AsianScientist (Jun. 1, 2017) – Japanese researchers have succeeded in deriving a theoretical formula that quantitatively predicts the wetting and spreading behavior of droplets that collide with the flat surface of a solid material. Their findings have been published in Scientific Reports.
Although the behavior of droplets colliding with a solid surface looks simple superficially, it is actually quite complicated due to various factors which influence each other such as surface roughness, fluid motion, and wettability (ease of liquid adherence) of the solid surface by the liquid.
Droplet collisions on solid surfaces is an important phenomenon for many industrial applications like ink jet printers, fuel injectors and spray cooling. The maximum wetting and spreading area of droplets after collision is one of the most important parameters that influences the quality and efficiency of such equipment.
Researchers from all over the world have attempted to make quantitative predictions about the extent of wetted areas through experimentation, theory and numerical analysis, but prediction—particularly during slow speed collisions—have not yet been realized.
In previous theoretical studies on the maximum wetting and spreading area of collision droplets, only the effect of the tangential direction was considered, ignoring effects in the perpendicular direction. Typically, two methods are used to make calculations, one when collision speeds are high and another when speeds are low. However, the conventional method used for high speed collisions generates large errors at low speeds and the conventional method used for low speed collisions returns large errors at high speeds.
To reduce calculation errors, researchers from Kumamoto University and Kyoto University derived a new formula to calculate the dissipation of energy when a droplet collides with a surface. The newly derived theoretical formula can quantitatively predicting the maximum wetting and spreading area when droplets collide with various types of solids, such as silicone rubber or super water repellent substrates. Furthermore, the researchers confirmed that it can be applied not only to milli-size but also to micro-size droplets.
“Recently, nanoscale circuit fabrication technology for semiconductor substrates using inkjet technology has attracted much attention,” said study leader Assistant Professor Yukihiro Yonemoto of Kumamoto University.
“Observations of nanoscale phenomena, however, require expensive experimental equipment, and prediction by numerical analysis requires specialized technology. By using a simple method to predict the maximum wetting spreading area of a droplet after collision, we can expect to realize more efficient circuit designs among other things.”
Droplets that strike the surface of a flat solid material will not only stretch and spread, but will also split into finer droplets (splash phenomenon) if the energy at the time of a collision is large. Researchers at Kumamoto University and Kyoto University are currently working on a theory that considers these phenomena to further extend the results of their research.
The article can be found at: Yonemoto & Kunugi. (2017) Analytical Consideration of Liquid Droplet Impingement on Solid Surfaces.
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Source: Kumamoto University; Photo: Yukihiro Yonemoto.
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