Scientists at the Massachusetts Institute of Technology have explained why sometimes a droplet of liquid does not merge with the surface of the liquid below it. If the droplet is very cold and the liquid at the bottom is warm enough, the droplet will "levitate" due to the flows caused by the temperature difference. The research results are presented in an article in the Journal of Fluid Mechanics.
Study leader Michela Gehry wondered why the temperature difference could prevent the droplet from mixing with the surface of the liquid. She designed a small box, about the size of an espresso cup, with acrylic walls and a metal bottom edge, which she took turns placing on a hot and cold plate. The box contained a bowl of silicone oil, and at the top was a syringe through which scientists could squeeze out droplets of silicone oil with the same viscosity as in the bowl. In each series of experiments, Gehry measured the temperature of the squeezed out oil droplet and the temperature of the surface of the liquid in the bath. Oils were selected with different viscosities - from close to water to 500 times more viscous.
The time interval between the droplet exiting the syringe and the time it merged with the surface of the liquid was carefully recorded on a camera filming 2000 frames per second. In one case, scientists were able to make a droplet hover in the air for ten seconds, maintaining a temperature difference of 30 ° C. This is roughly equal to the difference between hot coffee and cold milk that is added to it.
“We found that the weight of the droplet falling and the re-circulation force of the air layer can be balanced at one point, and in order to achieve this equilibrium, you need a minimum, or critical, temperature difference for the droplet to start levitating,” Gehry explains. At some point, the drop heats up completely, its temperature becomes equal to the surface temperature, and levitation stops.
A drop of cream falls into hot coffee / Massachusetts Institute of Technology
A group of scientists investigated this phenomenon from a mathematical point of view. Calculations have shown that the levitation time of a drop is related to the temperature difference as 2: 3. Physicists adapted equations describing the mixing of two liquids and simulated how the warm part of the liquid inside the droplet, which was heated from the surface below, behaves. This made it possible to understand how warm currents spread drop by drop, eventually heating it all up. Scientists believe that their calculations will help to better understand how chemicals and biological substances mix and spread in liquids, as well as to understand the behavior of droplets in conditions of zero gravity.
