Dynamic behaviors of water droplet moving on surfaces with different wettability driven by airflow

Abstract

The movement and deformation of droplet is widely applied in many fields. In this study, to investigate the dynamics of droplets moving on surfaces with different wettability driven by airflow, a series of experiments are conducted by using a high-speed camera, under conditions of airflow speeds increased from 0m/s to 22.2m/s with the same pattern per case, droplet volume varied from 20 to 80 μL, and different wettability surfaces (aluminum, PMMA, titanium, PTFE, PAL). Based on the observation of experiments, the change of the morphology of the droplet motion is divided into 4 stages. Stage I: Droplet oscillates at the original position and its wetting line does not move. Stage II: The droplet still oscillates and its wetting line slightly moves on the surface. Stage III: Droplet deforms dramatically and accelerates forward, and its wetting line elongates sharply. Stage IV: Several sub-droplets are split from the mother droplet. The droplet separation usually occurs after its wetting length reaches the extremum value. Specifically, the droplet separation is not observed on the PTFE and PAL surface, and the four stages of droplet motion reduce to three in these cases. According to the observation, droplet shapes during their movement are classified into four categories including compact, tail formation, film formation, and droplet separation. The relationship between the critical airflow speed at which the wetting line begins to move and the droplet diameter is studied, corresponding to different surfaces. To provide an insight into the phenomenon of dynamics and deformation of airflow-driven sessile droplet spreading, quantitative analysis of the characteristic parameters of the droplet including the wetting length, the dynamic contact angle, and the velocity of the center of droplet gravity are conducted. It is shown that the wetting length ratio of droplets moving on different surfaces varied in the range from 0 to 3.63. The maximum value of the difference between cosines of the upstream contact angle and downstream contact angle usually approaches the cosines of the receding contact angle and the advancing contact angle at the end of Stage II. This study provides experimental reference data for the study of the droplet motion driven by airflow.