Rheology of a dilute ferrofluid droplet suspension in shear flow: Viscosity and normal stress differences
Rheology of a dilute ferrofluid droplet suspension in shear flow: Viscosity and normal stress differences
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DOI：
10.1103/physrevfluids.5.123603
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发表时间：
2020-12
影响因子：
2.7
通讯作者：
S. Ishida;D. Matsunaga
中科院分区：
文献类型：
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作者：
S. Ishida;D. Matsunaga
我们使用三维晶格玻尔兹曼模拟和相场模型报告了简单剪切流下稀铁磁流体液滴悬浮液的流变学。在我们的模拟中，我们利用12M计算网格来完全解决液滴变形，并使用GPU并行化来加速计算。液滴变形由背景剪切流和外部磁场效应决定。铁磁流体液滴具有沿外场方向拉长的特性，并且向系统施加均匀的静磁场来控制液滴形状。通过改变外部磁场强度和方向，我们发现悬浮液的流变性可以得到极大的改变。当外场作用于速度梯度方向（速度方向）时，粘度随外场增加（减少）。仅通过施加外部磁场，比粘度就变为无外部磁场下粘度的12∼620%。磁力对于控制法向应力也很实用，因为当磁场施加到第 i 方向时，第 i 方向的法向应力会减小。因此，为了增大（减小）第一法向应力差N 1 ，应在速度方向（速度梯度方向）施加外部磁场。为了增大（减小）第二法向应力差N 2 ，应在速度梯度方向（涡度方向）施加外部磁场。通过施加磁场，我们还表明，在小雷诺数条件下，法向应力 N 1 、 N 2 甚至表现出与正常液滴溶液相反的符号（N 1 > 0、N 2 < 0）。我们的工作表明，铁磁流体液滴将成为一种实用的复杂流体，只需改变外部磁场强度和方向即可控制悬浮特性。
We report the rheology of a dilute ferrofluid droplet suspension under simple shear flow, using the three-dimensional lattice-Boltzmann simulation and the phase-field model. In our simulation, we utilize 12M computational grids to fully resolve the droplet deformation, and GPU parallelization is used to speed up the computation. The droplet deformation is determined by both the background shear flow and the external magnetic field effects. The ferrofluid droplet has a character to elongate in the direction of the external field, and a uniform static magnetic field is applied to the system to control the droplet shape. By changing the external field strength and direction, we found that the suspension rheologies can be drastically modified. The viscosity increase (decrease) with the external field when the external field is applied to the velocity gradient direction (velocity direction). Just by imposing the external magnetic field, the specific viscosity becomes 12 ∼ 620% of the viscosity under no external magnetic field. The magnetic force is also practical to control the normal stresses, since the normal stress in i th direction decreases when the magnetic field is applied to the i th direction. Therefore, in order to increase (decrease) the first normal stress difference N 1 , the external magnetic field should be applied to the velocity direction (velocity gradient direction). To increase (decrease) the second normal stress difference N 2 , the external magnetic field should be applied to the velocity gradient direction (vorticity direction). By applying the magnetic field, we also show that the normal stresses N 1 , N 2 even show opposite sign from the normal droplet solution ( N 1 > 0, N 2 < 0) under small-Reynolds-number conditions. Our work suggests that the ferrofluid droplet would be a practical complex fluid to control the suspension properties, just by changing the external magnetic field strength and directions.