Two engineers from the University of Houston have discovered that they can create upward fountains in water by shining laser beams at the surface of the water. Jiming Bao, professor of electrical and computer engineering at UH, and his postdoctoral student Feng Lin, attribute the discovery to a phenomenon known as the Marangoni effect, which causes convection and explains the behavior of water when there are differences in surface tension.
Although first described in the 1860s, the Marangoni effect is still gaining ground with science.
“Scientifically, no one had predicted or imagined this kind of upward deformation before,” Bao reports in The physics of materials today. “It is well known that an exterior Marangoni convection of low surface tension region will make the free surface of a depressed liquid. Here, we report that this established perception holds only for thin liquid films. Using surface laser heating, we show that in deep liquids a laser beam pulls the fluid above the free surface generating fountains of different shapes.
Here is a Marangoni visual: Sprinkle a bunch of pepper in a bowl of water. Then squeeze a drop of liquid detergent (dishes, laundry detergent, even a piece of soap or toothpaste) into the middle of the same bowl and watch the pepper splash, quickly dispersing to the sides of the bowl. This simple experiment illustrates the Marangoni effect, which appears in many applications of fluid dynamics.
In the most recent incarnation, laser-induced liquid fountains of the Marangoni effect have the potential to impact applications involving liquids or soft materials such as lithography and 3D printing, transfer of heat and mass transport, crystal growth and alloy welding, dynamic network and spatial modulation of light and microfluidics and adaptive optics.
Inspired by his previous work, the successful simulation of inward surface depression in a shallow liquid, Bao increased the depth of the ferrofluid in the current simulation. Ferrofluid is a so-called “magical” liquid and is best known for its amazing surface spikes generated by a magnet.
“Understanding distinct surface deformation in liquids of different depths helps unravel the dynamics of the surface deformation process,” Bao said.
Bao used a low power (
Laser fountains and the depth-dependent transition from surface indentation to laser fountain have never been reported in the literature, probably because they are not anticipated by any existing theory.
“We emphasize that there have been many attempts to understand Marangoni flow-induced surface deformation, but no existing theory can predict the deformation patterns of a liquid with arbitrary depth in a simple way,” said said Bao.