The Flow In The Evaporating Water Droplets

The evaporation of the liquid is ubiquitous in nature and human activities, and has a wide application in different industries including refrigeration, printing, coating, and production of novel materials. Previous studies show that the micro-region plays a key role in the heat and mass transfer of the liquid droplet. Due to the limitation of the resolution, experimental investigations on the liquid flow in the region near the contact line are scarce.In this study, a microfluid measurement system has been established by adopting the PTV technique. The system includes a fluorescent microscope, a CCD camera, and an image processing software, and utilizes fluorescent nanoparticles as tracing particles. The error analysis of the system shows that the Brownian motion of tracing particles plays a significant role in the accuracy of velocity measurements.By using the system, the Marangoni flow in the evaporating water droplets has been observed. The flow pattern of the droplet indicates that a stagnation point where the surface flow, the surface tension gradient, and the surface temperature gradient change their directions exists at the droplet surface. The deduced non-monotonic variation of the droplet surface temperature, which is different from that in some previous works, is explained by a heat transfer model considering the adsorbed thin film of the evaporating liquid droplet.The movement of the liquid in the region near the contact line of the droplet is closely relative to the evaporation and the heat transfer in the micro-region. The experimental results indicate that the velocity of the outward flow in the macro-region near the contact line is independent of the position, and increases with time. Analysis shows that the velocity of the outward flow relates to the height of the outward region. The result of the height measurement of the outward region using optical interference technique exhibits a consistency with the analysis in this work. A lower particle concentration in the liquid droplet results in a smooth de-pinning of the contact line while a higher concentration leads to an abrupt one. This is because that the deposition of the particles will increase the potential energy barrier for de-pining. As a result of the Marangoni flow, the collection of the particles on the droplet surface can lead a radial deposition.In addition, by observing the collision, the adsorption and the desorption of nanoparticles in a normal impacting liquid jet, the material removal process of the solid surfaces is simulated. It can provide means for investigating the material removal mechanism in the Chemical Mechanical Planarization.