A 'shock' discovery

Thalappil Pradeep has for long worked with materials in confinement. Confinement has a curious effect: ironically, instead of confining a material's abilities, it can give it new abilities relative to its bulk counterparts. Pradeep, Institute Professor in the Department of Chemistry at the Indian Institute of Technology Madras, got interested, about a decade ago, in confined water droplets of micrometre size. Likewise, researchers around the world had begun noticing that microdroplets of water are like tiny reactors, where many chemical reactions, impossible in the bulk, become possible or occur at higher speeds.

Studies have shown that reactions can be accelerated by factors of 104 to 106 in microdroplets. Indicatively, the industrial method for producing ammonia, the Haber-Bosch process, is carried out at 200-400 atmospheres of pressure and at temperatures of 400-650°C. But with nitrogen microdroplets, it can be produced at room temperature and atmospheric pressure. 

Now, Pradeep's group has established that microdroplets can break micrometre-sized mineral particles to produce nanomaterials. The discovery that a water droplet could break tough mineral particles was a "shock to everybody", says Pradeep. The researchers created a fine spray of water microdroplets that encased mineral particles of 5-10 micrometres. Within the microdroplets, mineral particles spontaneously transformed into nanoparticles of 5-10 nm in a few milliseconds.

Unlike with bulk water, which is largely inert, the environment within a microdroplet can be charged: a strong electric field can exist at the air-water interface. Microdroplets also have many unique physicochemical properties, including high acidity (due to excess protons) or basicity and possibility of partial solvation of reagents. In the most recent study, the researchers ascribe the production of nanomaterials to a phenomenon called proton-induced slip – in which atomic layers in the minerals slip between each other, assisted by protons. "The microdroplet is like a wet fire," says Pradeep. "It is a really active fellow."

For this study, researchers worked with pieces of ruby and quartz, and produced nanoparticles. This phenomenon, however, is not restricted to these minerals. "It is a generalisable approach to making nanoparticles of minerals. Thousands of minerals exist; here is a way to create their nanoparticles," says Pradeep.

"This work points to a new way to carry out chemical transformations, which has hardly been appreciated prior to this publication," says Richard Zare, Marguerite Blake Wilbur Professor of Natural Science at Stanford University in the U.S.

Microdroplets are common in nature, too. The water spray you feel on your face when standing next to a waterfall or on a sea shore contains microdroplets. Although not as highly charged as the ones in the lab, these natural microdroplets are also charged and contribute to the weathering of minerals and production of natural nanomaterials. Natural nanomaterials in the soil support the growth of plants and other organisms. Pradeep points to silica nanoparticles in the soil, which are taken up by rice plants and help them retain their structure and stand erect.

However, water microdroplets don't always produce nanomaterials. Every microdroplet spray falling on a piece of quartz or silica won't produce their nanoparticles. That happens only under specific conditions. The mineral-containing microdroplets are laced with a high positive charge; these charges repel each other, and push the droplet to break open, but the surface tension of water keeps it intact. Only when the repulsion between the charge is larger than the surface tension does the microdroplet break. With adequate charges and favourable conditions, the mineral particles break to produce nanomaterials.

With these laboratory learnings, researchers now have the know-how to appreciably accelerate the process of nanomaterial production in nature. Pradeep is confident that using this accelerated approach, the silica-rich sand in deserts can be converted into soil rich in active nanomaterials. Of course, it would require some investments, but it is possible, he says.