Researchers from Keio University used high-speed droplet impact simulation on wet rigid wall to analyze wall shear flow and water hammer after the impact
Physical cleaning techniques are a major application in removing particulate contamination, owing to their low environmental impact. One such approach is based on water jets that often involves fission into droplet fragments. Droplet-impact-induced wall shear flow can offer particle removal. Now, a team of researchers from Department of Mechanical Engineering of Keio University identified the underlying physics of what happens when liquid jet collisions strike surfaces to be cleaned. To study the impact of water droplets against a dry/wet rigid wall as a canonical example, the team used a fluid dynamics simulation. The findings were published in the journal Physics of Fluids on January 29, 2019.
Semiconductor manufacturing includes small-sized contaminant particles that are required to be removed from silicon wafers, owing to further miniaturization of devices. High-speed droplet impact is majorly preferred for removing particles of very small sizes that measure around 10 nanometers in order. However, this approach can also lead to surface erosion. Therefore, it is necessary to consider the effects of both viscosity and compressibility of the fluid on the impact dynamics, according to the researchers. The fluid is viscous and produces a mechanical friction, which has a key impact on particle removal. Moreover, the fluid is compressible and can produces a water-hammer shock at the impact to cause surface damage.
Computational fluid dynamics (CFD) accounts for both viscosity and compressibility and is a challenging approach. Therefore, the team performed the first known viscous and compressible flow simulation in order to analyze high-speed droplet impact dynamics. Simulation approach enables to identify a trade-off relationship between efficient cleaning performance and damage-free cleaning. According to the researchers, the new approach can be used to quantify friction force and water-hammer impact pressure. In the simulation, the team studied the case of initially introducing a film of water that covers the cleaning surface. The team found that this film can cushion the droplet impact and offer less erosive cleaning.