Sumy State University, 2 Rimsky-Korsakov Street, UA-40007 Sumy, Ukraine.
Phys Rev E. 2018 May;97(5-1):052611. doi: 10.1103/PhysRevE.97.052611.
Ferrofluid heating by an external alternating field is studied based on the rigid dipole model, where the magnetization of each particle in a fluid is supposed to be firmly fixed in the crystal lattice. Equations of motion, employing Newton's second law for rotational motion, the condition of rigid body rotation, and the assumption that the friction torque is proportional to angular velocity are used. This oversimplification permits us to expand the model easily: to take into account the thermal noise and interparticle interaction that allows us to estimate from unified positions the role of thermal activation and dipole interaction in the heating process. Our studies are conducted in three stages. The exact expressions for the average power loss of a single particle are obtained within the dynamical approximation. Then, in the stochastic case, the power loss of a single particle is estimated analytically using the Fokker-Planck equation and numerically using the effective Langevin equation. Finally, the power loss for the particle ensemble is obtained using the molecular dynamics method. Here, the local dipole fields are calculated approximately based on the Barnes-Hut algorithm. The revealed trends in the behavior of both a single particle and the particle ensemble suggest the way of choosing the conditions for obtaining the maximum heating efficiency. The competitiveness character of the interparticle interaction and thermal noise is investigated in detail. Two situations, when the thermal noise rectifies the power loss reduction caused by the interaction, are described. The first of them is related to the complete destruction of dense clusters at high noise intensity. The second one originates from the rare switching of the particles in clusters due to thermal activation, when the noise intensity is relatively weak. In this way, the constructive role of noise appears in the system.
基于刚性偶极子模型研究了外加强磁场中磁流体的加热，其中假定流体内每个粒子的磁化强度都牢固地固定在晶格中。运动方程采用牛顿第二定律的转动定律、刚体转动条件和摩擦力矩与角速度成正比的假设。这种简化允许我们轻松扩展模型：考虑热噪声和粒子间相互作用，从而能够从统一的角度估计热激活和偶极相互作用在加热过程中的作用。我们的研究分三个阶段进行。在动力学近似下，获得了单个粒子的平均功率损耗的精确表达式。然后，在随机情况下，使用福克-普朗克方程和有效朗之万方程对单个粒子的功率损耗进行了分析估计。最后，使用分子动力学方法获得了粒子系综的功率损耗。在这里，基于 Barnes-Hut 算法近似计算局部偶极场。对单个粒子和粒子系综的行为的趋势进行了研究，提出了获得最大加热效率的条件选择方法。详细研究了粒子间相互作用和热噪声的竞争特性。描述了两种情况，即热噪声纠正了相互作用导致的功率损耗降低的情况。第一种情况与高噪声强度下密集簇的完全破坏有关。第二种情况源于由于热激活导致的粒子在簇中的罕见切换，此时噪声强度相对较弱。通过这种方式，噪声在系统中发挥了建设性作用。
