... transport processes in porous materials, e.g. liquid drying, slow liquid flow or fast gas flow. Hardware. ⢠200mm-bore 7T horizontal NMR scanner (Bruker).
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NMR Methods for Characterization of Mass Transport in Porous Materials Scientists: Li Huang, Wolfgang Dreher In-vivo-MR Group, Faculty 02 (Biology/Chemistry), University of Bremen
Objectives NMR imaging technique can non-invasively measure the spatial distribution of fluids in porous materials. For opaque heterogeneous systems, NMR imaging is more versatile than other imaging techniques (e.g., X-ray µCT) by providing a variety of adaptable methods for different applications. This project aims at methodological improvements for characterizing mass transport processes in porous materials, e.g. liquid drying, slow liquid flow or fast gas flow.
Methods Suppression of background signals in Zero Echo Time NMR imaging Outer Volume Suppression module in NMR pulse sequence [1] 38mm-bore coil with 1H-free materials in manufacturing [2] Results Image artifacts due to background signals can be effectively suppressed (Fig. 1) and thus serial images can correctly reveal evolution of water filling within porous structures (Fig. 2). 1 On the Suppression of Background Signals originating from NMR Hardware Components. Application to Zero Echo Time Imaging and Relaxation Time Analysis. W. Dreher, I. Bardenhagen, L. Huang, M. Bäumer. Magn. Reson. Imag. 2016. 34(3):264-270. 2 Characterizing Macroscopic Mass Transport in Porous Media by Zero Echo Time MRI. L. Huang, W. Dreher. 13th International Conference on Magnetic Resonance Microscopy. 02.06.08.2015, Munich, Germany. Poster 097.
NMR Velocimetry for Slow Liquid Flow
Fig. 1. Background signals before (a) and after (b) Outer Volume Suppression module.
Methods Construction of a 22mm-bore coil Increased SNR Optimized use of vertical space in the scanner bore Modification of existing fast and robust RARE Phase Contrast NMR velocimetry pulse sequences Echo combination for better SNR and higher velocimetry accuracy Results Accurate 3D velocity maps with 3D sub-pore-scale spatial resolution of water flowing slowly (submillimeters to millimeters per second) through porous filters can be acquired within a few hours (Fig. 3). Velocity maps of slow water flow can be correlated to X-ray µCT images of particle deposition in porous filters for deep bed filtration studies (collaboration 1). Manuscript in preparation (to be submitted to J. Magn. Reson.) Fig. 3. Vertical NMR image of water with velocity vector fields (a) and an enlarged view of selected region (b). Slice-wise vertical volumetric flow rates (c). Transversal NMR image of water with velocity vector fields (d) and an enlarged view of an selected region (e). Slice-wise transversal volumetric flow rates (f).
Fig. 2. Photo of porous sample (a). Serial NMR images of water filling in sample (b-e) in different time points. Images of evaporated water (f-h) by subtracting subsequent images (c-e) from initial image (b).
NMR Velocimetry for Fast Gas Flow Methods A single Spin Echo Phase Contrast NMR velocimetry pulse sequence was optimized with respect to SNR considering signal losses by transversal relaxation and molecular diffusion effects. Results 3D velocity maps with 3D pore-scale spatial resolution of CH 4 gas flowing rapidly (decimeters per second) through porous sponges show streaming paths (Fig. 4), which will be compared to CFD simulations (collaboration 2). Manuscript in preparation Fig. 4. NMR image of CH4 gas with velocity vector fields overlaid.
Outlook for Successive Project (M. Mirdrikvand) Improved quantitative imaging of liquid density by Zero Echo Time NMR imaging considering off-resonance effect for increased spatial resolution Optimized 3D spectroscopic imaging for characterizing catalytic gas reactions (increased SNR, reduced echo time, reduced measurement time) Further improvements of NMR velocimetry applied to liquid or gas flow in porous media and verification by CFD simulation Characterization of diffusion and flow by NMR displacement imaging (“qspace imaging”) Evaluation of magnetization transfer imaging for characterizing reaction processes (e.g. catalyzed reduction of H2O2 with enzyme catalase)
Collaborations 1.Liquid velocimetry for deep bed filtration: G. Mikolajczyk, S. Odenbach 2.Gas velocimetry for CFD study: L. Kiewidt, J. Ulpts, J. Thöming
