ForMAX - a beamline for multiscale and multimodal structural characterization of hierarchical materials

ForMAX is equipped with a 3 m long room-temperature, in-vacuum undulator from Hitachi Metals. The maximum effective deflection parameter is $K=1.89$ at the minimum magnetic gap of 4.5 mm and the measured phase error is within specification for all operational gaps. In order to cover the energy range of 8-25 keV, we make use of the fifth to thirteenth harmonics of the undulator as shown in Fig. 2. Similar to other beamlines around the MAX IV 3 GeV storage ring Ursby et al. (2020); Johansson et al. (2021), the undulator exhibits narrow harmonic peaks, $\Delta E<100$ eV (full width at half maximum; FWHM). We summarize the main parameters of the undulator in Table 3.

The front end serves as the interface between the MAX IV 3 GeV storage ring and the ForMAX beamline and was provided by Toyama. It is part of personal and machine safety systems; it ensures safe access to the optical hutch and safe equipment operation. It includes safety and photon shutters, several fixed and movable masks, various diagnostics components including beam viewers, x-ray beam position monitors, thermocouples, and vacuum gauges, as well as vacuum valves to separate different vacuum sections and to safeguard the vacuum of the storage ring in case of vacuum loss in the beamline. The fixed masks remove a vast portion of the undulator radiation power, with the front end typically passing through $\approx 130$ W of radiation to the optics hutch at the projected 500 mA ring current. The movable mask, based on two L-shaped GLIDCOP slits with tantalum edges and located $\approx 19.5$ m downstream of the source, is used to define the angular acceptance of photon beam for the ForMAX beamline.

ForMAX’s primary optics consist of a double crystal monochromator provided by FMB Oxford, a double multilayer monochromator by Axilon, dynamically bendable vertical and horizontal focusing mirrors in Kirkpatrick-Baez geometry by IRELEC, a photon shutter by Axilon, and four diagnostics modules by FMB Oxford that host a fixed mask limiting the beamline’s acceptance angle to $\leq 24\times 36~{}\mu$rad² ($x\times y$) and a high-band-pass diamond filter for heat-load management, a white-beam stop, bremsstrahlung collimators, slits, beam viewers, and beam intensity monitors. In the following we will briefly discuss the monochromators and mirrors.

The major components of the experimental station shown in Fig. 3 - two beam-conditioning units (BCUs), an experimental table, a detector gantry, and a flight tube - have been custom designed at MAX IV. Due to the different, and some times mutually competing, technical requirements of SWAXS and SR$\mu$CT as outlined above, we gave special attention to the integration of these components into a single instrument. Because of the modular nature of the experimental station, as described below, we have installed a dedicated programmable logic controller (PLC) system to ensure its safe operation. In order to mitigate the effect of parasitic scattering in the small-angle regime, that hampers SAXS studies of weakly scattering biobased materials such as low-concentration suspensions of cellulose nanoparticles, all windows in the x-ray beam path of the ForMAX beamline are single crystalline. Finally, we have dedicated space between the BCUs to assemble a setup for x-ray multi-projection imaging (XMPI) Villanueva-Perez et al. (2018, 2023).

ForMAX offers a number of experimental techniques, each with specific requirements with respect to sample manipulation. In order to meet different user needs, ForMAX is equipped with three separate stacks of stages for sample manipulation:

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  For SWAXS experiments, we provide a high-load ($\leq 1500~{}$N) five-axis assembly from Huber as shown in Fig. 4A. It consists, from bottom to top, of motorized pitch Rx ($\pm 13^{\circ}$), roll Rz ($\pm 12^{\circ}$), lateral x ($\pm 25~{}$mm), longitudinal z ($\pm 25~{}$mm), and vertical y ($\pm 20~{}$mm) axes. In order to simplify mounting of sample holders or environments, we have added an optical breadboard with a $25\times 25~{}$mm² grid of centered ISO metric M6 threaded holes on top of the stages. The nominal distance between the top surface and the center of rotation is $49\pm 20~{}$mm, but this can be increased owing to the modular nature of the assembly of stages.

