What emulsifies fats and oils in milk?

Initially, the milk duct produces fat as globules, a membrane that prevents the fat from combining into clumps. These are still lighter than water, so it will eventually form a layer of cream on the top of milk.

When you take this natural mix out of the body the milkfat will separate in hours. It also heats up and proteins break down - this causes the milk fat (butter) to rise, giving what's called a cream line. This is how milk was had for centuries.

Now we humans invented a process call homogenization. What we do is force the milkfat and water-based milk through a very fine nozzle under high pressure. That gives rise to very very small globules (about 10 times smaller than is produced by the mother). These globules are so small that they can stay suspended indefinitely because small proteins called casein stick one half to the fat and one half to the watery component. The fat is still lighter or heavier than water, but the globules are so small they stay suspended by Brownian motion for a very long time.

source:
http://www.foodsci.uoguelph.ca/dairyedu/homogenization.html