Professor Amy Shen, who leads the OIST Unit, emphasized the importance of this research. "Even when basic household items are set aside, having a fundamental understanding of how EVP materials flow is very useful, especially in biomedical science and geophysics." For example, she explained, blood is an EVP material - it behaves like a solid at rest but flows like a liquid in arteries. What's more, she added, some 3D-printed tissues and scaffolds can have EVP properties, and, on the geophysics side, volcanic lava behaves like an EVP material albeit on a much larger scale.
Previous experimental research on EVP materials has measured their behavior under shear flow, obtained when layers of fluid slide past each other. But, when it comes to the industrial processing and uses of these materials, such as fiber-spinning and circuit-board printing, it's often the extensional flow - when the fluid is stretched - that's more important.
The study of purely extensional flows is a great challenge in experimental fluid dynamics, and the extensional flow of EVP materials has never previously been successfully measured in experiments. To achieve this for the first time, Dr. Simon Haward, the group leader from the Micro/Bio/Nanofluidics Unit, used a novel microfluidic apparatus known as a cross-slot geometry. The apparatus comprised four channels that were all at right angles to each other.
"Inside the cross-slot geometry, we used a Pluronic solution, a well-known EVP material," said Dr. Haward. "When we put pressure on the two inbound channels, which were located opposite to each other, the solution was pushed towards the center point and it came out of the other two channels. The resulting flow has a point at the center where the velocity goes to zero. In the two outbound channels, we generated an extensional flow where the fluid was stretched."