Water drops are ubiquitous around us and come in different sizes. They can be as large as a raindrop or as small as aerosol particles released from a spray can.
They can be even smaller — invisible to the naked eye — when they come as microdroplets. The latter are just a thousandth the size of a typical raindrop.
“We think that droplets are very tiny, and they are not important enough,” Thalappil Pradeep, a chemist at IIT Madras, told The Hindu.
But they can pack a punch.
Dr. Pradeep led a study recently published in the journal Science that showed microdroplets of water can break minerals down into nanoparticles. The team involved researchers from IIT Madras and the Jawaharlal Nehru Centre for Advanced Studies, Bengaluru.
“This outstanding work adds significantly to the growing body of evidence that water droplets enable chemical transformations that bulk water does not make possible,” Richard Zare, a chemist at Stanford University who wasn’t involved in the study, told The Hindu.
Eccentricity of water microdroplets
In a bucket of water, water molecules at the surface can participate more easily in chemical reactions than those in the bulk. But even at the surface, they’ll need to be supplied some energy before they can participate. The water molecules of microdroplets do one better: because they have so little room and are packed closely together, they’re more eager to participate in chemical reactions.
The water in microdroplets thus engage more readily in exotic chemical reactions that also proceed faster, up to a million-times in some cases. This isn’t possible with water molecules in bulk.
For the same reason, microdroplets are also good carriers of electric charge. Dr. Pradeep said they’re easy to encounter in this form. Go to the beach, and close to the shore, microdroplets from the spray of water could carry an excess of ions from the salt in the water and settle on your skin, he said.
A microdroplet can also become electrically charged in other ways. For example, when a larger droplet loses some water by evaporation and shrinks, the water molecules left behind are pushed closer together, and establish (weak) hydrogen bonds between themselves. This often results in a water molecule shedding one of its hydrogen atoms and becomes a negatively charged hydroxyl ion (OH–). The freed H+ is essentially a proton.
This process happens in bulk water as well — but because each molecule is surrounded by other water molecules, the protons can’t move around much. In microdroplets, the protons easily reach the surface, rendering the surface more acidic and creating fertile ground for chemical reactions.
Researchers have shown that amino acids use free protons on their surfaces as an intermediary to form peptide linkages.
The new study reported microdroplets have yet another ability.
An explosive experiment
Dr. Pradeep & co. were interested in whether water microdroplets could break bonds in crystals like silica (SiO2) and alumina (Al2O3) to create nanometre-sized pieces.
Spoorthi Bhat, then a PhD student under Dr. Pradeep and one of the paper’s coauthors, set up an experiment to confirm this hypothesis in crystals of quartz (silica), ruby, and fused alumina.
She pressed a battery terminal against the outside of a capillary tube. The terminal delivered a few thousand volts to mineral microparticles suspended in water inside the tube. The voltage elongated the suspension, squeezing it out of one end, and sending it flying through the air as a mist of microdroplets. They were still airborne when, in just 10 ms, the mineral microparticles broke up into nanoparticles.
The researchers had a few ideas about what could have caused this break up. The free protons could have squeezed themselves into crystal layers, which they scraped the mineral off from within if supplied some energy. The study suggests the electric fields produced by the charged surface could have provided this energy.
Surface tension — the force that keeps droplets spherical — could have been involved as well. In the experiment, a contest between surface tension, which is attractive, and like charges on the surface repelling each other could have set off shockwaves that blew up the microdroplets.
“This is a striking and non-intuitive result,” Shashi Thutupalli, a biophysicist at the National Centre for Biological Sciences, Bengaluru, who was not involved in the study, said to The Hindu. “It seems quite plausible that the high electric field within the droplets could cause the particle breakup.”
He added that the findings could be useful to the study of proto-cells, the precursors to cells as we know them today. Scientists are interested in proto-cells because they could have played an important part in the processes that first created life on the earth. “For me personally, the relevance of these results to the context of the origins of life is very exciting.”
He said the microdroplets could mimic proto-cells by being little compartments in which biochemical reactions play out.
Making a green paradise
The formation of nanoparticles from microparticles, Dr. Pradeep said, is “related to the origin of life, the problem of agriculture, … to issues as large as water itself. Another problem as big as water is food. It is in this context that soil is probably an interesting thing.”
Silica makes up half of sand. Plants absorb silica in the form of nanoparticles to help them become taller. The rice crop usually has high levels of silica.
Supplying soil with silica nanoparticles could thus have a positive impact on agriculture. “Here is a way to convert unproductive soil, unproductive fields or even desertified areas into productive areas,” Dr. Pradeep said.
He implored scientists to investigate whether water microdroplets react with minerals to form nanoparticles as part of atmospheric processes, in the form of ‘microdroplet showers’. Dr. Pradeep was optimistic they do.
Karthik Vinod is a freelance science journalist and co-founder of Ed Publica. He has masters’ degrees in astrophysics and science, technology and society.