If you've ever tried to lift a pizza slice covered in hot, melted cheese, you've no doubt encountered the long, cheesy strings that bridge one pizza slice from the next. Keep lifting the pizza slice and these cheese bridges eventually break, covering the plate, table (or even your lap) in long, thin strands of cheese. While this is just a minor inconvenience with pizza, it is a longstanding problem in industry, where liquids with similar properties to melted cheese - dubbed viscoelastic fluids - need to be cleanly and speedily dispensed.
Now, scientists from the Polymer Technology group at the department of Mechanical Engineering have teamed up with a Japanese research group at the Okinawa Institute of Science and Technology Graduate University (OIST). Together they developed a new technique that uses rotation to break these liquid bridges. Their findings, published in the Proceedings of the National Academy of Sciences (PNAS), could improve the speed and precision of dispensing viscoelastic fluids, in applications ranging from circuit board production and food processing to live tissue engineering and 3D printing.
Ketchup, silly putty and toothpaste
"Viscoelastic fluids, like ketchup, silly putty and toothpaste, have very strange properties - when squeezed slowly, they flow like a fluid, but at faster speeds, they act like an elastic solid," says co-first author, San To Chan, PhD student at OIST. "These unique properties make dispensing these fluids quite difficult."
Currently, the standard method in industry involves lifting the nozzle away from the surface on which the liquid has been deposited. Although this effectively breaks the bridge, it draws the deposited liquid up into a long, thin peak, known as a capillary tail. If the liquid bridge breaks in multiple places, small droplets of fluid, called satellite droplets, also form. Capillary tails and satellite droplets can contaminate products or short-circuit electronic chips.
"The higher the nozzle is retracted, the longer the capillary tail, so the greater the chance for contamination," Chan explained. "Since the nozzle can't be lifted too high, the liquid bridge is thicker and takes longer to break, which slows down the whole dispensing process."
From stretching to twisting
To overcome these challenges, the researchers devised a simple solution: instead of stretching the liquid bridge, it could be destabilized through twisting.
In the study, the research team tested this idea on viscoelastic silicone oil, which is 60,000 times more viscous than water. The scientists placed a droplet of silicone oil between an upper and lower plate. Using high-speed imaging, they found that when the liquid bridge was twisted by rotating the upper plate, it caused a crack halfway between the ends of the liquid bridge. The crack then spread inwards from the edge towards the center, cutting the bridge cleanly in two without forming capillary tails or satellite droplets.
Importantly, this process took about a second, compared to the ten seconds typically needed to dispense the same fluid using the conventional retraction method.
Edge fracture
Detailed numerical simulations by Frank van Berlo, co-author and PhD student at TU/e, were important to uncover the underlying mechanism that causes the liquid bridge to break when placed under torsion.
"The simulations are used to systematically investigate the effect of rotation on the flow and stresses in the liquid bridge", Van Berlo explains. His supervisor and co-author Patrick Anderson adds: "Frank has provided concrete information about how the liquid bridge reacted, validating what we had suspected: the crack was caused by edge fracture."
"This is particularly striking as edge fracture has been characterized as a really undesirable phenomenon that scientists try to stop from occurring," says Simon Haward, group leader at OIST. "This is the first time that edge fracture has been found to have a beneficial application.
Future research
In the next phase of their research, the researchers plan to experiment with different viscoelastic fluids to confirm that the same effect applies. They also plan to increase the speed of the dispensing process further, potentially by combining both rotating and retracting the upper plate.
Professor Anderson: "For future 3D printing faster and more precise liquid dispensing could lower energy consumption, and fewer contaminated products could mean that less raw material is used."
