Laboratoire de Génie Chimique, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France.
Laboratoire de Physique de la Matière Condensée, Nice, France.
Phys Rev E. 2017 Jun;95(6-1):062611. doi: 10.1103/PhysRevE.95.062611. Epub 2017 Jun 29.
In applications where magnetic particles are used to detect and dose targeted molecules, it is of major importance to prevent particle clustering and aggregation during the capture stage in order to maximize the capture rate. Elongated ferromagnetic particles can be more interesting than spherical ones due to their large magnetic moment, which facilitates their separation by magnets or the detection by optical measurement of their orientation relaxation time. Under alternating magnetic field, the rotational dynamics of elongated ferromagnetic particles results from the balance between magnetic torque that tends to align the particle axis with the field direction and viscous torque. As for their translational motion, it results from a competition between direct magnetic particle-particle interactions and solvent-flow-mediated hydrodynamic interactions. Due to particle anisotropy, this may lead to intricate translation-rotation couplings. Using numerical simulations and theoretical modeling of the system, we show that two ellipsoidal magnetic particles, initially in a head-to-tail attractive configuration resulting from their remnant magnetization, can repel each other due to hydrodynamic interactions when alternating field is operated. The separation takes place in a range of low frequencies f_{c1}<f<f_{c2}. The upper frequency limit f_{c2}τ_{r}≈0.04 (where τ_{r} is the rotation time scale) depends weakly on the ratio of magnetic field to particle magnetization strength, whereas f_{c1} tends to zero when this ratio increases.
在应用中，磁性粒子被用于检测和计量靶向分子，在捕获阶段防止粒子的聚集和团聚对于最大化捕获率非常重要。由于长形铁磁粒子具有较大的磁矩，因此它们比球形粒子更有趣，这有利于通过磁铁分离或通过光学测量其取向弛豫时间来检测。在交变磁场下，长形铁磁粒子的旋转动力学是由磁转矩与粘性转矩之间的平衡产生的，磁转矩倾向于使粒子轴与磁场方向对齐。对于它们的平移运动，则源于直接磁粒子-粒子相互作用和溶剂流介导的流体动力相互作用之间的竞争。由于粒子各向异性，这可能导致复杂的平移-旋转耦合。通过对系统进行数值模拟和理论建模，我们表明，两个初始处于由于剩余磁化而头尾相连的吸引力配置的椭球形磁性粒子，由于交变磁场作用下的流体动力相互作用，可能会相互排斥。分离发生在低频率范围 f_{c1}<f<f_{c2}。上限频率 f_{c2}τ_{r}≈0.04（其中 τ_{r}是旋转时间尺度）对磁场与粒子磁化强度之比的依赖性较弱，而当该比值增加时，f_{c1}趋于零。
