活性流体流变行为的布朗动力学模拟研究

BROWNIAN DYNAMICS SIMULATION OF RHEOLOGICAL BEHAVIOR OF ACTIVE FLUIDS

活性流体在用于开发新材料方面具有巨大潜力,满足这一需求就要定量掌握活性流体所表现的特殊力学行为,特别是流变行为.扩展布朗运动方程,建立自驱动活性粒子的运动模型,基于反向非平衡法确定活性流体的黏度,考察活性粒子体积分数、直行速度和转向扩散系数对活性流体流变行为的影响规律,确定活性流体特殊流变行为的形成机理.结果表明,活性流体的流变曲线可被划分为黏度下降区、过渡区和牛顿区;活性粒子体积分数越高,活性流体的非牛顿特性越显著,活性粒子的直行运动引起活性流体在低剪切速率区域黏度下降,直行运动和转向运动的耦合作用导致中剪切速率区域流变曲线非单调变化,活性粒子频繁发生转向运动会导致活性流体非牛顿特性受到抑制;活性流体的宏观流变学特性和粒子的涨落直接相关,活性粒子体积分数越高、直行速度越快和转向扩散系数越小,活性流体中活性粒子越容易产生显著的涨落;低剪切速率区域内活性粒子涨落明显,随着剪切速率增大,活性粒子的涨落逐渐被削弱,粒子的聚集结构不断被破坏,最终体系的流变行为类似一般被动流体.

Active fluids hold great potential for the development of new materials, but realizing this potential requires a quantitative understanding of the mechanical behavior that these fluids exhibit, especially the rheological behavior. The Brownian motion equation is extended to establish the kinematic model of self-propelled particles. The viscosity of active fluid is determined based on the reverse non-equilibrium molecular dynamics scheme. The effects of volume fraction,forward locomotion velocity and rotational diffusion coefficient of active particles on the rheological behavior of active fluid are investigated, and the formation mechanism of special rheological behavior of active fluid is determined. The results show that the rheological curve of the active fluid can be divided into viscosity reduction regime, transition regime and Newtonian regime. The higher the volume fraction of the active particle is, the more significant the non-Newtonian properties of the active fluid are. The forward locomotion of the active particles leads to the reduction of the viscosity of the active fluid in the low shear rate region. The coupling effect of forward locomotion and rotational locomotion leads to the non-monotonic change of the rheological curve in the moderate shear rate region, and the frequent rotational locomotion of the active particles leads to the inhibition of the non-Newtonian properties of the active fluids. The fluctuation of active particles makes the active fluids have special rheological behavior. The higher the volume fraction of active particles, the faster the forward locomotion velocity and the smaller the rotational diffusion coefficient are, the easier the active particles produce obvious fluctuations in the active fluid is. The fluctuation of active particles is obvious in the low shear rate region. With the increase of shear rate, the fluctuation of active particles is gradually weakened, and the aggregation structure of particles is constantly destroyed. Finally, the rheological behavior of the system is similar to that of the general passive fluids.