微通道疏水表面滑移的耗散粒子动力学研究

了解疏水表面的滑移规律对其在流动减阻方面的应用至关重要.利用耗散粒子动力学(dissipative particle dynamics, DPD)方法研究了微通道疏水表面的滑移现象.采用固定住的粒子并配合修正的向前反弹机制,构建了DPD固体壁面边界模型,利用该边界模型模拟了平板间的Couette流动.研究结果表明,通过调整壁面与流体间排斥作用强度,壁面能实现从无滑移到滑移的转变,壁面与流体间排斥作用越强,即疏水性越强,壁面滑移越明显,并且滑移长度与接触角之间存在近似的二次函数关系.无滑移时壁面附近密度分布均匀,有滑移时壁面附近存在低密度区域,低密度区域阻碍了动量传递,致使壁面产生滑移.

Fluid slip over hydrophobic surfaces in microchannels: a dissipative particle dynamics study

The understanding of fluid slip over a hydrophobic surface is of great importance for reducing the drag for fluid flows. Dissipative particle dynamics (DPD) is used to investigate the mechanism of fluid slip at the solid-fluid interface in microchannels. A wall model adopting three layers of frozen particles is proposed for DPD simulation. In addition, a modified bounce-forward reflection is proposed to reintroduce fluid particles back into the fluid region when they "penetrate" into the wall region in the calculation due to the soft potential employed. Then the Couette flow is simulated by using the proposed wall model. The simulation results show that the no-slip or slip of the fluid at the wall can be achieved by adjusting parameter a_wf. The parameter a_wf represents the interaction between wall particles and liquid particles. Our simulation results show that the distributions of the velocity, density, temperature and shear stress compare well with the corresponding analytical solutions with a_wf=9.68, and there is no fluctuation of the fluid density near the wall. This indicates that the no-slip is obtained, and hence the wall is hydrophilic. With a_wf > 9.68, the wall becomes hydrophobic and the fluid can slip at the wall. The wall becomes more hydrophobic with larger a_wf, and the stronger hydrophobicity leads to greater slip. The slip velocity and slip length can be used to describe the fluid slip. According to the Navier slip boundary model, the slip velocity and slip length are determined by fitting a straight line (linear fitting) to the velocity profile in the central portion of the channel. The results show that the slip velocity or the slip length is a quadratic function of the parameter a_wf, namely, the slip velocity or the slip length is a quadratic function of the contact angle. A physical mechanism of the fluid slip over hydrophobic surfaces is also proposed. The density profile is uniform for the no-slip condition, but there is a layer of low density fluid near the wall when the fluid can slip at the wall surface. This low density region can disrupt the momentum transfer between the wall and the fluid, which induces the fluid slip at the wall surface.