This little applet shows the hydrophobic effect. The green balls are less charged and heavier than the blue balls. AS the defualts are set up, the balls should separate into two layers over time. This is despite the fact that there is no repulsion between the two types.
In fact the green balls, representing oil, are more attracted to the polar balls, representing water, than they are to each other. However the water is more attracted to itself than to them, so like high school students at a dance, the attractive molecules pair off with each other and the unattractive ones dance with each other rather than with no-one.