Why is milk smoother and flatter than water?

[Jorgen1224]

I have conducted an experiment involving projecting a laser beam onto a surface and observing speckles that move relative to us. Those speckles become less visible (maybe smaller?) when the surface is smooth and that is especially the case with milk. I have dyed milk and water to see whether in both of these substances speckles appear and they do, but only in a sample with water. They are barely visible, but everyone in my group agrees that they are still apparent and we don't understand why.


There is not a lot of information about these aspects of milk on the internet yet I found this "Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red." [https://www.uoguelph.ca/foodscience/book-page/physical-properties-milk].

Milk was 3.2% fat
Laser: 532nm and about 100mW
I used black fabric dye and made sure that it mixed well. While it made water totally black, milk became dark blue.

I posted it here since I'm not sure whether it's more about optics or particle physics.

 

[HallsofIvy]

The molecules in milk, mostly fat, are much more complicated than water molecules. They wrap together and link together much more than water molecules.

 

[DaveC426913]

I'm not convinced this is a fair test.
I'm not convinced that dyed water is equivalent to dyed milk in terms of the conditions being tested.

Just one spurious example: I'll bet the specific gravity of the water is not the same as the milk.

You could repeat the experiment, and refine the conditions, eliminating confounding factors until you found one factor that is the crux. As HoI suggests, the proteins and fats are likely the cause.

I'm also not convinced that your conclusion about "flatness" makes sense. But maybe I don't understand your setup. Are the speckles you refer to reflections off the surface?

 

[Vanadium 50]

I have dyed milk and water to see whether in both of these substances speckles appear and they do, but only in a sample with water.

That is not at all clear.