Oil and water truly don’t mix, and this is especially true if you try to throw water on an oil fire to quench the flames. Instead, it just goes BOOM!
That explosive reaction makes this a favourite demo for science lectures. The Royal Institution in London did it in 2012 to promote its annual Christmas Lectures, with chemist Peter Wothers donning a flame retardant suit to to do the honours on the roof of the RI building. Wothers heated a mere 150 ml of oil over a Bunsen burner until a small flame developed, then poured a small cup of water on the flame. BOOM! Per the RI Channel:
The reaction is so violent because water and oil don’t mix. When the water is poured into the beaker of burning oil, it sinks to the bottom and, due to the intense heat, vaporizes into steam almost instantaneously. With this phase change from a liquid to a gaseous state the water expands by up to 1700 times, and forces the fire above it upwards. This oxygenates the oil and creates the huge flame...”
Impressive as this phase shift is on the large scale, it’s positively hypnotic to watch at the smaller scale, in slow motion.
Back in 2009, scientists at the G.W. Pritchard Labs at Penn State University decided to take a closer look at this phenomenon, with some truly eye-popping results. They took close-up video footage at the various stages of the reaction, using high speed video at 3000 frames per second.
When that first little drop hits the hot oil, because the oil’s temperature is higher than the boiling point of water, you get a small initial expansion effect as the water undergoes a rapid phase transition from liquid to vapor.
At some point, that phase shift hits a critical point, and you get that dramatic explosive over-expansion.
Eventually the drop will collapse due to higher pressure of the surrounding oil, but then it will re-expand. And collapse. And re-expand. It’s a vicious cycle.
The cycle ends when it hits peak instability: the vapour bubble will eventually rise high enough through the oil to break free of the surface. That’s when you get that loud pop, and hot oil scattered everywhere. And possibly third-degree burns.
Image credits: The G.W. Pritchard Labs, Penn State University.
Brennen, C.E . (2002) “Fission of collapsing cavitation bubbles,” Journal of Fluid Mechanics 472: 153-166.