Micelles are aggregates (or supramolecular assemblies) from amphiphilic molecules or surfactant substances that are stored spontaneously in a dispersion medium – usually water. This process is called auto-assembly. Micelles are formed from a certain concentration of substances, which is called the critical micelle concentration (CMC). They are formed because surfactants tend to separate phases: the hydrophilic head regions of the surfactant molecules are directed towards the adjacent water molecules, while the hydrophobic tail parts are stored together and form a separate phase. Such a micelle is typically a few nanometers in size. The micelle formation is also known as the clumping of asphalt in petroleum products.