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  For scanning SWAXS experiments, we provide another assembly with five degrees of freedom by Huber, see Fig. 4B. The base consists of motorized lateral x ($\pm 25~{}$mm), vertical y ($\pm 10$ or $\pm 45~{}$mm, depending on resolution and speed requirements), and longitudinal z ($\pm 25~{}$mm) axes for 2D scanning and adjustment of the sample along the x-ray beam path. On top of these we have mounted a yaw Ry axis that, combined with the translation stages below, allows SWAXS tomographic imaging. Finally, we have added a large-range, custom pitch axis Rx ($\pm 45^{\circ}$) for SWAXS tensor tomography experiments. A manual five-axis goniometer head (Huber, model 1002 or 1005) on top of the assembly enable fine alignment of the sample.

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  In SR$\mu$CT experiments we employ a five-axis assembly from Lab Motion as shown in Fig. 4C. It consists, from bottom to top, of a motorized longitudinal z axis ($\approx 380~{}$mm range) for propagation-based phase-contrast imaging, a vertical y axis ($\pm 20~{}$mm) for helical imaging, an air-bearing tomographic yaw axis Ry, coupled with a rotary union accomodating a fluid slip ring, and horizontal xz axes ($\pm 5~{}$mm each) for sample alignment. The electrical slip ring is equipped with 15 spare wires for integration of sample environments. The maximum rotation speed of the yaw stage is 720 revolutions per minute, allowing SR$\mu$CT experiments with temporal resolution up to $\approx 20~{}$Hz. The assembly is modular and is typically operated without the vertical axis and the rotary union. In order to facilitate mounting of sample holders or environments, we have installed an optical breadboard with a $12.5\times 12.5~{}$mm² grid of centered ISO metric M6 threaded holes on top of the stages.

We further note that we can combine the air-bearing tomographic rotation stage with linear scanning SWAXS stages in a modular setup, allowing combined high-resolution SR$\mu$CT and 2D/3D scanning SWAXS experiments without the need to re-mount the sample upon changing experimental modality.

For SWAXS experiments, ForMAX is equipped with two megapixel, hybrid photon-counting detectors that provide high resolution, high dynamic range, and low noise. The SAXS detector is a vacuum-compatible Dectris EIGER2 X 4M Donath et al. (2023). The WAXS detector, in turn, is a custom X-Spectrum Lambda 3M Pennicard et al. (2013). In Table 4 we provide technical details about both detectors.

The range of scattering vector modulus q covered by the SAXS detector depends on the x-ray energy, sample-to-detector distance (SDD), positioning of the SAXS detector in the scattering plane, and the size of the central beam stop (at the moment 4-5 mm diameter); assuming that the SAXS detector is centered on the direct x-ray beam, the accessible SAXS q range varies from $q\approx 0.01\dots 0.5$ nm^-1 at minimum x-ray energy and maximum SDD to $q\approx 0.25\dots 10$ nm^-1 at maximum energy and minimum SDD.

The custom WAXS detector warrants a more detailed discussion. In order to facilitate scanning SWAXS experiments from fibrous materials, it has a hole in the center to pass through the SAXS signal (and the direct x-ray beam), while simultaneously allowing us to collect WAXS data in all directions of the scattering plane as exemplified in Fig. 5A. Moreover, it is mounted onto an evacuated nose cone and connected to the flight tube via a bellow, thereby minimizing parasitic air scattering in the SAXS regime. At the nominal SDD of 135 mm, we can collect WAXS data at scattering angles $2\theta=7\dots 20^{\circ}$ in all directions of the scattering plane, yielding the energy-dependent range of accessible scattering vector moduli $q=(4\pi/\lambda)\mathrm{sin}(\theta)$ shown in Fig. 5B. We note that there is $\approx 100$ mm path of air between the sample and the entrance window of the flight tube when using the WAXS detector, adding to the parasitic background scattering in the SAXS regime. Finally, due to the thickness of the full-field microscope, SDD$\geq 235$ mm in the combined SWAXS and SR$\mu$CT experiments, essentially halving (i) the energy-dependent minimum and maximum q of Fig. 5B and (ii) the accessible scattering angles $2\theta$ in the SAXS regime due to shadowing of the flight tube entrance window, hence in practice limiting these experiments to x-ray energies $\geq 20$ keV.

