Magnetic properties of ferrofluid change over time: Implications for magnetic pore fabric studies

The anisotropy of magnetic susceptibility of ferrofluid-impregnated samples is an efficient and powerful proxy for pore space anisotropy and preferred flow directions. One of the main assumptions in pore fabric studies is that all pores >10-20 nm are homogeneously filled with ferrofluid, and that the ferrofluid has constant properties throughout the pore space and over time. If only part of the pore space is filled, this can lead to artifacts. Additionally, because magnetic anisotropy of a given pore space depends on fluid susceptibility, quantitative interpretations may be affected by the interval between impregnation and measurement time, or by the age of the ferrofluid during impregnation, unless fluid properties remain constant. A careful investigation of the time variation of ferrofluid properties and magnetic pore fabrics in synthetic and natural samples shows time-dependence of susceptibility and hysteresis properties, related to dissolution of particle surfactants, and deterioration of colloidal stability. The latter leads to particle aggregation and sedimentation, which changes the anisotropy properties, and also affects impregnation behavior. Natural samples impregnated by oil-based ferrofluid also experience a 2.5-3-fold increase in mean susceptibility, with important consequences for the susceptibility-based determination of impregnation efficiency. Based on our results, we recommend that ferrofluid properties are determined at the time of impregnation, and that samples are measured shortly after impregnation.