Visualization Research On The Flow Characteristic In Contact Line Region Of Evaporating Droplet

A liquid drop evaporating on a solid surface is a very normal phenomenon, it has been widely used in different industries. Generally speaking, applications of evaporation of sessile drop can be classified into two types, heat transfer enhancement and controlled deposition. These two types of applications directly relies on the understanding of the flow characteristic in the drop during evaporation. It seems that the drop evaporation phenomenon is a simple process, however, it’s actually a extremely complicated multi-scale and multi coupling process, which internal flow characteristic has not been completely revealed.The visualization research can used to directly observe the internal flow status of evaporating drop and provide data for theoretical research and numerical simulation, so it help us to understand the drop evaporating principle comprehensively. Howver, a great difficult in related researches is that visualization experiments are hard to be carried out due to the small scale and many restricts in drop evaporation process.In our research, A micro flow observation apparatus which based on an inverted fluorescence microscope is build up. Furthermore, A MnPTV system is build up based on the objective TIRF illumination system. The work includes building, verification, algorithm transplant and error analysis.The internal flow status of evaporating drop is the result of many influential factors. By observing it, the special phenomenon of drop evaporating in different phases can be found, which will guide the theoretical research. By using the micro flow observation apparatus, we qualitative observed the flow status in the region near the triple-phase contact line of drops. Drops are generated from deionized water and some kinds of alcoholic solutions. We found that the drop evaporation process of all kinds can be divided in three phase, which are spreading phase, contact line pinned phase and shrink phase. The spreading phase is actually a dynamic wetting process, during which the base circle expands. The contact line pinned phase is the main phase of evaporation. We directly observed the Marangoni flow in this phase, and three zone were divided based on the flow status. The shrink phase is the phase when the contact line retreats. There are two shrink modes of contact line, stick-slip mode and contact angle constant mode. In addition, there exists an intensive turbulence in the early stage of drop evaporation of alcoholic solutions. This phenomenon is generally believed to be cause by the Marangoni flow generated by temperature gradient.By using the MnPTV system, we quantitatively measured the flow status in the region which is 40μm away from the contact line and 400nm in height from the substrate. Data like the height averaged velocity, velocity profile, thickness of this region and data variation tendency with time are obtained. The result indicates that as time increases, the height averaged velocity increases exponentially, and the thickness decreases linearly. Besides, based on the variation tendency of velocity profile, the variation of internal flow were analyzed.