A team of researchers from Japan's Okinawa Institute of Science and Technology Graduate University (OIST) discovered an unusual liquid spiral vortex that is created by the particular flow of liquids through channels in the intersections of cross-shaped devices.
The team made its findings by moving water through a cross-shaped device by pushing it into two channels that face each other at the same time. When the two streams of water meet at the intersection of the two channels, they are compressed by the force of the flow and then extended by the pull of the remaining two pathways, creating a unique spiral vortex.
"On the surface it is a really simple geometry," said Amy Shen of OIST's Micro/Bio/Nanofluidics Unit and senior author of the study. "But no one has ever captured or visualized such striking flow structures like this before."
Shen and her team were able to capture this unique liquid spiral vortex by placing fluorescent dye into the water that flowed through just one of the two pathways. In addition, they performed numerical simulations that allowed them to predict the spiral vortex and, in combination with their experimental results, they were able to use these simulations to classify the unusual behavior.
"We are starting to think that this kind of instability would exist in any kind of intersecting geometry," said Simon Haward, first author of the study and group leader of OIST's Micro/Bio/Nanofluidics unit.
Further examination revealed that as the water's flow rate was increased, the spirals appear and, as it decreased, they disappear. However, when they altered the aspect ratio of the channel - the depth of the channel divided by its width - they noticed changes in how the spiral formed and collapsed. In particular, smaller aspect ratios led to the spiral forming and collapsing at the same flow rate, whereas larger aspect ratios corresponded to collapses at a lower flow rate than the formation.
The team's observations of the unique spiral vortex phenomenon will help scientists better understand and optimize basic fluid transport processes.
"In any channel, we can predict when the spiral will form, how big it will grow and the mixing quality that will result," Haward said.
Shen believes the liquid spiral vortex can also help understand other research areas because "in microfluidic devices, it can be difficult to begin the mixing process. Our results suggest that if we make the channel dimensions deeper, then it is easier to induce mixing."
The findings were published in the March 9 issue of Physical Review E.
