How do lipid bilayers form?
Thermodynamic self assembly.
Lipid bilayers are 5nm thick structures primarily composed of phospholipids. The molecules are amphiphillic containing a hydrophilic phosphate head and a pair of hydrophobic hydrocarbon tails. They arranged such that the hydrophobic regions form the 'core' of the bilayer while the heads from the surface.
To understand how they self assemble to form the structure described above, it is important to know the concept of hydrophobic interactions and entropy.
When several drops is lipids are put into water, their inability to interact with water by hydrogen bonding means that the three dimensional network of hydrogen bonded water molecules is disrupted. To maintain the network, the water molecules around lipids orient themselves to generate a cage around lipids which causes then to have decreased mobility. A decrease in mobility is energetically unfavourable as it decreases randomness or in scientific terms, entropy.
To increase entropy, the tiny cages formed around the lipid droplets merge, casing the hydrocarbons to hydrophobically interact with one another and fewer water molecules are involved in cage formation (the surface area to volume concept).
A similar case is applicable to explain the formation of the lipid bilayer. The hydrophobicity of the exposed tails disrupts the water network, so they are brought to interact with each other. The cage around these is formed by the covalently bonded phosphate heads.
Since the process follows the principle of thermodynamics and does not require catalytic intervention: thermodynamic self-assembly.