Falling drops capture air
If a falling drop comes into contact with a surface then an air bubble often becomes entrapped. This phenomenon is often undesirable, for example, in the case of inkjet printing. Researchers from the FOM Foundation and the University of Twente have demonstrated that such an air bubble has a maximum volume, the size of which depends on parameters like the drop size and speed of impact, for example. The researchers' findings will be published in the last 2012 edition of the scientific journal Physical Review Letters.
A falling drop compresses the air underneath it. Consequently the pressure under a falling drop is higher than the pressure above it. A dimple forms on the underside of the drop. "At the moment such a drop comes into contact with a surface that dimple turns into an air bubble that is trapped between the drop and the surface. The pressure profile therefore deforms the bubble before this even comes into contact with the surface," explains Wilco Bouwhuis, PhD student under FOM workgroup leader professor Detlef Lohse of the MESA+ Institute for Nanotechnology of the University of Twente.
Two mechanisms
The quantity of air entrapped can be influenced. "For large drops and high impact speeds, the inertia of the fluid ensures that the deformation is attenuated. Therefore the faster a large drop collides with the surface the smaller the quantity of air it entraps. This is a well-known scientific principle," says Bouwhuis. "For small drops with low collision speeds, the surface tension of the liquid-air surface appears to ensure a similar effect: the slower the drop falls the less air becomes entrapped."
Lohse's group wanted to find out what happens in between these two regimes. They investigated at which impact speed the volume of entrapped air was greatest for different liquids and different drop diameters. They performed experiments as well as computer simulation analyses.
Inkjet
"Both the experiments and the simulations we performed with ethanol drops correlated extremely well with the theory," says Bouwhuis. "We can also use the theory to determine the maximum air bubble volume in other drops." The researchers elaborated the theory for inkjet drops, as entrapped air has a detrimental effect on inkjet printing technology. A striking result emerged. FOM workgroup leader Lohse: "For inkjet drops with a radius of about 10 micrometres, the theory indicates that the entrapped air bubble has a maximum size at a fall rate of 1 metre per second. That happens to be exactly the regime in which inkjet printers operate! Our results could also help to minimise or prevent the entrapment of air in a range of other applications."
For editors
Contact details
Detlef Lohse
+31 (0) 53 489 80 76
Wilco Bouwhuis
+31 (0)53 489 30 84
Reference
‘Maximal air bubble entrainment at liquid drop impact’. Wilco Bouwhuis, Roeland C.A. van der Veen, T. Tran, Diederik L. Keij, Koen G. Winkels, Ivo R. Peters, Devaraj van der Meer, Chao Sun, Jacco H. Snoeijer, Detlef Lohse. Physical Review Letters, Volume 109, issue 26
ArXiv: 1205.4761 [physics.flu-dyn] (2012)