Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51875105, 51875106), the Industry-Academy Cooperation Project of Fujian Province (Grant No. 2020H6025), and the Scientific Research Program of the Jinjiang Science and Education Park Development Center Fuzhou University, China (Grant No. 2019-JJFDKY-54).
Received Date:15 January 2021
Accepted Date:07 February 2021
Available Online:29 June 2021
Published Online:05 July 2021
Abstract:Droplets’ impinging on a solid surface is a common phenomenon in industry and agriculture. With the development of micro and nano technology, the quantitative descriptions of impinging behaviors for nanodroplets are expected to be further explored. Molecular dynamics (MD) simulation is adopted to investigate the behaviors of water nanodroplets impinging on cooper surfaces which have been decorated with square nanopillars. The dynamical characteristics of nanodroplets are analyzed at 5 different pillar heights, 6 different surface characteristic energy values, and a wide range of droplet velocities. The results show that there is no obvious difference among the dynamical behaviors for nanodroplets, whose radii are in a range from 35 to 45 ?, impinging on a solid surface. With the increase of droplet velocity, the wetting pattern of steady nanodroplets first transfers from Cassie state (V0 = 2–3 ?/ps) to Wenzel state (V0 = 4–10 ?/ps), then it returns to the Cassie state (V0 = 11–13 ?/ps) again. Nanodroplets bounce off the solid surface when V0 > 13 ?/ps. The relationship between the maximum spreading time and droplet velocity is presented. Inflection points in the curve of the relationship are discovered and their formation mechanism is studied. The spreading factors of steady states for nanodroplets with velocity lower than 9 ?/ps are nearly the same; however, they decrease gradually for nanodroplets with velocity higher than 9 ?/ps. In addition, the increasing height of square nanopillars facilitates the transition from Wenzel state to Cassie state and reduces the spreading radius of steady nanodroplets. The mechanism, which yields Wenzel state when the nanodroplets impinge on solid surface with lower height nanopillars, is investigated. In the spreading stage, spreading radii of nanodroplets impinging on surfaces with different height nanopillars are almost identical. The influence of nanopillar height mainly plays a role in the retraction stage of droplets and it fades away as the height further increases. Moreover, the higher surface characteristic energy benefits the spreading of nanodroplets and reduces the retraction time. Especially, nanodroplets do not experience retraction stage, and the spreading stage is kept until the nanodroplets reach a stable state when the surface characteristic energy is increased to 0.714 kcal/mol. Compared with the spreading factor, the centroid height of nanodroplet is very sensitive to the change of surface characteristic energy. Keywords:nanodroplet/ molecular dynamics simulations/ impinge/ dynamic behavior
V0处于低速区的液滴, tmax随V0的增加逐渐减小. 当V0增加到6—9 ?/ps中速区, 图线出现拐点, tmax有增加的趋势. 为探究拐点出现的原因, V0为2—15 ?/ps的液滴其最大铺展状态展示如图7. 速度处于2—6 ?/ps低速区的液滴撞击固体表面后在柱顶铺展, 但由于受到钉扎效应[40, 41]的影响, 铺展半径几乎没有增加, 而接触角却显著增大. 图 7 不同速度液滴的最大铺展状态 Figure7. The maximum spreading states of droplets with different velocities.
选定V0为3 ?/ps的液滴, 改变方柱高度H, 探究其对液滴动态行为的影响. 设置方柱高度分别H1 = 10.845 ?, H2 = 14.460 ?, H3 = 18.075 ?, H4 = 21.690 ?和H5 = 25.305 ?. 图10和表2展示了液滴撞击后的稳定状态及润湿模式. 当方柱高度为H1和H2时, 液滴完全充满方柱间隙, 呈现Wenzel态; 当方柱高度为H3, H4和H5时, 液滴渗入方柱间隙的深度随着高度的增加而减小. 图 10 液滴撞击不同方柱高度固体表面的稳定状态 Figure10. Steady states of droplets impinging on solid surfaces with nanopillars of different height.
方柱 高度
H1
H2
H3
H4
H5
10.845 ?
14.460 ?
18.075 ?
21.690 ?
25.305 ?
润湿 模式
Wenzel
Wenzel
Cassie
Cassie
Cassie
表2液滴撞击不同高度柱状表面后的稳定态润湿模式 Table2.Wetting patterns of steady state of droplets impinging on surfaces with different height nanopillars.
图11记录了方柱高度为H5时, 不同时刻液滴底部的z向坐标. 在模拟过程中zmin = 20 ?. 据此, 可得到液滴渗入方柱间隙最大深度为8.920 ?, 与固体表面底层最大距离为16.385 ?, 大于LJ势的截断半径10 ?, 因此可以认为方柱高度为H5的固体表面, 液滴的动态行为不受固体表面底层影响. 对方柱高度为H1, H2, H3和H4的表面, 假设液滴不受固体表面底层影响, 其在动态过程中与固体表面底层最大距离分别为: 1.925 ?, 5.540 ?, 9.155 ?和12.770 ?. 可见当方柱高度为H1和H2时, 液滴能够完全充满方柱间隙处于Wenzel态, 是因为液滴与固体表面底层的距离过近, 明显小于LJ势的截断半径, 此时固液间有着非常大的吸引力. 图 11 不同时刻液滴底部z坐标 Figure11. Time evolution of z coordinates for the bottom of droplets.
图12展示了液滴撞击不同高度柱状固体表面时质心高度h的变化. 对于H1和H2, h在经历前期的下降后几乎保持水平且两者基本重合. 表明当液滴能够完全充满方柱间隙时, 方柱高度的变化对h没有影响. 但当方柱高度达到临界值18.075 ?时, 液滴最终呈现Cassie态, h有突然的增加. 当方柱高度H > 18.075 ?时, 对应液滴质心高度的增加量等于方柱高度的增加. 由图13可知, 方柱高度为H4和H5, 液滴达到稳态时β约为0.800, 小于其他三种情况, 表明随着H的增加, 液滴的铺展能力减小. 方柱高度的变化对βmax几乎没有影响且液滴在达到最大铺展状态前β基本一致. 因此, 方柱高度对铺展过程的影响主要作用在液滴的回缩阶段. 图 12 不同方柱高度对液滴质心高度的影响 Figure12. Effects of nanopillars with different height on centroid height of droplets.
图 13 方柱高度对铺展因子的影响 Figure13. Dependence of spreading factor on the height of nanopillars.
图14展示了不同高度的方柱对液滴稳定状态下铺展半径R的影响. 随着H的增加, 液滴稳定时R逐渐下降且下降程度逐渐减小, 表明H虽然对液滴的铺展过程有影响, 但影响逐渐减弱. 图 14 方柱高度对铺展半径R的影响 Figure14. Dependence of spreading radius on the height of nanopillars.
表3不同${\varepsilon _{\rm{s}}}$固体表面对应的${\varepsilon _{{\rm{s \text- o}}}}$及液滴接触角$ \theta $ Table3.Corresponding ${\varepsilon _{{\rm{s \text- o}}}}$ and contact angles of droplets for solid surfaces with different ${\varepsilon _{\rm{s}}}$.
图16展示了铺展因子β的变化. 当${\varepsilon _{\rm{s}}} \!\leqslant\! {\varepsilon _{{\rm{s3}}}}$时, β 在达到最大值后有减小的过程, 但该过程经历的时间随着${\varepsilon _{\rm{s}}}$的增加逐渐减小, 与上述结论相互验证. 图 16 液滴撞击具有不同特征能方柱表面时的铺展因子 Figure16. Time evolution of spreading factor of droplets impinging on surfaces with different characteristic energy.