非牛顿流体剪切稀化特性的分子动力学模拟

Molecular dynamics simulation on shear thinning characteristics of non-Newtonian fluids

非牛顿流体在微流控系统及生物医学等领域具有广泛应用.采用分子动力学模拟方法,以羧甲基纤维素钠分子和水分子构成的系统为研究对象,模拟了不同剪切速度下羧甲基纤维素钠溶液的构型演变,采用溶质分子的均方位移和近表面水分子层的相对剪切速度表征溶液黏度的变化,着重分析了氢键作用对溶液黏度变化的影响.结果表明:羧甲基纤维素钠溶液中水分子与溶质分子相互吸引形成更致密的氢键网络,使得溶液黏度增加.当对溶液施加剪切作用时,溶质分子上与碳相连的氢原子与水氧原子之间的径向分布函数峰值减小,二者形成的氢键作用减弱.在剪切作用下,溶质分子运动加剧,水分子对溶质分子运动的阻碍作用减小;同时距剪切边界越近的水分子的移动速度越接近剪切速度,随着距离增加,水分子层的移动速度大幅度减小;以上结果在宏观上可表现为体系的黏度降低.剪切速度增加,羧甲基纤维素钠溶液的剪切稀化现象愈加显著.

Non-Newtonian fluids are widely used in microfluidic systems and biomedical fields.In this paper,based on molecular dynamics simulation,taking the system composed of sodium carboxymethyl cellulose molecules and water molecules as the research object,the configuration evolutions of sodium carboxymethyl cellulose solution are simulated under different shear rates.Change of the solution viscosity is characterized by mean square displacement of sodium carboxymethyl cellulose molecules and the relative velocity between water layer and shear boundary.The effect of hydrogen bonding on the viscosity of the solution is analyzed emphatically.The results show that water molecules and solute molecules attract each other to form a more compact hydrogen bond network,which increases the viscosity of the solution;the peak value of the radial distribution function between the hydrogen atoms attached to carbon and the water oxygen atoms decreases when shear action is applied to the solution,and the hydrogen bond between the two atoms is weakened;the mobility of solute molecules increases and the blocking effect of water molecules on the movement of solute molecules weakens under the shear action;at the same time,the shorter the distance to the shear boundary,the closer to the shear velocity the velocity of water molecules is,and with the increase of distance,the velocity of water molecular layer decreases greatly.These results are macroscopically understood as the viscosity of the system decreasing.As the shear rate increases,the shear thinning of the sodium carboxymethyl cellulose solution becomes more significant.