For SR$\mu$CT experiments, ForMAX is equipped with a high-resolution full-field microtomography detection system encompassing two main components $-$ an optical microscope and a sCMOS camera. The transmitted x-ray beam is converted by a scintillator into visible light, that in turn is magnified by the optical microscope and recorded by the sCMOS camera. The white-beam optical microscope from Optique Peter has motorized triple objective lens and dual camera port configurations for simple switching of magnification and sCMOS camera, respectively. We can operate the microscope with $2\times$, $5\times$, $7.5\times$, $10\times$, and $20\times$ objectives, depending on the required effective pixel size and field of view.

Currently we employ two sCMOS cameras for high-resolution imaging at limited speed, the Hamamatsu ORCA Lightning and the Andor Zyla. We are also equipped with a high-speed Photron FASTCAM Nova sCMOS camera to allow $\approx 20$ Hz SR$\mu$CT, i.e., the maximum temporal resolution allowed for by the tomographic rotation stage. We summarize the technical details of the sCMOS cameras in Table 5.

We have evaluated the performance and resolution of the presented SR$\mu$CT system. For this evaluation, we used the full-field microscope with $5\times$, $10\times$, and $20\times$ magnification coupled to the Andor Zyla camera (physical pixel size 6.5 $\mu$m), resulting in 1.3, 0.65, and 0.325 $\mu$m effective pixel size, respectively. The reconstructed slices for the different magnifications of a wood sample are presented in the right column of Fig. 6. We also evaluated the resolution over the 3D reconstructed volume using the Fourier Shell Correlation (FSC) together with the half-bit threshold criterion van Heel (1987); van Heel and Schatz (2005) as depicted in the right column of Fig. 6. We observe that the ForMAX instrument retrieves Nyquist-limited resolution for the $5\times$ and $10\times$ magnifications, i.e., 2.6 and 1.3 $\mu$m resolution. For the $20\times$ magnification, the retrieved resolution was around 3 pixels, which corresponds to 0.975 $\mu$m. Thus, the ForMAX instrument is ideal to characterize objects in 3D with micrometer resolution.

ForMAX’s control system is based on Tango Chaize et al. (1999), an open-source control system that is in use at several European synchrotron facilities. On top of Tango we employ Sardana Coutinho et al. (2011), a software environment for, e.g., controlling motors, acquiring signals, and running macros. We have optimized the scan routines for the specific needs of ForMAX, such as reducing the overhead per line in continuous xy mesh scans to $<1$ second for scanning SWAXS applications. From a hardware point of view, the majority of our motorized axes are based on stepper motors controlled by IcePAP Janvier et al. (2013) and we make use of PandABoxes for synchronization of the experiments Zhang et al. (2017).

All detectors are integrated into the beamline control system via dedicated detector servers, utilizing detector-specific software development kits (SDKs) running on detector control units (DCUs) for detector control and data readout. Low-level image processing such as flat-field correction is either applied by default (for hybrid pixel detectors; SWAXS) or in the image reconstruction (for sCMOS cameras; SR$\mu$CT), while low level acquisition parameters such as acquisition time, number of frames, and photon-counting threshold energy are accessible to the user. Finally, the data are streamed via high-speed Ethernet connections to MAX IV’s central data storage and saved together with metadata in the hierarchical data format 5 (HDF5). In the case of the high-speed Photron sCMOS camera, the data will temporarily be saved on the local data storage of the camera, before transfer to the central data storage. The data are stored for at least seven years.

