Abstract
The recent development of a superhydrophobic surface enhances the droplet shedding under a shear flow. The present study gives insights into the effects of shear flow on a pinned droplet over a superhydrophobic surface. To experimentally simulate the change in the size of a sessile droplet on an aerodynamic surface, the volume of the pinned droplet is expanded by water supplied through a pore. Under a continuous airflow that provides a shear flow over the superhydrophobic surface, the size of a pinned water droplet shed from the surface is experimentally characterized. The air velocity ranges from 8 to 61 m/s, and the size of pinned droplets shed at a given air velocity is measured using an instantaneous snapshot captured with a high-speed camera. It is found that the size of the shedding pinned droplet decreases as air velocity increases. At higher air velocities, shedding pinned droplets are fully immersed in the boundary layer. The present findings give a correlation between critical air velocity and the size of pinned droplets shed from the pore over the superhydrophobic surface.
Highlights
A sessile water droplet on an aerodynamic surface can be shed by airflow
We focus on the measurement of a pinned droplet shedding on a superhydrophobic surface exposed to a continuous shear flow
To understand the relationship between the air velocity and the size of a pinned droplet on a superhydrophobic surface, we simulated the incipient motion of a pinned droplet under continuous shedding on a superhydrophobic surface, we simulated the incipient motion of a pinned droplet flow at various air velocities, and applied a numerically estimated boundary layer thickness
Summary
A sessile water droplet on an aerodynamic surface can be shed by airflow. Under the flow, the droplet experiences an aerodynamic drag. Moghtadernejad et al [22] investigated the droplet shedding under the impact of shear flow on hydrophilic and superhydrophobic surfaces Even though they covered a wide velocity range, it is still necessary to discuss the incipient motion of a droplet under a continuous airflow instead of the impact of a shear flow to characterize the critical velocity. This will give great insights into aerodynamics, as well as interface science, of droplet behavior on a superhydrophobic surface under continuous shear flow. The relationship between the boundary layer thickness and critical droplet size is considered
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