This alkali metal stuff with water is fun!
The weapon of choice is still the HS camera, which shoots 240fps (about 4 milliseconds per frame). The resolution isn’t great, but it gets the job done. It can go up to 1000fps, but the resolution drops below 100 pixels.
Both sodium and potassium explode even in modest quantities in water. Thus far I have been exclusively limited to less that 250 mg (1/4 gram, about the size of a regular aspirin tablet). Much bigger than this and the hazards amplify horribly. So the fact finding mission is done on really small quantities.
Sodium tends not to catch fire, then the hydrogen cloud above the metal ignites causing the whole thing to explode with the characteristic pop of a hydrogen explosion. Not quite sure how that gets the metal to do what it does, or so symmetrically, or why this generates so much light etc compared to the potassium reaction. The sodium tends to explode within 2-3 seconds, while the potassium takes considerably longer (~8 seconds). Interestingly, if sodium doesn’t explode on the ignition of the hydrogen gas, it tends to just fizz around on the surface.
Sodium is also much more buoyant than potassium and typically floats almost completely clear of the water. I figure this is a very significant factor in the behavior of the two metals with water.
This is for comparison what the potassium explosion looks like.