Thursday 24 April, 2014

Drops splash even before they touch surface

Published On: Sun, Nov 20th, 2011 | Atmospheric chemistry | By BioNews

Splashing, which plays a pivotal role in the transport of pollutants and the spread of diseases, may occur even before the drop comes in actual contact with the surface, a new study has revealed.

While the sight of a droplet striking and splashing off of a solid surface is a common experience, the actual physical ingredients and mechanisms involved in splashing aren’t all that well understood.

A team of Brown University and Harvard University researchers has discovered that there is indeed more involved in splashing than previously believed, the Newswise reported.

“In recent studies, it was highlighted that the surrounding air plays an important role in the splashing process. Decreasing the ambient pressure inhibits splashing,” said Julie Albagnac, a postdoctoral research associate in engineering at Brown.

“The observations led to a hypothesis of the existence of a thin layer of air trapped between the drop and the surface.”

To better understand droplet splashing, the researchers initially wanted to observe and measure this layer of trapped air.

They hypothesized that splashing takes place while the drop is still spreading on an air film.

Unfortunately, they found that experimental equipment available today isn’t sophisticated enough to catch a length as small as the air layer thickness (hundreds of nanometers) during the short time of existence of this air layer (hundreds of nanoseconds).

So they instead studied the time evolution and behaviour of droplets splashing with various impact velocities under a variety of ambient pressures to examine different scenarios involving splashing.

“This study shows a new signature of a precursor to the splashing through the wriggling of the contact line at the interface between the droplet and the surface,” said Albagnac.

The splashing/spreading of a droplet doesn’t seem to be an on/off situation, according to the team.

A transition regime exists between the spreading and splashing, which they observed by changing either the impact velocity or the ambient pressure while the other is fixed.

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