Lipids tend to coat a water surface, keeping their hydrophic tails away from water. They line up spontaneously to make sheets.
Lipids are examples of colloids -- paints, grease, soap bubbles, foam -- assemblies of many molecules that are able to be easily changed, and "flow". They are often naturally white because they scatter light at their surfaces. You might get a feel for this by rubbing the surface of an old mirror with sandpaper and seeing what happens. Scratching makes more surface area; this means that light will scatter from those surfaces in different directions.
Students can experiment with this by using the water activities in Choreographed Science.
Lipids can form spheres, too.
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