Institute of Continuous Media Mechanics-The Division of Perm Federal Research Center, Russian Academy of Sciences, Ural Branch, Perm 614013, Russia.
J Chem Phys. 2017 Sep 28;147(12):124903. doi: 10.1063/1.4989752.
The theory of orientational motion of a Brownian magnetic nanoparticle embedded in a viscoelastic medium and subjected to a time-dependent uniform magnetic field is developed. The rheology of the viscoelastic environment of the particle is modeled by the Jeffreys scheme, which under variation of a minimal number of parameters is able to resemble a wide range of soft materials: from a weakly structured (nearly Newtonian) polymer solution to a gel. It is shown that in the Jeffreys model, the diffusional orientational motion of a particle is a combination of two modes, which could be associated with a fast motion within the polymer mesh cell and a slow displacement that involves deformation of the mesh, respectively. The dependencies of the reference times of both relaxation modes on the Jeffreys viscous and elastic parameters and temperature are found. It turns out that in substantially viscoelastic media, the rate of the slow mode (it dominates in relaxation) quadratically depends on the matrix temperature. This effect does not have analogs in linearly viscous systems. For an ensemble of magnetic nanoparticles in viscoelastic and gel Jeffreys matrices: (1) the dynamic magnetic susceptibility is derived and evaluated both within an exact approach and in a simple approximation; (2) the problem of magnetic relaxometry, i.e., evolution of magnetization after step-wise turning off the field, is solved; (3) the specific power loss caused by viscous dissipation generated by the particles under an ac field is analyzed as a function of the rheological parameters. Results (1) and (2) provide simple models for magnetic nanorheology; consideration (3) advances the physics of magnetic hyperthermia in viscoelastic and gel-like media.
本文发展了一种理论，用于描述嵌入黏弹性介质中的布朗磁纳米粒子在外加随时间变化的均匀磁场中的定向运动。该粒子黏弹性环境的流变学通过杰弗里斯方案建模，该方案通过变化最小数量的参数，能够模拟广泛的软物质：从弱结构（近牛顿）聚合物溶液到凝胶。结果表明，在杰弗里斯模型中，粒子的扩散定向运动是两种模式的组合，分别与聚合物网格单元内的快速运动和涉及网格变形的缓慢位移有关。发现了两种弛豫模式的参考时间与杰弗里斯黏性和弹性参数以及温度的依赖关系。结果表明，在显著黏弹性介质中，慢模式（在弛豫中占主导地位）的速率与基质温度呈二次方关系。这种效应在线性黏性系统中没有类似物。对于黏弹性和凝胶杰弗里斯基质中的磁性纳米粒子集合：（1）推导并评估了动态磁化率，分别采用精确方法和简单近似方法；（2）解决了磁弛豫计问题，即在逐步关闭磁场后磁化强度的演化；（3）分析了由于粒子在交流场中产生的粘性耗散而引起的特定功率损耗作为流变学参数的函数。结果（1）和（2）提供了用于磁纳米流变学的简单模型；考虑（3）推进了黏弹性和类凝胶介质中磁热疗的物理。
