An emulsion is a blend of materials which do not normally mix, such as oil and water. Various types of emulsions have different textures, behaviour and properties, which make them useful for food, pharmaceutical, cosmetic and agricultural products.

For the first time, researchers have been able to controllably split water and oil emulsions through osmosis – the same process that moves water between cells in your body, or from soils into the roots of plants.

To explain their work, published in Chemical Science, scientists from Tsinghua University in China used the example of making a salad dressing: "When you make a dressing, the oil and vinegar never mix," said corresponding author Professor Guangtao Li. "However, imagine a drop of oil in vinegar which contains a lot of salt inside.

"The presence of the salt draws vinegar into the oil droplet. When the vinegar goes into the droplet, it forms a series of patterns depending on the amount of salt inside the oil. Then you can enjoy a unique and beautiful salad!"

The researchers’ breakthrough was possible by using inorganic salts to draw water into oil from surrounding ionic liquids by generating an osmotic pressure – just like in the salad dressing. They found that one mixture in particular – organic salt LiTf 2 N and ionic liquid 1-alkyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide – showed unique properties.

The separation process could be controlled with unprecedented flexibility and range, resulting in the one-step fabrication of diverse, complex emulsion structures.

This was possible due to the extent of the inorganic salt’s solubility in the ionic liquid, and also the easily-tailorable molecular structure of the ionic liquid. Fine-tuning and control of separation direction could then be achieved by adding further salt into the liquid. Interestingly, this process was reversible, by re-drawing water molecules taken up.