Being a guy, I generally pee standing up. Today’s morning urination was particularly thunderous. The initial stream hit the water in the bowl and SOMETHING splashed back up and out of the bowl, landing squarely on the top of my foot.
So what was the liquid on my foot made of? Water, urine or some combination? Can anyone explain the physics of how the two liquids combine and why the downward force of the urine cause a backwards splsh?
Your urine has mass doesn’t it. And this morning it had a fair amount of velocity above normal right? Seems whilst you were micturating the initial urine displaced the water with as much force required to displace a few drops on your foot. My guess, it was water.
It was lemonade.
…And around the corner, chocolate cake. 
After extensive study, I contend that around the corner, fudge is made.
As for the OP, many people have studied droplet formation, but I don’t know of any work that looks at mixing, i.e. whether the “jetting” that occurs during dropet impact is composed of the droplet fluid or the fluid from the impacted surface.
Actually there have been some studies like that, but they have been analyses of asteroid impacts…
In fact, there’s even a real time Java-based modelling program on the web somewhere, but I don’t have the Google-fu to find it.
But I’d wager that pipper’s morning micturations are on a somewhat lesser scale than planetary collisions.
We hope.
If anyone’s got some dark food coloring and a decent water gun, we should be able to answer this fairly easily.
From my old Chemistry classes, I managed to remember a few things, even though my grades may suggest I learned nothing at all in the first place, but I digress.
Always pour Acid into Water, never vice-versa, since any splash would be the fluid that is receiving the other. Simple to recall since an inverted “A” fits nicely into a capital “W”, but an inverted “W” overflows the capital “A”.
My memory of the reason for the acid into water rule was that when mixing the two there is heat and if something bad happens it is better to have a container of msotly water crack than one filled with mostly acid.
High school was a long time ago though and I am not a chemist so I look forward to someone knowledgeable coming in with the dope on both the acid and the urine issue.
I have worked in a lab every day for the last 17 years.
I have heard both the Uncle Bill and Coil explanations for the Always Add Acid (or base) to water rule…that the fluid dynamics tend to make any splashes consist mostly of the receiving liquid, and that you would rather have a container of water (or dilute acid or base) spill than one of concentrated acid or base.
Also since most laboratory solutions are predominantly water and heat is released when you mix an acid and water it is better to have a larger mass of hopefully cold water there to help soak up the heat.
So getting back to pipper’s question: mostly or entirely water in the splash back.
My understanding of the water/acid rule was that it was possible to generate enough energy from the reaction that it would sometimes cause the water to actually heat to the boiling point, go gaseous and splatter. It was therfore better to be splattering a solution of mostly water than acid, hence the saying “Do what you oughta, add acid to water.”
Getting back to my original question: Have you ever seen the high speed films of a water droplet hitting a pool of water? Where immediately after the drop hits the water, a somewhat smaller drop “bounces back” and basically flies up out of the ‘hole’ that the first drop made? It’s the composition of that second “bounce back” drop that is the key. Unfortunately, all the pictures I’ve ever seen have been where the liquid of the falling drop is the same as the liquid in the pool.
Maybe this weekend I’ll try and get some food coloring and try out Catalyst’s idea.
And bughunter: Never underestimate my morning emiction. Remember that little earthquake Chicago had a month or so back? That was me taking a whizz. The flooding of the Des Plaines river? That was me too…
Totally. This is what science is about. Someone should try this.