Researchers at Tsinghua University Describe How Nanodroplets Deform Upon Surface Strike, Potential to Improve 3D Printing

The demand for, and interest in, nanotechnology is growing, which means that a reliance on nanoscale (one billionth of a meter) printing is not going away. Researchers at ORNL developed a guided simulation process to improve 3D printing at the nanoscale this summer, and Nanoscribe recently 3D printed optics at the nanoscale. While research continues on nanoprinting, another interesting area of research is nano spraying, which basically deposits tiny drops of liquid onto a surface. Researchers with Tsinghua University in Beijing came up with a new theory, which describes how tiny droplets that are sized on the nanoscale deform and break up upon striking a surface.

This new theory, and the researchers’ model, were both recently published in a paper, titled “Spreading and breakup of nanodroplet impinging on surface,” in Physics of Fluids, an American Institute of Physics (AIP) journal. This is not the first time we’ve written about the accomplished Chinese university, which celebrated its 105th anniversary in the spring of 2016: Tsinghua University researchers have worked on everything from 3D metal printing to 3D bioprinting uniform blocks of embryonic stem cells, and a group of former university students even created a unique pancake 3D printer. Nanoscale printing and coating are extremely important when it comes to tiny devices and structures, as well as potentially larger objects like robots, and this new model could improve the overall quality of these techniques.

According to Chen Min, a professor in the Engineering Mechanics Department at Tsinghua University, when someone is spraying coatings, the quality of the coating is enhanced when the droplets are smaller and faster upon hitting the surface. But there is a delicate tightrope to walk when attempting to enhance the coating quality with size and speed of the droplets: at certain impingement speeds, the coating can actually be ruined, because the droplets will break up and splatter. So, it goes without saying that researchers will need to learn more about the conditions that cause droplet surface deformation if they want to improving the nanoprinting and spraying techniques.

Most often, computer simulations are used when conducting this type of research, because of how difficult it is to experiment with nanosized droplets. The Tsinghua University researchers have successfully modeled nanodroplet breakups through a computer simulation, which will help improve 3D printing and spraying at the nanoscale. Chen, along with fellow researchers and study co-authors Xin-Hao Li and Bu-Xuan Li, used a technique called molecular dynamics simulation, which they used to simulate “every molecule that makes up a droplet of water.” Every droplet is about 8.6 nanometers in diameter and is made up of 12,000 molecules of water, and will hit the surface at speeds of “a few hundred meters per second.” The computer simulation shows exactly what happens when the entire collection of 12,000 water molecules hits a flat surface, and how the nanodroplet behaves upon impact.

According to the abstract for the team’s research paper, “Two modes of breakup are observed during the nanodroplet impinging on the surface: (1) touch-bottom of the surfaceo f the liquid film and (2) propagation of finger-like projections on the flow frontier. The touch-bottom breakup is possibly the dominant mode in cases with large We and small Re. The criterion is proposed to be that the amplitude of the capillary wave is larger than the average height of the droplets at the maximum spreading state. This criterion gives a well prediction comparing to the results obtained in molecular dynamics simulations.”

Chen said, “We developed an analytical model to describe the deformation process and another to describe the breakup process.”

Chen explained that while the breakup model is completely new, the deformation model has actually improved on previous work the team completed together. The new breakup model, which Chen says is ready for use in applications, pairs the simulation results with theory already in existence, and gives researchers a formula they can use while calculating the breakup of a water droplet. It’s also helpful when describing the safety hazard that results when ice forms after water droplets collide with aircraft. Suspended in clouds, these droplets are generally 20 to 50 micrometers, which are bigger than the droplets depicted in the research team’s simulations. However, this model is useful anyway, because there isn’t much information available about how droplets of water impinge on aircraft.

However, a limitation of this research is that the model is not intended for bigger droplets: it’s only been verified to work for droplets at the nanoscale.

Bu-Xuan Li said, “The reason is that the way a droplet breaks up is different in macro and nanoscale.”

Another limitation is that the breakup model only applies to fluids like water, generally referred to as a Newtonian fluid. But, the team is hard at work to create a non-Newtonian model for thicker fluids like crude oil, or Oobleck, which comes from mixing water and cornstarch; it’s also in the title of a Dr. Seuss children’s book. As an example, if someone is 3D printing polymers, like human tissue and organs, or polymers, a non-Newtonian model would be necessary. Discuss in the Nano Spraying forum at 3DPB.com.

[Source: American Institute of Physics]