The effect of particle wettability on the stick-slip motion of the contact line
The effect of particle wettability on the stick-slip motion of the contact line
复制标题
DOI：
10.1039/c8sm02129e
复制
发表时间：
2018-12-21
期刊：
影响因子：
3.4
通讯作者：
Sun, Ying
中科院分区：
文献类型：
--
作者：
Kim, Dong-Ook;Pack, Min;Sun, Ying
接触线动力学是决定蒸发胶滴沉积模式的关键。使用高速干涉测量，我们可以直接在现场观察接触线的粘滑运动，并能够解析喷墨打印、蒸发的皮升液滴的瞬时形状，其中包含不同润湿性的纳米颗粒。结合沉积图案的死后光学轮廓术，可以确定瞬时颗粒体积分数，从而确定颗粒沉积速率。结果表明，接触线的粘滑运动是颗粒润湿性的强函数。对于不太亲水的纳米粒子(即，粒子在水-空气界面的接触角约为74度)，观察到粘滑运动，这导致了多环沉积，而对于更亲水的纳米粒子，观察到接触线的连续后退(即，约34度)，这留下了单环图案。根据流体阻力、颗粒间相互作用和表面张力对接触线附近颗粒的作用力平衡以及颗粒润湿性的变化，建立了一个模型来预测钉扎接触线所需的颗粒数量。当粒子的润湿性从theta的润湿角大约为74度增加到theta的大约34度时，钉扎所需的粒子数量预计会增加三倍。这一发现解释了为什么润湿性较大的颗粒形成单环图案，而润湿性较低的颗粒形成多环图案。此外，颗粒的沉积速率依赖于颗粒的润湿性，并且随时间变化。
Contact line dynamics is crucial in determining the deposition patterns of evaporating colloidal droplets. Using high-speed interferometry, we directly observe the stick-slip motion of the contact line in situ and are able to resolve the instantaneous shape of the inkjet-printed, evaporating pico-liter drops containing nanoparticles of varying wettability. Integrated with post-mortem optical profilometry of the deposition patterns, the instantaneous particle volume fraction and hence the particle deposition rate can be determined. The results show that the stick-slip motion of the contact line is a strong function of the particle wettability. While the stick-slip motion is observed for nanoparticles that are less hydrophilic (i.e., particle contact angle theta approximate to 74 degrees at the water-air interface), which results in a multiring deposition, a continuous receding of the contact line is observed for more hydrophilic nanoparticles (i.e., theta approximate to 34 degrees), which leaves a single-ring pattern. A model is developed to predict the number of particles required to pin the contact line based on the force balance of the hydrodynamic drag, interparticle interactions, and surface tension acting on the particles near the contact line with varying particle wettability. A three-fold increase in the number of particles required for pinning is predicted when the particle wettability increases from the wetting angle of theta approximate to 74 degrees to theta approximate to 34 degrees. This finding explains why particles with greater wettability form a single-ring pattern and those with lower wettability form a multi-ring pattern. In addition, the particle deposition rate is found to depend on the particle wettability and vary with time.