Institute of Microelectronics, Peking University, Beijing, PR China.
J Phys Chem B. 2010 Apr 29;114(16):5269-74. doi: 10.1021/jp912190v.
A macroscopic evaporating water droplet with suspended particles on a solid surface will form a ring-like structure at the pinned contact line due to induced capillary flow. As the droplet size shrinks, the competition between the time scales of the liquid evaporation and the particle movement may influence the resulting ring formation. When the liquid evaporates much faster than the particle movement, coffee ring formation may cease. Here, we experimentally show that there exists a lower limit of droplet size, D(c), for the successful formation of a coffee ring structure. When the particle concentration is above a threshold value, D(c) can be estimated by considering the collective effects of the liquid evaporation and the particle diffusive motion within the droplet. For suspended particles of size approximately 100 nm, the minimum diameter of the coffee ring structure is found to be approximately 10 microm.
在固体表面上带有悬浮颗粒的宏观蒸发水滴会由于诱导的毛细流动而在固定接触线处形成环形结构。随着液滴尺寸的缩小，液体蒸发和颗粒运动的时间尺度之间的竞争可能会影响最终的环形结构形成。当液体蒸发速度远快于颗粒运动速度时，咖啡环结构可能会停止形成。在这里，我们通过实验表明，存在一个用于成功形成咖啡环结构的液滴尺寸下限 D(c)。当颗粒浓度超过一个阈值时，可以通过考虑液滴内液体蒸发和颗粒扩散运动的集体效应来估计 D(c)。对于大小约为 100nm 的悬浮颗粒，发现咖啡环结构的最小直径约为 10 微米。
