Water droplets might seem simple at first. But when nearing evaporation, a desperate power struggle of competing physical forces can emerge, with explosive effects. In a new Proceedings of the National Academy of Sciences publication, researchers have taken a closer look at the physics of charged water droplets on frictionless surfaces, observing spontaneous jets of microdroplet emissions. Their insights may open new opportunities in nanoscale fabrication and electrospray ionization.
Professor Dan Daniel, head of the Droplet and Soft Matter Unit at the Okinawa Institute of Science and Technology (OIST) says, "From raindrops to spray coatings, mass spectrometry to microfluidics, sneezes to spacecraft plumes, charged droplets can show up in a surprising wealth of settings. Our observations enable new physical understanding of evaporating charged droplets, with a range of potential industrial applications."
Research that began 144 years ago
In 1882, Lord Rayleigh, a giant of late 19th century and early 20th century physics and a pioneer in the field of fluid mechanics, laid down the theory behind charged droplet stability. He identified a threshold now known as the Rayleigh limit, beyond which an evaporating charged droplet undergoes a process called Coulomb fission. It would, in simple terms, explode.
Rayleigh's predictions have since been extensively validated for suspended droplets. "Yet in most scenarios of evaporation, droplets aren't suspended: they're resting on a surface," says Daniel. "Remarkably, no one had reported Coulomb fission for droplets on a surface, until we studied it."
Friction-free conditions for emissions
Daniel and his team created a simple set up, depositing tiny, millimeter-sized water droplets on a plastic surface covered in silicone oil. "The oil was essential," notes Daniel. "It eliminates friction so the droplet can change shape, expanding and retracting. Otherwise, the droplets would evaporate uniformly, with no bursts of water jets."
Studying water droplets of different sizes over different timescales, Daniel and colleagues saw the droplets periodically blasting out jets of tiny microdroplets within millionths of a second.
Investigating the mathematical basis for this behavior, instead of observing a single threshold like Rayleigh did, they discovered two - one upon which the droplet stretched and elongated, and another which triggered microdroplet release. "This causes a time delay between elongation and microdroplet release," says Daniel, "potentially allowing finer control over the electrospray process."
These thresholds mark a delicate balance between charge and surface tension. As the water droplets first move through and out of the researcher's plastic pipettes, charges are transferred at the interface between the materials, leaving the droplets positively charged.
During evaporation, this charge is concentrated, until the first threshold is reached. Here, the shape of the droplet changes. It elongates, with a sharp cone shape forming on one side, where the charges accumulate. Upon reaching the second threshold, a jet of water erupts from this cone, spraying forth highly charged microdroplets of water. This reduces the charge of the main water droplet back below the thresholds, until evaporation concentrates charges again and the cycle repeats, over and over.
Tunable nanoengineering
By changing the viscosity of the silicone oil layer, the researchers were able to change the size of the microdroplets. "The thicker and more viscous the oil, the larger the microdroplet size," says Professor Marcus Lin of the University of Tokyo, former postdoctoral researcher within the Droplet and Soft Matter Unit and first author of the paper. "This ability to tune size opens up new possibilities for nanofabrication."
Looking forward, the researchers also hope this new understanding might forge a path for greener scientific techniques. They highlight electrospray ionization, which typically uses a high voltage source to create a continuous spray of droplets. As the droplets get smaller, individual ions separate out and can be measured using mass spectrometry, a staple technique within chemistry labs.
"We show Coulomb fission can happen here even without initial high energy input and driven entirely by evaporation, paving a path towards greener electrospray techniques," highlights Daniel. This same method could also be harnessed for inkjet printing or spray coatings.
This work was started at the King Abdullah University for Science and Technology (KAUST) and continued as Daniel's lab transitioned to OIST, and as Lin joined the University of Tokyo as an assistant professor.
