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.

中文翻译：

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铁磁流体的磁性随时间变化：对磁孔织物研究的意义

铁磁流体浸渍样品的磁化率各向异性是孔隙空间各向异性和优选流动方向的有效且有力的代表。孔隙结构研究的主要假设之一是所有 >10-20 nm 的孔隙都均匀地充满了铁磁流体，并且铁磁流体在整个孔隙空间和随着时间的推移具有恒定的特性。如果仅填充部分孔隙空间，则可能导致伪影。此外，由于给定孔隙空间的磁各向异性取决于流体磁化率，定量解释可能会受到浸渍和测量时间之间的间隔或浸渍期间铁磁流体的年龄的影响，除非流体特性保持不变。对合成和天然样品中铁磁流体特性和磁孔织物的时间变化的仔细研究表明，磁化率和磁滞特性随时间变化，与颗粒表面活性剂的溶解和胶体稳定性的恶化有关。后者导致颗粒聚集和沉降，从而改变各向异性性质，也影响浸渍行为。用油基铁磁流体浸渍的天然样品的平均磁化率也增加了 2.5-3 倍，这对基于磁化率的浸渍效率测定具有重要意义。根据我们的结果，我们建议在浸渍时确定铁磁流体的特性，并在浸渍后不久测量样品。