Ferrofluid Impregnation Efficiency and Its Spatial Variability in Natural and Synthetic Porous Media: Implications for Magnetic Pore Fabric Studies

Abstract

Magnetic pore fabrics (MPF) are an efficient way to characterize pore space anisotropy, i.e., the average pore shape and orientation. They are determined by impregnating rocks with ferrofluid and then measuring their magnetic anisotropy. Obtaining even impregnation of the entire pore space is key for reliable results, and a major challenge in MPF studies. Here, impregnation efficiency and its spatial variability are systematically tested for natural (wood, rock) and synthetic (gel) samples, using oil- and water-based ferrofluids, and comparing various impregnation methods: percolation, standard vacuum impregnation, flowthrough vacuum impregnation, immersion, diffusion, and diffusion assisted by magnetic forcing. Seemingly best impregnation was achieved by standard vacuum impregnation and oil-based ferrofluid (76%), and percolation (53%) on rock samples; however, sub-sampling revealed inhomogeneous distribution of the fluid within the samples. Flowthrough vacuum impregnation yielded slightly lower bulk impregnation efficiencies, but more homogeneous distribution of the fluid. Magnetically assisted diffusion led to faster impregnation in gel samples, but appeared to be hindered in rocks by particle aggregation. This suggests that processes other than the mechanical transport of nanoparticles in the pore space need to be taken into account, including potential interactions between the ferrofluid and rock, particle aggregation and filtering. Our results indicate that bulk measurements are not sufficient to assess impregnation efficiency. Since spatial variation of impregnation efficiency may affect MPF orientation, degree and shape, impregnation efficiency should be tested on sub-samples prior to MPF interpretation.

Magnetic Properties of Ferrofluid Change Over Time: Implications for Magnetic Pore Fabric Studies

“…manuscript submitted to Journal of Geophysical Research -Solid Earth 4 (Biedermann, 2019(Biedermann, , 2020Jones et al, 2006). Further, cracks can largely affect the measured MPFs (Humbert et al, 2012;Pugnetti, Zhou, & Biedermann, 2022). Differences in magnetic susceptibilities of the used ferrofluids, and the frequency-dependence of ferrofluid susceptibility lead to further variability (Biedermann, 2019;Biedermann, Pugnetti, & Zhou, 2021;Jones et al, 2006).…”

Toward Better Understanding of the Ferrofluid Impregnation Process and Potential Artefacts—A Prerequisite for Reliable Interpretation of Magnetic Pore Fabrics

“…For oil-based ferrofluid, susceptibility-based impregnation efficiency is almost always lower than mass-based impregnation efficiency. Similar to the explanation in Pugnetti et al (2022), this may be related to increased particle aggregation and filtering of the oil-based ferrofluid, so that the oil carrier liquid enters the pore space, while the magnetic nanoparticles remain at the surface. This interpretation is further supported by the observation that the particles in oil-based ferrofluid tend to aggregate and sediment over time (Biedermann & Parés, 2022;Biedermann et al, 2021).…”

“…This impregnation method was introduced here after Pugnetti et al. (2022) observed improved impregnation in the presence of a magnetic gradient. A ferrofluid reservoir is placed at the bottom of the sample, and a strong permanent magnet (1.3 T) above the sample, in an experiment adapted from the setting by Borglin et al.…”

“…Because several authors reported difficulties impregnating the center of their samples (Almqvist et al., 2011; Robion et al., 2014), more work is needed to improve impregnation protocols. Strategies include using smaller samples (Parés et al., 2016), or improving the impregnation method itself (Pugnetti et al., 2022). A major challenge is that ferrofluid magnetic properties change over time, i.e., changes in susceptibility and hysteresis properties associated with particle aggregation and sedimentation have been observed (Biedermann & Parés, 2022; Biedermann et al., 2021).…”

Quantitative Comparison of 3D Pore Space Properties With Magnetic Pore Fabrics—Testing the Ability of Magnetic Methods to Predict Pore Fabrics in Rocks

“…Pores and throats smaller than magnetic nanoparticles (10–20 nm diameter) are not captured by MPF. In practice, the threshold of pores that are impregnated depends on pore throat geometry and wettability (Robion et al., 2014), and 100 nm has been put forward as a more realistic threshold (Pugnetti et al., 2022). Empirical relationships have been established between MPFs and pore fabrics: (a) the maximum and minimum principal susceptibility axes are sub‐parallel to the average orientations of major and minor pore axes (Hrouda et al., 2000; Jezek & Hrouda, 2007; Jones et al., 2006; Pfleiderer & Halls, 1990, 1994), (b) the degree of magnetic anisotropy increases when pore shapes become more anisotropic (Jones et al., 2006; Pfleiderer & Halls, 1990, 1993; Robion et al., 2014), and (c) oblate or prolate MPFs are related to flattened or elongated pore shapes (Jones et al., 2006; Pfleiderer & Halls, 1990).…”