黏弹性液滴撞击疏水壁面的动力学研究

Dynamics of viscoelastic drops impacting onto a hydrophobic solid substrate

本文数值研究了在较低雷诺数和韦伯数下黏弹性液滴撞击疏水壁面的动力学过程.与牛顿流体相比,前人实验发现黏弹性流体对液滴撞击壁面后的铺展和回缩动力学过程有很大影响.为了探讨撞击过程的黏弹性效应,我们采用Oldroyd-B本构模型与扩散界面模型相结合的方法进行直接数值模拟,其中液滴黏弹性是通过德博拉数(De数,是松弛时间与特征时间的比值)来进行表征.基于数值结果,我们发现在撞击壁面之后,液滴的铺展过程几乎不会受到De数的影响,均满足铺展时间t_m与最大铺展半径R_m的尺度率关系:t _m～(We)^(1/2)R_m^(3/2).在回缩过程中,牛顿流体液滴会发生反弹现象,而黏弹性液滴则随着De数的增大,其回缩速度逐渐减小,直至反弹现象被完全抑制.通过对数值结果的深入分析,揭示了液滴黏弹性导致反弹抑制现象的机制,即在液滴的回缩过程中,液体的黏弹性效应导致了界面周围出现向下的弹性力,从而阻碍了液滴的反弹运动,且De数越大,反弹越容易被抑制.

In this paper, the impact of a viscoelastic drop onto a hydrophobic solid substrate is numerically investigated at low Weber number and Reynolds number. Previous experiments showed that viscoelastic fluids could make a big difference to drop spreading and retraction after impact, compared to Newtonian fluids. To quantitatively study the effect of viscoelasticity, Oldroyd-B model and diffuse interface methods are adopted in the direct numerical simulation of viscoelastic drops impacting onto a hydrophobic solid surface. The viscoelasticity is represented by the Deborah number, which is the ratio of the relaxation time of the viscoelastic fluid to the flow characteristic time. Based on the numerical results, we find that the spreading process is hardly affected by the viscoelasticity, and follows the scaling law： tm -We-Rm 3/2, where Rm is the maximum radius of the wetted area and tm is the corresponding time. During the drop retraction, the Newtonian drop would bounce up from the substrate. By contrast, the retraction velocity of the viscoelastic drops gradually slow down with the increase of Deborah number, thereby eventually suppressing the drop bouncing. The mechanism of anti-rebound effect due to the viscoelasticity can be interpreted by the occurrence of the downward elastic stress near the interface, and the bigger the Deborah number is, the more likely the bouncing is inhibited.