Department of Mechanical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea.
Soft Matter. 2017 Sep 27;13(37):6529-6541. doi: 10.1039/c7sm01224a.
A coarse-grained lattice gas model is developed to study pattern forming processes in drying drops containing surfactant. By performing Monte Carlo simulations of the model, the coupled dynamics of surfactant and liquid evaporation and the resulting oscillatory dynamics at the contact line are elucidated. We show that the coupled drop dynamics and the resulting final deposition patterns can be altered by adsorption kinetics. For slow adsorption rates, surfactant molecules recirculate along with colloidal particles and the area covered by the surfactant on the surface grows from the contact line as the initial concentration of the surfactant increases. This leads to coffee-ring patterns with wide rim areas upon drying or to multi-ring patterns depending on the surfactant concentration. For fast adsorption rates, a surfactant skin covers the entire surface area during the early phase of evaporation. This suppresses the coffee ring effect, and uniform patterns are obtained independent of surfactant concentration. The results suggest that the distribution of surfactant on the surface is critical in determining final deposition patterns and that understanding of the skin-forming process of the surfactant on the surface can help in manipulating the delicate pattern forming process of particles in evaporating drops.
我们开发了一种粗粒度格子气模型来研究含有表面活性剂的干燥液滴中的图案形成过程。通过对模型进行蒙特卡罗模拟，阐明了表面活性剂和液体蒸发的耦合动力学以及在接触线处产生的振荡动力学。我们表明，通过吸附动力学可以改变耦合的液滴动力学和由此产生的最终沉积图案。对于缓慢的吸附速率，表面活性剂分子与胶体颗粒一起再循环，并且随着表面活性剂初始浓度的增加，表面上的表面活性剂覆盖区域从接触线生长。这导致在干燥时具有宽边缘区域的咖啡环图案，或者根据表面活性剂浓度形成多环图案。对于快速的吸附速率，在蒸发的早期阶段，表面活性剂的皮肤覆盖整个表面积。这抑制了咖啡环效应，并且独立于表面活性剂浓度获得了均匀的图案。结果表明，表面活性剂在表面上的分布对于确定最终的沉积图案至关重要，并且对表面活性剂在表面上形成皮肤过程的理解有助于控制蒸发液滴中颗粒的精细图案形成过程。
