What is more entertaining that watching milk fall into coffee? The milk always forms such beautiful spiraling formations within the cup of coffee… and there are some real physics behind the mixing going on right there in your morning joe!
I enjoyed my own view of coffee mixing this morning… through the slow motion eyes of my camera! Check it out. Differences in temperature and density may contribute to the way my organic creamer mixes into my glass of chilled expresso.
Did you know that it is very difficult to mix fluids together in very small volumes, like at the nanoscale (10^-9 liter volumes)? This is because at very small scales, for example in micro- or nano-sized fluid channels built into microfluidic chips, fluids tend to demonstrate laminar as opposed to turbulent flows. This means that when two fluids enter a channel from two inlet channels, they tend to stay side-by-side as they flow into the merged channel, mixing mostly only by diffusion of molecules from one fluid stream into the other.
Microfluidic researchers have put much effort into designing structures that can help fluids mix together on the nanoscale, like serpentine channels, step-like grooves on channel bottoms, and votex-like structures.
Schematic 3D diagrams of the three representative types of microfluidic mixers investigated in this study: Y channel mixer (A), 3D serpentine mixer (B), and vortex mixer ©. 2004 PNAS, the National Academy of Sciences.
Fluids behave very differently indeed on the nanoscale! It would take you quite a while to mix milk into your coffee on the nanoscale!