It’s a bad idea to try to put out an oil fire with water, and this video will show you why. It will also show you a drop of water crazily growing and shrinking, over and over, inside some hot oil.
Here we see water drops in hot oil. Starting at 1:58, the drops undergo a very weird process that looks like a visual stutter, or someone reversing and replaying the video several times in a row. What’s actually happening is a battle between the heat causing the water to expand and the pressure of the oil causing the water to collapse. According to Fuck Yeah Fluid Dynamics:
“The initial water droplet expands as it shifts from liquid to vapor (top). At a critical volume, the expansion occurs explosively (middle), causing the bubble to over-expand relative to the pressure of the surrounding fluid. The higher pressure of the oil around it collapses the drop, which then re-expands, creating the cycle we see in the final two animations. This oscillation triggers a Rayleigh-Taylor type instability along the bubble’s interface, causing the surface corrugations observed.”