Thermodiffusion in ferrofluids regarding thermomagnetic convection
Thermodiffusion in ferrofluids regarding thermomagnetic convection
复制标题
关于热磁对流的铁磁流体中的热扩散
DOI：
10.1016/j.crme.2013.02.005
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发表时间：
2013
期刊：
影响因子：
1.8
通讯作者：
S. Odenbach
中科院分区：
文献类型：
--
作者：
Lisa Sprenger;A. Lange;S. Odenbach
磁性液体通常由分散在煤油、油或水等载体液体中的磁铁矿或钴纳米颗粒组成。这些颗粒的直径接近10 nm，并被覆盖了约2 nm的表面活性剂层[1，2]。表面活性剂需要保持颗粒的稳定分散，并与特定的载液相匹配。在零磁场的情况下，流体不具有磁性，因为单域粒子的磁矩是随机分布的。当流体暴露在外部磁场中时，这些磁矩与磁场方向对齐，从而使流体变得磁化。这种行为被称为超顺磁性，其特征是磁化曲线表示磁化程度与磁场强度的关系[1，2]。这一曲线可以在实验上测量，并提供了磁性液体的特征值，如颗粒的平均磁性直径、饱和磁化强度和由后者得出的体积浓度。在该上下文中，热磁对流表示由磁性液体层中的空间变化的磁化作用驱动的传输现象。垂直变化是由施加到层的上边界和下边界的温差引起的。磁化过程中出现的梯度导致流体内部磁场的梯度。流体中的微小扰动，如体积元素的绝热位错，就会导致位错元素的磁化强度与其周围的磁化强度不同。这种差异与内部磁场梯度相互作用，所产生的力指向初始扰动的方向[3-7]。已经进行了测量[6，7]，目的是确定在固定磁场强度下在磁流体层中启动对流所需的临界温差。通过测量该层上边界的热通量来检测该层的起始位置。从传导到对流的热传输机制的变化产生了增强的通量[4-6]。当磁场强度为25kA/m，磁场方向与温差平行时，与油基磁流体的零场情况相比，在较低的临界温差处发生对流[6]。因此，假设磁场增强了对流。在相同的装置中，使用煤油基磁流体，抑制对流
Magnetic fluids are usually composed of magnetite or cobalt nanoparticles dispersed in a carrier liquid such as kerosene, oil or water. The particles have diameters close to 10 nm, and are coated with a surfactant layer of about 2 nm [1, 2]. The surfactant is needed to keep the particles stably dispersed, and is matched with the specific carrier fluid. In the case of a zero magnetic field, the fluid does not behave magnetically, since the magnetic moments of the single-domain particles are distributed stochastically. When the fluid is exposed to an external magnetic field, these magnetic moments align with the field direction so that the fluid becomes magnetised. This behaviour is called super-paramagnetic, and is characterised by the magnetisation curve indicating the dependence of the magnetisation on the strength of the magnetic field [1, 2]. This curve can be measured experimentally and provides characteristic values of the magnetic fluid such as the average magnetic diameter of the particles, the saturation magnetisation, and the volume concentration derived from the latter. Thermomagnetic convection in that context denotes a transport phenomenon driven by a spatially varying magnetisation in a layer of a magnetic fluid. A vertical variation is caused by a temperature difference applied to the upper and lower boundaries of the layer. The occurring gradient in the magnetisation results in a gradient in the internal magnetic field of the fluid. A small perturbation in the fluid such as the adiabatic dislocation of a volume element then leads to a difference in the magnetisation of the dislocated element with its surrounding. This difference interacts with the internal magnetic field gradient and the resulting force is directed in favour of the direction of the initial perturbation [3–7]. Measurements [6, 7] have been carried out with the aim to determine the critical temperature difference needed at fixed magnetic field strengths to start convection in a layer of ferrofluid. The onset is detected by measuring the heat flux over the layer’s upper boundary. A change in the mechanism of heat transport from conductive to convective yields an enhanced flux [4–6]. At field strength of 25 kA/m and a parallel orientation of the magnetic field to the temperature difference, convection sets in at a lower critical temperature difference than in the zero field case for an oil-based ferrofluid [6]. Therefore, it is assumed that the magnetic field enhances convection. In the same setup using a kerosene-based ferrofluid, suppression of convection