Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India.
Soft Matter. 2017 Feb 7;13(5):969-977. doi: 10.1039/c6sm02429g. Epub 2017 Jan 12.
Evaporation of sessile droplets has been a topic of extensive research. However, the effect of confinement on the underlying dynamics has not been well explored. Here, we report the evaporation dynamics of a sessile droplet in a confined fluidic environment. Our findings reveal that an increase in the channel length delays the completion of the evaporation process and leads to unique spatio-temporal evaporation flux and internal flow. The evaporation modes (constant contact angle and constant contact radius) during the droplet lifetime however exhibit global similarity when normalized by appropriate length and timescales. These results are explained in light of an increase in vapor concentration inside the channel due to greater accumulation of water vapor on account of increased channel length. We have formulated a theoretical framework which introduces two key parameters namely an enhanced concentration of the vapor field in the vicinity of the confined droplet and a corresponding accumulation lengthscale over which the accumulated vapor relaxes to the ambient concentration. Using these two parameters and modified diffusion based evaporation we are able to show that confined droplets exhibit a universal behavior in terms of the temporal evolution of each evaporation mode irrespective of the channel length. These results may turn out to be of profound importance in a wide variety of applications, ranging from surface patterning to microfluidic technology.
固着液滴的蒸发一直是广泛研究的课题。然而，限制对基础动力学的影响尚未得到很好的探索。在这里，我们报告了在受限流场中固着液滴的蒸发动力学。我们的研究结果表明，通道长度的增加会延迟蒸发过程的完成，并导致独特的时空蒸发通量和内部流动。然而，当通过适当的长度和时间尺度归一化时，液滴寿命期间的蒸发模式（恒定接触角和恒定接触半径）表现出全局相似性。这些结果可以用由于通道长度增加导致水蒸气在通道内积聚而导致蒸气浓度增加来解释。我们提出了一个理论框架，该框架引入了两个关键参数，即受限制的液滴附近蒸气场的增强浓度以及蒸气积聚达到环境浓度的相应积累长度尺度。使用这两个参数和改进的基于扩散的蒸发，我们能够表明，受限液滴在每个蒸发模式的时间演化方面表现出普遍的行为，而与通道长度无关。这些结果可能在从表面图案化到微流控技术等各种应用中变得非常重要。