In parallel with data streaming and storage of the as measured scattering data, our SWAXS data pipeline reduces the data to a more user-friendly format. The data reduction is carried out using the python implementation of MatFRAIA Jensen et al. (2022), based on a matrix-multiplication algorithm for radial and azimuthal integration, and is faster than the maximum frame rate of the EIGER detector. We reduce the SWAXS data into both 1D $I(q)$ and 2D $I(q,\varphi)$ formats, where $I$ denotes the scattering intensity and $\varphi$ the azimuthal angle. We emphasize that the fast data reduction into so-called ’cake plots’, $I(q,\varphi)$, is particularly convenient for monitoring anisotropic scattering from fibrous materials in (scanning) SWAXS experiments. For calibration and masking of the detectors we utilize PyFAI Kieffer et al. (2018), which is well known in the user community. Finally, in order to facilitate monitoring of the experiment, we plot either the radial integral $I(q)$ or the ’cake plot’ $I(q,\varphi)$ in both SAXS and WAXS regimes in live mode. In Fig. 7 we present a snapshot from the beamline control computer, exemplifying the live plotting of reduced SWAXS data.

In SR$\mu$CT experiments, we take a different approach for the data pipeline. In line with community convention, we collect projections as well as flat- and dark-field images using dedicated scan routines and save all data in common HDF5 files. In order to further improve user friendliness, we are currently in the process of implementing live tomographic reconstructions for SR$\mu$CT experiments.

ForMAX allows a wide range of SWAXS, scanning SWAXS, and SR$\mu$CT experiments, each with their unique requirements with respect to on-line data analysis. In order to support all these different experiments, we provide up-to-date Jupyter notebook templates for our users. For SR$\mu$CT experiments, the script for tomographic reconstruction includes the option to perform phase retrieval for single-distance propagation-based phase contrast tomography, in addition to standard absorption contrast tomography reconstruction. We plan to continuously implement further developments, including the aforementioned live tomographic reconstructions for SR$\mu$CT experiments.

SAXS tensor tomography (SASTT), that combines concepts of scanning SAXS with SR$\mu$CT to retrieve not solely scattering intensity measures but the full 3D reciprocal space map within each voxel of the tomogram, is a special case due to the high demands on image reconstruction. Data acquisition must be matched with sufficient computational resources to allow reconstructions of the 3D reciprocal space map, ideally already during the experiment, to evaluate the quality of the measurements. At ForMAX, we have implemented Jupyter notebook templates for projection alignment and apply the software package Mumott (mumott.org) to perform SASTT reconstructions. In the future, we plan to continuously update these notebooks to remain up to date and match further developments and improvements of the reconstruction algorithm. Details about Mumott can be found in a recent publication by Nielsen et al. (2023).

The dynamically bendable mirrors provide means to vary the lateral and vertical beam size at the sample position in a large range. In the unfocused mode, we obtain an FWHM beam size of $\approx 0.8\times 1.3~{}$mm² ($x\times y$) using the DCM, while the larger bandpass of the MLM yields a beam size of $\approx 1.3\times 1.5~{}$mm². In the other extreme, we can focus the beam down to $\approx 55\dots 60\times 10\dots 15~{}\mu$m² at the sample position using either monochromator. In this case, the lateral beam size is limited by source size and imaging geometry, while the vertical beam size is limited by slope errors of the mirrors. The dependence of the beam size on the ID harmonic is negligible.

In order to further decrease the beam size at the sample position, we have installed compound refractive lenses (CRLs) $\approx 1.5$ m upstream of the nominal sample position. At the moment we employ a stack of 16 radiation-resistant SU-8 polymeric lenses from Microworks, optimized for 16.3 keV x-rays and yielding a FWHM beam size of $\approx 10\times 2~{}\mu$m² at the sample position. This is similar to the beam size typically available at third-generation SWAXS beamlines with microfocus capability Buffet et al. (2012); Smith et al. (2021).

The natural beam size at ForMAX limits SR$\mu$CT experiments on large samples. This limitation can be partly overcome by stitching images, but at the expense of temporal resolution. We will soon also install an optional, overfocusing SU-8 x-ray prism lens from Microworks $\approx 5.4$ m upstream of the nominal sample position, yielding an energy-dependent x-ray beam size of $\approx 5\times 5~{}$mm² or larger at the sample position in combination with the DCM.

