There is a river in Costa Rica where God dipped his brush when he was painting the sky. As a consequence of this artist’ routine action, the waters got tainted in an intense turquoise colour giving it the name of the river. And the river became famous and now is overpopulated by tourists that visit Tenorio Volcano National Park (fact-that-i-find-fun: my grandfather is from a tiny, tiny village in Galicia called Tenorio. He will be happy of me talking about ‘his’ Tenorio too on the internet).


Once the nonsense introduction is there, I can start talking about colloids. Because yeah, these beautiful celeste waters are just another example of light interaction with colloidal suspensions in this world. And why should I not express my weird love and passion about colloids?


Bigger than organic molecules but smaller than a simple cell. To feel in your own skin what we call nanoscale in terms of size: you are the cell and one mole of your skin is one colloid. Moles have a range of sizes too, as nanoparticles do (a mainstream way of calling them). They usually are suspended happily (neither colliding or falling down, aka chilling and enjoying being a colloid) in another medium, like water, air or blood. Having this capability, apart from the size, is what makes something a colloid. Many things can perturb this peace, usually related to changes in the media properties, like concentrations, pH or temperature, which can make the colloid collapse and get you wet with that unexpected rain. Because yes, clouds are water colloids that aggregate in colder temperature and become rain drops. Rain drops that will make plants bloom and release the colloidal pollen particles that will be blown away by the wind. That air where colloidal viral particles are being suspended by the enough amount of time to visit your nose and kill you. The nose that will smell those disgusting parfum particles of your neighbour. 


And this river is full of aluminosilicate colloids that are happily dispersed in the Buenavista River (check the diagram and the table). We can barely see them (“barely” = we need a microscope), so the water seems transparent. The moment this river gets mixed with the more acidic Sour Creek River, also transparent, these particles stop chilling and the drama starts happening. They lose the charges that keep them away-enough from each other and start colliding, creating random micro-massive aggregates that start dispersing the light in a different way. In this case, it is blurry and bluish, becoming finally the famous Celeste. After 14 km its transparency is recovered. The particles were steep falling during their way, leaving behind a white dust in the riverbed as a final proof of their existence.