The study of liquid droplets and their post-collision behavior is of great importance in many fields, including agriculture, engineering and medicine. Prediction of droplet behavior is used in spray painting and pesticide spraying, inkjet printing technology, and aerosol production during precipitation. Therefore, a deeper understanding of this phenomenon is necessary not only to advance our knowledge of fluid physics, but also technology.

In this regard, a particularly intriguing phenomenon is the splashing of droplets when they hit solid surfaces. Several studies on liquid film behavior have helped shed light on droplet spraying. However, no consensus has emerged as to when a drop can be expected to splash. Additionally, wetting behavior, or the ease with which a liquid adheres to smooth, rough solid surfaces, is equally important to understand.

In the background, a group of scientists from Japan and China recently conducted a study to address this issue. The research team, led by Associate Professor Yukihiro Yonemoto from Kumamoto University, Japan, in collaboration with Professor Tomoaki Kunugi from Zhejiang University, China, has proposed a new model that can predict when a droplet will splash after hitting a hard surface. Their study was published in volume 12 of Scientific Reports on March 24, 2022.

When a droplet collides with a solid surface, an unstable liquid film appears below the impinging droplet. To account for this instability, the team modified the energy balance equation that predicts the spreading contact area for smooth and rough surfaces.

To develop the theoretical model for predicting the spray state, the team considered the pressure balance of the liquid film. The analytical results obtained from the combination of the modified energy balance equation and the pressure balance equation were in good agreement with the critical Weber number (a dimensionless quantity characterizing the flow of liquid on the surface) for sprays obtained experimentally for the points of liquid water-ethanol mixture.

The results showed that the spray condition depended not only on the viscosity of the liquid, but also on the wettability and roughness of the solid surface. Furthermore, the splash criterion was driven by a competition between hydrostatic and hydrodynamic pressures, which were the driving forces, and capillary pressure and viscous stress, which were the opposing forces. The splash happened when the driving forces won.

In addition to predicting the spray conditions, the spray model also predicted the size of dispersed secondary droplets and the number of liquid finger-like structures that appeared after the liquid film was destabilized. The model showed that the thickness of the liquid film, which was raised after the droplet collision, was related to the size of the secondary droplets. Further, the size of these secondary points and the number of fingers were correlated. They were also affected by the wettability/surface roughness of the solid surface in addition to the liquid properties. This study was published online in volume 50 of Colloid and Interface Science Communications on August 1, 2022.

“Our results may pave the way for a better understanding of the basic physics of edge or liquid film fragmentation, as well as find applications in important engineering fields related to printing, coating and spraying,” commented Dr. Yonemoto.

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Materials provided by Kumamoto University. Note: Content may be edited for style and length.

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