Amphiphilic molecules with hydrophilic heads and hydrophobic tails self-assemble into bilayers, micelles, and vesicles driven by the hydrophobic effect.
Amphiphiles (shown as head+tail molecules) undergo Brownian motion. Hydrophobic tails avoid the aqueous region (blue background), while hydrophilic heads prefer it. This drives spontaneous assembly into bilayers and vesicles — the fundamental structure of cell membranes.