Dye, Surface Tension, Milk, Detergent
Why does the dye spread out and swirl when you put a few
drops of dye to some full cream milk and add a little detergent solution?
Dyes are organic compounds. Although they contain groups that tend to make
them water soluble -- food coloring and Easter egg dyes for example --
they are also hydrophobic. Both milk and various detergents contain
certain types of chemicals called surfactants (an acronym for "surface
active agents"). Surfactants have a molecular structure that has both a
hydrophobic (water-hating) "end" and a hydrophylic (water-loving) "end".
In water and/or milk they form bodies, called micelles, in which the
hydrophobic ends "clump together" inside the micelle and the hydrophylic
ends are pointed toward the water phase. Hydrophobic compounds such as
dyes (and other 'oily' substances) migrate to the inside of the micelle.
This increases their "solubility" in the detergent/water solution. In fact
this is the mechanism for the way that soap removes dirt from your hands
Milk all by itself contains some components that have this surface
active behavior too, so in general you would find that an oily substance, a
dye or otherwise, would tend to disperse more readily in milk than in pure
water alone. You would find for example that a couple of drops of olive oil
will disperse much more readily in milk than in water without surfactant.
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Update: June 2012