dense suspensions and fluidised beds of particles, liquid foam [1-7] and froth [8,9], ferrofluids, as well as liquid metals [10-13] and solidification processes. Similarly, radiographic techniques are particularly suitable for flow investigations in systems without direct optical access, such as in liquid metal batteries [14-17], electrolysers or fuel cells.

Instrumentation

For X-ray radiography, we operate and use two different X-ray tubes in the X-ray laboratory at HZDR:

• Industrial X-ray tube ISOVOLT 450M1/25-55 (GE Sensing & Inspection Technologies);
  max. 320 kV, max. 10 mA, max. 3200 W
• Microfocus X-ray tube XWT-225-CT (X-RAY WorX);
  max. 225 kV, max. 3 mA, max. 350 W

For neutron radiography, we use the imaging facilities at the Swiss neutron spallation source (SINQ) of the Paul Scherrer Institute (PSI):

• NEUTRA (thermal neutrons)
• ICON (cold neutrons)

Transmission imaging

Radiography using X-rays or neutrons is based on the measurement principle of transmission imaging. When penetrating matter, the projection of the transmitted beam intensity yields radiographs, which are typically visualised as grey-scale images. Beer-Lambert’s law describes the attenuation of X-rays or neutrons in first approximation: the transmittance I/I₀ is defined as the ratio of the transmitted to incident intensity of the X-ray or neutron beam, and depends exponentially on the effective attenuation coefficient µ of the penetrated sample and its thickness s in the beam direction.

Fig. 1: X-ray and neutron radiography: general measurement arrangement for transmission imaging.

The attenuation coefficients of X-rays and neutrons are both element- and energy-specific. In addition, neutron attenuation coefficients are also isotope-specific. For example, heavy water (deuterium oxide, D₂O) has a much lower neutron attenuation coefficient than normal water (H₂O). All in all, the attenuation characteristics are utilised to distinguish different gas, liquid or solid phases and to quantify phase fractions based on radiographs.

Example: radiographic measurements of liquid fraction and velocity in flowing foams

In liquid foams, the X-ray or neutron attenuation is mainly related to the water content, which equals the liquid fraction φ_l of the foam sample multiplied by its thickness s_f in the beam direction. Neutrons are strongly absorbed in water; their penetration depth is limited to less than 10 mm. Therefore, neutron images provide high-contrast mappings of the liquid fraction distribution in foams, and even small local variations of the liquid fraction can be quantified. To increase the penetration depth, normal water can be partially substituted with heavy water, thus taking advantage of its lower neutron attenuation coefficient. Using X-rays instead of neutrons, the penetration depth in water is increased by more than one order of magnitude. This extends the measurement capabilities of X-ray radiography to liquid foams at high liquid fraction and large sample thickness (Fig. 2) [1].

Fig. 2: X-ray and neutron attenuation in liquid foam, depending on its liquid fraction φ_l and sample thickness s_f in the beam direction. The coloured areas mark the ranges of the transmittance I/I₀ that are preferable (0.20 < I/I₀ < 0.80) or still acceptable (0.05 < I/I₀ < 0.95) for X-ray or neutron radiography. The open and closed symbols denote the minimum and maximum liquid fraction in previous foam studies using X-ray or neutron radiography and X-ray synchrotron micro-tomography, as summarised in [1].

Based on the liquid fraction mapping, time-resolved radiography of flowing foam points towards the flow pattern and velocity distribution in a given cross-section. To measure the local foam velocity, X-ray particle tracking velocimetry has proven as a suitable approach. For this purpose, we have developed tracer particles that are neutrally buoyant in liquid foam flows and have radiopaque features. In this way, as shown in Figs. 3b and c, measurements of the liquid fraction and velocity in flowing foams can be combined and performed simultaneously [2].

Fig. 3: Liquid foam flow through a vertically aligned diverging nozzle: (a) optical measurement of the near-wall velocity, (b) X-ray radiographic mapping of the liquid fraction and (c) X-ray particle tracking velocimetry inside the nozzle (d) using customised tracer particles (e) with radiopaque features [2].

Publications

[1] Knüpfer, L., Lappan, T., Skrypnik, A., Ziauddin, M., Sommer, A.-E., Marquardt, T., Drenckhan, W., & Heitkam, S. Measurement techniques for velocity and liquid fraction in flowing foams. Advances in Colloid and Interface Science, 339, 103421. https://doi.org/10.1016/j.cis.2025.103421

[2] Skrypnik, A., Lappan, T., Knüpfer, L., Ziauddin, M., Arnal Tribaldos, I., Shevchenko, N., & Heitkam, S. Liquid foam flow through a vertically aligned diverging nozzle. In preparation.

