The European Physical Journal E ( IF 1.8 ) Pub Date : 2022-05-05 , DOI: 10.1140/epje/s10189-022-00200-7 Viesturs Sints 1 , Mitradeep Sarkar 2 , Jesse Riedl 2 , Gilles Demouchy 2, 3 , Emmanuelle Dubois 2 , Régine Perzynski 2 , Dmitry Zablotsky 1 , Gunars Kronkalns 1 , Elmars Blums 1
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Abstract
The effect of an excess of surfactant on the thermophoresis of a sterically stabilized ferrofluid is investigated experimentally by forced Rayleigh scattering (FRS). The experiments are performed with a stable magnetic fluid sample to which controlled amounts of surfactant are added. A decrease in the thermally induced transport of magnetic nanoparticles is observed while increasing the temperature T. The positive Soret coefficient \(S_{\mathrm{T}}\) decreases by adding 2 vol% of surfactant at room temperature. As shown by FRS relaxation, this decreasing is mainly associated with a reduction of the interaction between the carrier fluid and individual nanoparticles. No significant effect of extra surfactant on the sign of \(S_{\mathrm{T}}\) is observed at higher T’s (up to \(\sim \,85\,^{\circ }\)C). Dynamic light scattering at room temperature reveals the presence of a small amount of clusters/aggregates in the samples, which are hardly detectable by FRS relaxation. The presence of these small clusters/aggregates is confirmed by a rheological probing of the fluid properties. Whatever T, a small amount of added surfactant first causes a decrease of the ferrofluid viscosity, associated with a 10% decreasing of the flow activation energy. Further on, viscosity and activation energy both recover at higher excess surfactant concentrations. These results are analyzed in terms of saturation of the surfactant layer, concentration of free surfactant chains and heat of transport of the nanoparticles.
Graphic abstract
中文翻译：
表面活性剂过量对油性表面活性剂稳定的铁磁流体的热泳、质量扩散和粘度的影响
摘要
通过强制瑞利散射 (FRS) 实验研究了过量表面活性剂对空间稳定铁磁流体的热泳的影响。实验是使用添加了受控量表面活性剂的稳定磁流体样品进行的。在增加温度T的同时观察到磁性纳米颗粒的热诱导传输减少。正 Soret 系数\(S_{\mathrm{T}}\)通过在室温下添加 2 vol% 的表面活性剂而降低。如 FRS 弛豫所示，这种降低主要与载液和单个纳米颗粒之间相互作用的降低有关。额外的表面活性剂对\(S_{\mathrm{T}}\)的符号没有显着影响在更高的T处观察到（直到\(\sim \,85\,^{\circ }\) C）。室温下的动态光散射显示样品中存在少量簇/聚集体，FRS 弛豫几乎无法检测到这些簇/聚集体。这些小簇/聚集体的存在通过流体特性的流变探测得到证实。无论T多少，添加的少量表面活性剂首先会导致铁磁流体粘度降低，这与流动活化能降低 10% 相关。此外，粘度和活化能都在较高的过量表面活性剂浓度下恢复。根据表面活性剂层的饱和度、游离表面活性剂链的浓度和纳米粒子的传输热分析这些结果。
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