## Thursday, January 6, 2011

### When You Drip Milk In Coffee, Which One Splashes Up?

Preliminary discussion and experiments:

Question:
dripping drops into a pool of liquid. when you drip liquid A into a pool of liquid B, one drop at a time, you get a tiny splash upon impact.

Is the splash that you see a small amount of liquid B being flung into the air, or is it liquid A bouncing off of the surface of liquid B?

http://sfbay.craigslist.org/forums/?ID=91386638

Blackskimmmer:
lets look:

maybe the website describes the experimental setup and we could use two immiscible fluids of different colors to find out?

cool pics of splashes:

http://courses.ncssm.edu/hsi/class2000/splashes/pictures2.htm

find the equations for this!

Blackskimmmer:
more pics and an article about bubbles in champagne:

http://courses.ncssm.edu/hsi/class2005/splashes/a/Balloon_Index.htm

http://courses.ncssm.edu/hsi/class2005/splashes/a/water_index.htm

http://www.europhysicsnews.com/full/13/article3/article3.html

Toober:
Perhaps a moving frame of reference

When I learned Navier Stokes as a chemical engineer, we always resorted to solvable boundary conditions within a static frame of reference (like a pipe wall, or sphere boundary, or rectangular channel). But I don't think that stasis is required. Or, another way to solve it is to treat the drop as a Newtonian sphere yet allow for "differentiable" elastic liquid smearing at the surface of impact. That's the ticket.

2)
well, my personal observation of dripping half-n-half into a cup of black coffee, is that the splash is white.

3)
iamlucky13:
Here's an example

First of all, yes these are real. It's from a photo contest site I sometimes browse. The photographer spent hours setting them up.

The ones to check out are the milk drops into coffee. You can see some mixing in the center column, but the outer edges appear to be mostly milk, suggesting that the milk drop rebounds against the coffee.

http://www.dpchallenge.com/portfolio.php?USER_ID=52549&collection_id=20111

iamlucky13:
Oops...correction

Reading her comments in a the "Octopus" photo, it sounds like the white milk umbrella forms when a second drop impacts the rising coffee column from a previous drop.

So the splash appears to be mixed coffee and milk.

4)
Blackskimmmer:
my results: mixing

water into soy milk from 6" to a foot, white spikes a cm or so
soy milk into water from ditto white spikes a cm or so
soy milk into olive oil from a foot, really viscous white blobs (the olive oil was in a 2cm deep bottle cap
olive oil into soy milk 6" to a foot, white spikes a cm or so

well, use imagination and interpret results. some mixing or at least the matrix fluid is clinging to the surface of the dropped fluid as it bounces back out.

this is fun, i'll try more variations when i get some time