The imaging techniques available at ForMAX rely on a large incident photon flux. In Fig. 8 we present the measured x-ray photon flux at the sample position for both monochromators. We collected the data with the minimum undulator gap (4.5 mm) and maximum acceptance angle ($24\times 36~{}\mu$rad²), as typically employed for photon-hungry SR$\mu$CT and scanning SWAXS experiments. We measured the flux of a strongly attenuated x-ray beam at $\approx 9$ and 20 keV using the photon-counting EIGER detector, an approach that yielded reliable results owing to the efficient harmonic rejection using the Si and Rh stripes of the mirrors at these energies. The measured fluxes agree within a factor of three with results based on ray-tracing simulations with XRT Klementiev and Chernikov (2014), assuming ideal undulator and optics.

Let us briefly discuss the available x-ray flux at ForMAX compared to competitive beamlines at third-generation synchrotron sources. In terms of SWAXS, the x-ray flux at the latter for typical experimental conditions in the enrgey range of ForMAX is generally $\approx 10^{13}$ ph s^-1 or smaller, see e.g. Smith et al. (2021). The smaller x-ray beam divergence at ForMAX allows for a photon flux (using the DCM) of up to an order of magnitude larger than these values, greatly facilitating scanning SWAXS experiments. Moreover, the MLM is available for niche experiments requiring an even higher flux. In terms of SR$\mu$CT, in turn, the flux at ForMAX (using the MLM) is comparable to one available at third-generation sources Stampanoni et al. (2006); Rau et al. (2011); Vaughan et al. (2020), albeit in an up to two orders of magnitude smaller beam cross section. We note that while the small beam size at ForMAX limits the capability of full-field imaging of large samples, as alluded to above, the very high photon density instead carries the potential for ultrafast imaging.

In this section, we present an initial quantification of the coherent properties at the ForMAX beamline. Specifically, we evaluate the effects of coherence in the formation of holographic fringes in an in-line holography experiment. We envision performing an exhaustive analysis of the coherent properties of ForMAX Goodman (1985); Vartanyants and Singer (2010) for different energies and imaging configurations, but this is out of the scope of the present paper.

We performed in-line holography at 9.1 keV, imaging a broken $\mathrm{Si_{3}N_{4}}$ membrane that exhibited several sharp edges with random orientations Dierks and Wallentin (2020). Fig. 9A depicts the hologram ($I$) recorded 19 cm downstream of the sample, using the SR$\mu$CT detection system with an effective pixel size of 0.325 $\mu$m and a response function (also known as the Point Spread Function; PSF) with a FWHM corresponding to 3 pixels ($\sigma_{\mathrm{PSF}}$=0.41 $\mu$m), as estimated in section III.1 for the 20x magnification objective. The Fourier transform of the recorded hologram ($\hat{I}$) can be written as Zabler et al. (2005)

where $f$ is the frequency, $\phi$ the wave’s phase after the object, $\lambda$ the wavelength, $z$ the propagation distance between the sample and the detector, $R$ the detector’s point-spread function, and $\gamma_{C}$ the degree of coherence. The sinusoidal term in Eq. (1) is also known as the contrast-transfer function Guigay (1977) and the visibility of its oscillations as a function of the frequency is limited by the coherence and the PSF. The power spectrum ($|\hat{I}|^{2}$) of the recorded hologram in logarithmic scale is depicted in Fig. 9B. We clearly observe an asymmetry between the visibility of the CTF oscillations in the vertical and lateral directions due to coherent effects.

For an initial quantification of the coherence effects, we performed a fit of Eq. (1) to the power spectrum, describing the PSF and the degree of coherence by a Gaussian function with the standard deviation

where $\sigma_{C}$ is due to the degree of coherence. Because of the difference in phase space of the source in the principal directions, we fitted the data independently in vertical and lateral directions as presented in Fig. 9C. On the one hand, the vertical $\sigma_{\mathrm{TOT}}\approx 0.40~{}\mu$m is comparable to $\sigma_{\mathrm{PSF}}$, implying that the blurring of the fringes in the vertical direction is dominated by the PSF. This observation is compatible with the small vertical electron source size, and hence large vertical coherence length, of Table 1. On the other hand, the lateral $\sigma_{\mathrm{TOT}}\approx 0.71~{}\mu$m corresponds to $\sigma_{C}\approx 0.58~{}\mu$m, suggesting that the lateral visibility is mainly limited by the degree of coherence. This finding is in line with the larger lateral electron source size, and hence smaller lateral coherence length, of Table 1.

