Experimental Study Of The Internal Flow Inside A Sessile Droplet Undergoing Lateral Vibration

The resonant frequencies of sessile liquid drops undergoing a lateral vibration have been extensively studied in the past due to their relevance to various technological processes,i.e.liquid-liquid extraction,ceramic powder synthesis,and crystal growth in microgravity.A lot of researchers have carried out studies on the motion of the droplets from the macroscopic point of view by numerical simulation and experimental investigation.However,it is lack of deep understanding about the internal flow of a droplet standing on a hydrophobic substrate undergoing a lateral vibration.So it is of great significance to study the internal flow of the droplet undergoing lateral vibration.In this paper,a fluorescence microscope was used to observe the flow path and the distribution of the fluorescent dye inside a laterally vibrating droplet.Combined with CFD simulation results,the internal flow pattern of the droplet was discussed.Then the fluid mixing inside the droplet was evaluated to compare the velocity of the main flow at different conditions.The main work is summarized as follows:The hydrophobic glass was made by solution immersion method,and the Yong's contact angle of the obtained hydrophobic solid substrate was measured to be 110?±1? using a contact angle analyzer.Then the apparatus figure was designed and the experimental setup was established according to the experimental requirements.By tracing the motion of the fluorescent dye,the internal flow pattern inside a sessile droplet with different viscosity undergoing a lateral vibration was studied.It was found that the main flow inside the oscillating drop is the fluid flows downward along the central axis and ascends upward along the edge to form two main flowing circuits.The results demonstrated that extra four flow circuits exits inside the laterally oscillating droplet besides the main flow.The diffusion of substrate momentum within the Stokes layer resulted in the two flow circuits near the bottom substrate,and the Laplace force due to the droplet deformation induced the two counter-current flow circuits inside both side of the drop.The experimental results showed good agreement with the CFD calculation result.Moreover,the velocity of internal flow inside the oscillating droplet was investigated by evaluating the mixing time of the partially staining droplet.The results showed that the droplet mixed slowly only depending on a diffusion process without vibration.,whereas the droplet was completely mixed in a short time with the vibration.In addition,when a droplet was excited around its natural frequency,the main flow inside the oscillating droplet was faster,improving the droplet mixing.The fluid mixing inside the droplet was investigated at different conditions.The mixing time was longer and the internal flow inside the droplets was slower when the viscosity increased.With the increase of the amplitude,the mixing time gradients decreased and then tended to be steady.Also the mixing time increased with the increase of the droplet volume.