Indeed, oil and water separate from each other by simple mixing. However, if enough energy is applied in the form of ultrasound, oil droplets less than 1 micron in size are formed in pure water and continue to exist for several weeks or months.
It turned out that the answer to this riddle lies at the interface between oil droplets and water. Water molecules prefer to give and receive electrical charges from their neighbors through an interaction known as hydrogen bonding. However, when they are next to oil molecules on the droplet surface, the molecules can no longer find enough neighbors for hydrogen bonding. Instead, these water molecules donate their electrical charges to oil molecules on the droplet’s surface.
The study allowed scientists to better understand the interactions between molecules.