A key feature of ForMAX is the possibility of zooming into hierarchical materials, as illustrated in Fig. 10. In a first instance, we acquire a high-resolution 3D image by SR$\mu$CT, yielding microscopic structural characterization of the sample. This is exemplified in Fig. 10A for a sample of aspen sapling mounted in tangential geometry. Moreover, the tomogram allows us to identify regions of interest (RoI) for scanning SWAXS mapping of nanoscale structures. In the second instance, we focus the x-ray beam onto the sample position and collect spatially resolved SWAXS data on either the RoIs or the full sample, as illustrated in Figs. 10B and C-D for SAXS and WAXS, respectively. Here, the anisotropy in the SAXS data is due to scattering from cellulose fibrils, whereas the main WAXS signal is due to diffraction from their crystalline parts. We note that whereas the SAXS and WAXS data of Figs. 10 provide access to structural properties such as microfibril size and orientation, the WAXS data of Figs. 10C and D also allow the mapping of other crystalline compounds within the sample, in this case calcium oxalate crystals. For bio-based materials, different crystalline agents are often present in the samples, and with the combination of spatial SR$\mu$CT and SWAXS data, the regions of interest within the sample can be reconstructed using various scattering contrasts.

The feature of zooming into hierarchical materials is still under development. Potential means for improving user friendliness include, e.g., a graphical user interface for selecting RoIs from the 3D SR$\mu$CT data. Nevertheless, ForMAX provides already in its present state unique means for multiscale and multimodal structural characterization of soft and/or bio-based materials in the nm to mm range.

As noted in the introduction, scanning SWAXS imaging provides structural characterization across seven orders of magnitude in length scales in a single experiment. SAXS tensor tomography (SASTT) is particularly useful for hierarchical materials, since the statistically averaged local orientation of fibrils, fibers, or filaments accessible in these experiments is directly linked to the mechanical properties of the sample. In the scope of commissioning the beamline, we acquired a SASTT dataset from carbon fiber bundles that were carefully arranged in the shape of a small knot. A similar test sample has already been used in the initial SASTT commissioning experiments at the cSAXS beamline, Swiss Light Source Liebi et al. (2015). The purpose of such a measurement is to ensure the proper mapping of 3D reciprocal space (scattering directions) and real space directions, $xy$ scanning at different rotation and tilt orientations, into the reconstruction algorithm. We successfully reconstructed the first dataset already during the beamtime, due to the readily available computing resources at the MAX IV high performance cluster (HPC).

The input for the reconstruction consists of a dataset with 276 two-dimensional projections with $55\times 76$ pixels ($x\times y$) at a pixel size of $25~{}\mu$m, computed from a total of 1.15M detector frames. Each pixel of every projection consists of detector data which was reintegrated into 32 azimuthal bins in the range of $q=0.3\dots 0.5$ nm^-1 and further symmetrically averaged to remove detector gaps. The remaining 16 azimuthal bins are used as input for the SASTT reconstruction with Mumott version 1.2 (https://zenodo.org/records/8404162). Another important step in the workflow is projection alignment. We used a computational procedure to align all projections for different orientations of the sample ($Rx=0\dots 180^{\circ}$ for $Ry=0^{\circ}$, $Rx=0\dots 360^{\circ}$ for $Ry>0^{\circ}$), that first generates a tomogram for $Ry=0^{\circ}$ and next back projects the projections of all other tilts and uses phase_cross_correlation from the skimage.registration python package for image registration and computation of the required shifts. We used the integrated dark field signal as input for the alignment procedure, due to the weak absorption signal from the carbon fibers. The same procedure was further used to mask out the sample holder and frame from some of the projections. The computed vertical and horizontal shifts are in total $\leq 300~{}\mu$m (see Fig. 11), showing that the experimental setup is very stable.