[3] Skrypnik, A., Cole, K., Lappan, T., Brito-Parada, P. R., Neethling, S. J., Trtik, P., Eckert, K., & Heitkam, S. (2024). Neutron radiography of an anisotropic drainage flow. Physical Review E, 109(1), 014609. https://doi.org/10.1103/PhysRevE.109.014609

[4] Lappan, T., Herting, D., Ziauddin, M., Stenzel, J., Shevchenko, N., Eckert, S., Eckert, K., & Heitkam, S. (2023). X-ray particle tracking velocimetry in an overflowing foam. Applied Sciences, 13(3), 1765. https://doi.org/10.3390/app13031765

[5] Skrypnik, A., Knüpfer, L., Trtik, P., Tholan, V., Parkes, S., & Heitkam, S. (2023). Neutron radiography of liquid foam structure near a vertical wall. Soft Matter, 19(44), 8552–8560. https://doi.org/10.1039/D3SM00983A

[6] Ziauddin, M., Schleicher, E., Trtik, P., Knüpfer, L., Skrypnik, A., Lappan, T., Eckert, K., & Heitkam, S. (2023). Comparing wire-mesh sensor with neutron radiography for measurement of liquid fraction in foam. Journal of Physics: Condensed Matter, 35(1), 015101. https://doi.org/10.1088/1361-648X/ac9d16

[7] Lappan, T., Franz, A., Schwab, H., Kühn, U., Eckert, S., Eckert, K., & Heitkam, S. (2020). X-ray particle tracking velocimetry in liquid foam flow. Soft Matter, 16(8), 2093–2103. https://doi.org/10.1039/c9sm02140j

[8] Heitkam, S., Lappan, T., Eckert, S., Trtik, P., & Eckert, K. (2019). Tracking of particles in froth using neutron imaging. Chemie Ingenieur Technik, 91(7), 1001–1007. https://doi.org/10.1002/cite.201800127

[9] Heitkam, S., Rudolph, M., Lappan, T., Sarma, M., Eckert, S., Trtik, P., Lehmann, E., Vontobel, P., & Eckert, K. (2018). Neutron imaging of froth structure and particle motion. Minerals Engineering, 119, 126–129. https://doi.org/10.1016/j.mineng.2018.01.021

[10] Birjukovs, M., Peteris Zvejnieks, Lappan, T., Klevs, M., Heitkam, S., Trtik, P., Mannes, D., Eckert, S., & Jakovics, A. (2024). Particle tracking velocimetry and trajectory curvature statistics for particle-laden liquid metal flow in the wake of a cylindrical obstacle. Experiments in Fluids, 65(5), 67. https://doi.org/10.1007/s00348-024-03793-1

[11] Birjukovs, M., Zvejnieks, P., Lappan, T., Sarma, M., Heitkam, S., Trtik, P., Mannes, D., Eckert, S., & Jakovics, A. (2022). Particle tracking velocimetry in liquid gallium flow around a cylindrical obstacle. Experiments in Fluids, 63, 99. https://doi.org/10.1007/s00348-022-03445-2

[12] Lappan, T., Sarma, M., Heitkam, S., Mannes, D., Trtik, P., Shevchenko, N., Eckert, K., & Eckert, S. (2021). X-ray and neutron radiographic experiments on particle-laden molten metal flows. In J. Lee, S. Wagstaff, A. Anderson, F. Tesfaye, G. Lambotte, & A. Allanore (Eds.), Materials Processing Fundamentals 2021 (pp. 13–29). Springer International Publishing. https://doi.org/10.1007/978-3-030-65253-1_2

[13] Lappan, T., Sarma, M., Heitkam, S., Trtik, P., Mannes, D., Eckert, K., & Eckert, S. (2020). Neutron radiography of particle-laden liquid metal flow driven by an electromagnetic induction pump. Magnetohydrodynamics, 56(2/3), 167–176. https://doi.org/10.22364/mhd.56.2-3.8

[14] Duczek, C., Horstmann, G. M., Ding, W., Einarsrud, K. E., Gelfgat, A. Y., Godinez-Brizuela, O. E., Kjos, O. S., Landgraf, S., Lappan, T., Monrrabal, G., Nash, W., Personnettaz, P., Sarma, M., Sommerseth, C., Trtik, P., Weber, N., & Weier, T. (2024). Fluid mechanics of Na-Zn liquid metal batteries. Applied Physics Reviews, 11(4), 041326. https://doi.org/10.1063/5.0225593

[15] Nash, W., Sarma, M., Lappan, T., Trtik, P., Solem, C.K.W., Wang, Z., Duczek, C., Beltrán, A., Weber, N., & Weier, T. (2025). Diaphragm performance of high-temperature Na-Zn cells evaluated by neutron imaging. Journal of Energy Storage, 114, 115542. https://doi.org/10.1016/j.est.2025.115542

[16] Sarma, M., Lee, J., Nash, W., Lappan, T., Shevchenko, N., Landgraf, S., Monrrabal, G., Trtik, P., Weber, N., & Weier, T. (2024). Reusable cell design for high-temperature (600 °C) liquid metal battery cycling. Journal of The Electrochemical Society, 171(4), 040531. https://doi.org/10.1149/1945-7111/ad3b78

[17] Lee, J., Marquez, G. M., Sarma, M., Lappan, T., Hofstetter, Y. J., Trtik, P., Landgraf, S., Ding, W., Kumar, S., Vaynzof, Y., Weber, N., & Weier, T. (2023). Membrane‐free alkali metal – iodide battery with a molten salt. Energy Technology, 11(7), 2300051. https://doi.org/10.1002/ente.202300051