Researchers have determined why droplets travel over a heated oily surface 100 times faster than on bare metal.
When you fry something, and a few drops of waterfall into the pan, you will notice that they literally slide over the film of hot oil. This seemingly common phenomenon was first analyzed by researchers at the Massachusetts Institute of Technology. Understanding how droplets move through hot oil is important for the development of microfluidic devices, heat transfer systems, and other useful functions.
A drop of boiling water on a hot surface sometimes levitates on a thin vapor film – a well-studied phenomenon called the Leidenfrost effect. Since the droplet literally hangs on a pillow of steam, it can move across a surface with little friction.
If the surface is covered with hot oil, which has much more friction than the vapor film, then it can be mistakenly assumed that the hot droplet will move much more slowly. But a series of experiments at the Massachusetts Institute of Technology showed that the opposite effect occurs: a drop in oil travels much faster.
This effect accelerates the drop by a factor of 10-100 compared to the speed of movement on metal. This property can potentially be used for self-cleaning or anti-icing systems, or for moving small amounts of liquid through tiny tubes of microfluidic devices: they are used for biomedical and chemical research.
After much analysis, the authors were able to determine that under the right conditions – high temperature, oil viscosity and thickness – the oil literally forms a thin layer covering the outside of each drop of water. When the droplet is heated, tiny vapor bubbles form at the interface between the droplet and the oil. As these tiny bubbles randomly accumulate along the base of the droplet, asymmetry develops and the reduced friction under the bubble prevents the droplet from attaching to the surface and, conversely, pushes it.