We reconstruct the 3D reciprocal space in each voxel using band-limited Friedel symmetric spherical functions expressed in spherical harmonics up to a maximum order of 6, which results in 28 coefficients for each voxel that are used to reconstruct the 3D reciprocal space. The orientation of the main structure is determined from the eigenvector associated with the smallest eigenvalue of the rank-2 tensor. We have checked the robustness of the reconstruction by visual comparison of 2D orientation, anisotropy, and degree of orientation between the measurements and simulated projections of the reconstructed data. Finally, we calculate the degree of orientation as the ratio between the mean (isotropic component) and standard deviation (RMS of anisotropic component).

We display the results of the reconstruction in Fig. 12. In Fig. 12A, we directly compare the input data of the mean intensity with a synthetic projection computed from results of the reconstruction. Since there is essentially no difference between measured and synthetic projections, which is the goal of the reconstruction, we move on to inspect the tomogram in more detail. Fig. 12B displays two central cuts, a zx and a zy slice through the tomographic reconstructed mean intensity, which gives direct insights into the arrangement of the fibers within the knot. The top left of the image exhibits a region of higher intensity where the fiber bundles from top/bottom overlap, while the opposing side of the image shows two open loops of less densely packed material. Besides the mean intensity, SASTT reconstructions also offer the unique possibility to assess the 3D reciprocal space map within each voxel, as shown in Fig. 12C for selected voxels of the tomogram (scaled with the same colormap for better comparison). The high intensity region clearly shows a ring-like reciprocal space map, which is expected for fiber-like structures. Finally, in Fig. 12D we visualize the combined information of the carbon fiber knot using the visualization software ParaView (www.paraview.org). Cylinder glyphs with fixed aspect ratio point in the direction of the carbon fibers. We use the mean intensity, a measure of the material’s density, to scale as well as color-code the glyphs. Note that we have masked the output with a 3D array taken from the mean intensity to exclude low scattering regions and mask out data from air/background voxels.

In situ rheological or mechanical testing is a common approach to address, for example, flow-induced assembly of nanoparticles into advanced materials or the relationship between structural and mechanical properties in fibrous materials. We foresee that such studies will be popular among our user community. However, while combined rheological and small-angle scattering experiments are rather mature Eberle and Porcar (2012), a deep understanding of flow-induced assembly of nanoparticle suspensions into novel hierarchical materials requires simultaneous rheological and multiscale structural characterization Kádár et al. (2021, 2023). This is particularly important for the assembly and development of new materials from biomass, for which the importance of flow cannot be overstated. Likewise, while in situ uniaxial tensile or compressional load is commonly excerted during SWAXS or SR$\mu$CT experiments, materials engineering applications may require more complex load geometries and loading profiles or extensive load cycling in well controlled temperature and relative humidity. Again, this is of great importance for biobased materials, that are viscoelastic even in their solid state.

In parallel with the construction and commissioning of ForMAX, we have therefore also developed x-ray methodology to further expand the possibilities for multiscale and multimodal structural characterization during rheological and mechanical testing. Based on this development work we can, together with our sister beamline CoSAXS Kahnt et al. (2021), provide users with the following capability:

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  Simultaneous rheological and SWAXS experiments, as exemplified in Figs. 13A and 13C for a cellulose nanocrystal suspension subjected to laminar Couette flow in a concentric polycarbonate cup-bob geometry. Other geometries, including plate-plate geometry that allows simultaneous mesoscale structural characterization by polarized light imaging (see Fig. 13D), and environmental control are also available. Finally, we note that the MLM provides the prospect of supreme temporal resolution in such studies.

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  We’re addressing the need for more complex in situ load experiments by developing combined dynamical mechanical analysis (DMA) and SWAXS in an atmosphere of controlled temperature and humidity, see Fig. 13B. Inspired by recent development of combined rheological and SR$\mu$CT experiments Dobson et al. (2020), we’re currently expanding the DMA-SWAXS experiments towards multiscale structural characterization by introducing simultaneous SR$\mu$CT capability, using the rheometer in co-rotation mode as the tomographic rotation stage.