These figures show how a nanodroplet breaks up when it impinges on the solid wall through molecular dynamic simulation in computer. There are 12,195 water molecules represented by the green particles in this figure (the droplet originally has a diameter of 8.6 nm). CREDIT: Li, Li and Chen
The growing interest and demand for nanotechnology will result in an increasing need for nanoscale printing and spraying, which relies on depositing small drops of liquid onto a surface. A new theory explaining how such a nanosized droplet deforms and breaks up when it strikes a surface has been recently developed by researchers from Tsinghua University in Beijing.
The model has been discussed in their publication appearing this week in
Physics of Fluids, from AIP Publishing. This model will help researchers to enhance the quality of nanoscale printing and coating, important to almost everything starting from printing and coating small devices and structures to 3D printing machines and robots.
Min Chen, a professor in the Engineering Mechanics Department at Tsinghua University, stated that when it comes to spraying coatings, for instance, the quality of the coating improves when smaller droplets fall on the surface faster. However, at specific impingement speeds, the droplets will break up and splatter, damaging the coating.
An improved understanding of the conditions responsible for bringing about deformation of the droplets when they hit a surface is important for enhancing printing and spraying techniques. Additionally, knowledge on how the droplets break is also essential to improve these techniques. However, researchers depend mostly on computer simulations because of the extreme difficulty that exists in experimenting with nanosized droplets.
A technique called molecular dynamics simulation was used by Bu-Xuan Li and Xin-Hao Li together with Chen in order to simulate every single molecule that makes up a droplet of water. Each droplet, made up of almost 12,000 molecules, is about 8.6 nm in diameter and hits the surface at speeds of a few hundred meters per second. The computer simulates the reactions that take place when the collection of water molecules hits a flat surface.
We developed an analytical model to describe the deformation process and another to describe the breakup process.
Min Chen, Professor, Tsinghua University
The deformation model improves based on the team's earlier work,
“but the breakup model is totally new.”
The breakup model merges theory with the results obtained from the simulations in order to provide a formula that researchers will be able to use to calculate when a droplet will breakup. Chen states that the model is now ready for use in applications
One limitation refers to the fact that the breakup model is only proven to work for droplets at the nanoscale, and not for larger droplets.
The reason is that the way a droplet breaks up is different in macro and nanoscale.
Additionally, this breakup model only applies to the so-called Newtonian fluids like water. Currently, researchers are working on coming up with a model for non-Newtonian fluids, such as crude oil or the gooey mixture of water and cornstarch, sometimes known as Oobleck. For instance, a non-Newtonian model would be required for 3D printing biomaterials and polymers, such as human organs and tissue.
The model can also be used to explain how water droplets collide with aircraft and produce ice, which is a safety hazard. These water droplets, suspended in clouds, usually range from 20 to 50 micrometers, bigger than those in the simulations. According to Chen, this model is still useful as not much is known about the impinging of these water droplets on aircraft.