Hypothesis

Recently, the naoncapillary devices with the channel width about 2–3 water molecules have been fabricated. Water transport through these nanoslits showed unexpectedly fast flow, revealing the failure of Washburn’s equation.

Experiments

Liquid penetration into a nanocapillary made of two parallel walls is explored by many-body dissipative particle dynamics. Both partial wetting and total wetting walls are considered and the no-slip boundary condition is satisfied.

Findings

The wicking velocity generally obeys Washburn’s equation, but the dynamic contact angle (CA) has to be employed. The dynamic CA (θ_D) relies on the penetration rate and is always larger than the equilibrium CA. The breakdown of Washburn’s equation occurs under two conditions, (i) the channel width close to molecular size and (ii) the positive spreading coefficient is large enough. Both cases come about when the wicking velocity in a nanoslit exceeds the maximum value corresponding to cos(θ_D) = 1. The failure of Washburn’s equation is attributed to the invalidity of Young-Laplace equation associated with undefined meniscus. The extended meniscus will be developed as a wall of the nanoslit continues to extend outside the exit mouth. The shapes of extended menisci are discussed for both partial wetting and total wetting surfaces.

中文翻译：

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通过纳米级亲水毛细管的渗透动力学：超越沃什伯恩方程和扩展的半月板

假设

最近，已制造出通道宽度约为2-3个水分子的微毛细管装置。通过这些纳米缝隙的水传输显示出出乎意料的快速流动，这揭示了沃什伯恩方程的失败。

实验

通过多体耗散粒子动力学探索了液体渗透到由两个平行壁构成的纳米毛细管中的方法。同时考虑部分润湿壁和全部润湿壁，并满足防滑边界条件。

发现

芯吸速度通常服从Washburn方程，但是必须采用动态接触角（CA）。动态CA（θ _d）依赖于渗透速率，总是比平衡CA.较大 Washburn方程的分解发生在两个条件下：（i）通道宽度接近分子大小，（ii）正扩展系数足够大。这两种情况下，当来在纳米裂缝芯吸速度超过对应于COS（最大值θ大约_d）= 1. Washburn方程的失败归因于与不确定弯月面相关的Young-Laplace方程的无效。随着纳米缝隙的壁继续延伸到出口之外，弯月面将被扩展。讨论了半湿润和半湿润表面的扩展半月板形状。