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With support from this grant, we have obtained the following three achievements in the past funding period. The first is two-dimensional imaging of flow in porous metallic materials using optically-detected MRI, with a spatial resolution of nearly 1 mm and a temporal resolution of 0.1 s. The highest signal appears when the entire sample of the porous material is occupied by the fluid. The corresponding signal amplitude is a measurement of effective storage volume for the porous material, which is different from the total pore volume of the material. The concept of effective storage volume is more relevant to practical applications than total pore volume because the former represents the accessible portion of the overall pore volume. The time-resolved flow images indicate how fast fluids stored in the porous material can be extracted. It is interesting that our results show fluids in porous metals with large pore sizes, for example 100-micron diameter, have a faster evacuating time than they have in void volume. This shows the difference in flow pattern in porous materials from laminar flow.

The second achievement is that we measured nuclear relaxation time of protons as a function of average pore size. This study is carried out by varying the flow rate under continuous-flow condition. Nuclear relaxation time has never been studied before due to lack of an applicable technique. An abnormally long relaxation time is observed for a steel sample with average pore size of 5-micron diameter (specified by the manufacturer). Our measurements are valuable for modeling nuclear relaxation in long and connected pores.

The third achievement is the investigation of nuclear relaxation enhancement by a gadolinium chelate, Gd(TTHA) (TTHA: triethylenetriamine hexaacetic acid). We discovered that in the Earth’s magnetic field of 50 microtesla, the contrast has a strong and monotonic pH-dependence. In a comparative study, the relaxation enhancement by commonly-used Gd(DTPA) (DTPA: diethylenetriamine pentaacetic acid) is found not responsive to different pH values. This result will allow Gd(TTHA) to be used as a molecular probe for pH properties of porous materials, where electrical and optical probes are not applicable. Our study is consistent with existing studies of Gd(TTHA) and Gd(DTPA) in high magnetic fields, but provides more quantitative information that can be used for pH probing.

These results have significant impact on my career. We have already published some of the results in a prestigious journal, Optics Letters. This paper is the only one so far on optically-detected MRI that comes from a group other than my postdoctoral’s group. It clearly demonstrated my capability of independently carrying out research on the technique that I invented as a postdoctoral fellow. The results are also a significant improvement over my previous work at Berkeley, in terms of both detection sensitivity and instrument capability. Two more manuscripts are in final preparation stage, which will be attributed primarily to this grant. Furthermore, these achievements lay a solid foundation for my group to further pursue more complicated and detailed projects in the field of optical detection of MRI.

This grant also has important impact on the students involved in related projects. Two students worked full-time during the summer, which was only possible with this financial support. Each of them has played a leading role in their respective project, with the achievements mentioned above. Their improved skills during this period of training will be a decisive factor towards their PhD pursuing. In addition, a third graduate student, who was supported in the previous year, successfully earned her master’s degree this year.

Furthermore, this grant has played a major role in the relatively new field of optically-detected MRI. Our results have convincingly shown the two distinctive advantages of low-field MRI, which are improved penetration through metallic materials and enhanced contrast as a function of local environment. The low sensitivity of conventional Faraday detection in low fields is overcome by using atomic magnetometers. Porous metallic materials can now be studied for their storage capacity, flow dynamics, pore size distribution, and pH property inside the pores. These parameters are essential to the widespread applications of this type of materials in energy-related fields. As for the technique, optically-detected MRI is inexpensive, portable, and does not require liquid helium and nitrogen. These properties will facilitate its applications under various conditions. With the results that are made possible by this grant, optically-detected MRI technique will be a unique tool to study fluids in porous materials.