Nucleation characteristics of supercooled sessile water droplets under shear airflow

The icing of sessile droplets under shear airflow is a widespread phenomenon with nucleation serving as its initial stage. However, existing studies focus on droplet nucleation under natural convection, and limited attention is paid to forced airflow. In this study, nucleation experiments on supercooled sessile droplets on cold surfaces under shear airflow are conducted to systematically investigate the effects of airflow velocity and temperature on nucleation characteristics under linear cooling conditions. The tested airflow velocities and temperatures range from 0 to 4 m s^-1 and -5 to 25 °C. Experimental results indicate that supercooled sessile water droplets primarily undergo heterogeneous nucleation at the solid-liquid interface, with nucleation temperature following an approximately normal distribution. Compared to natural convection, the average nucleation temperature under shear airflow increases but the standard deviation decreases. As the nucleation temperature decreases, the nucleation rate increases while the influence of airflow on the nucleation rate shifts from suppression to promotion. Since nucleation occurs at the solid-liquid interface, airflow temperature has a negligible impact on nucleation rate. Furthermore, a theoretical model is developed to clarify the influence of different factors on droplet nucleation rate under shear airflow. Shear airflow induces internal flow within the droplet, increasing both the kinetic pre-factor and the nucleation energy barrier. The elevated energy barrier dominates at higher nucleation temperatures with nucleation suppressed, while the enhanced kinetic pre-factor prevails at lower ones with nucleation promoted. These findings offer guidance for the design and optimization of anti-icing surfaces by improving understanding of nucleation behavior under flow conditions.