Magnet C, Kuzhir P, Bossis G, Meunier A, Nave S, Zubarev A, Lomenech C, Bashtovoi V
University of Nice-Sophia Antipolis, CNRS, Laboratory of Condensed Matter Physics, UMR 7336, 28 avenue Joseph Vallot, 06100 Nice, France.
Department of Mathematical Physics, Ural Federal University, 51 Prospekt Lenina, Ekaterinburg 620083, Russia.
Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Mar;89(3):032310. doi: 10.1103/PhysRevE.89.032310. Epub 2014 Mar 31.
When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, the size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in detail. In experiments, a dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50 μm) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces-the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter α≥2), the Brownian motion seems not to affect the cloud behavior.
当将微米级可磁化颗粒引入纳米级磁性颗粒的悬浮液中时，纳米颗粒会在微粒周围聚集，并形成沿外加磁场方向延伸的厚各向异性云状物。这种现象促进了双峰磁性悬浮液的胶体稳定性，并允许对用于生物分析和水净化的纳米颗粒进行高效磁分离。在本工作中，详细研究了在外部均匀磁场和流体同时作用下纳米颗粒云的尺寸和形状。在实验中，将平均直径为60nm的氧化铁纳米团簇的稀悬浮液推过一个薄狭缝通道，通道壁上附着有平均直径为50μm的镍微球。使用光学显微镜在稳态下观察纳米团簇云的行为。在足够强的流体存在下，在纵向和横向磁场中，云的尺寸都随着流速的增加而单调减小。这可以定性地解释为流体动力增强，将纳米团簇从云中冲走。在纵向磁场中，流体导致前后云的不对称。为了解释流体和磁场对云的影响，我们基于云表面应力和颗粒通量的平衡开发了一个简单模型。该模型应用于磁场与流体平行的情况，能够较好地捕捉流体对云的尺寸和形状的影响，并揭示出控制云尺寸的唯一无量纲参数是流体动力与磁力之比——梅森数。在本工作中考虑的强磁相互作用（偶极耦合参数α≥2）下，布朗运动似乎不影响云的行为。
