Additive manufacturing can produce, instead of a tidy reproduction of the desired shape, an object marred by plastic strings, as shown in the three-dimensional print above. Those strings are difficult to prevent when dispensing plastics, polymers, and other viscoelastic fluids, which behave as viscous fluids at low speeds and as elastic solids at high speeds.
As a fluid is dispensed from above, it bridges the gap between the target substrate and the nozzle. To break the connection, one can simply lift the nozzle to thin the bridge until it splits. But that retraction takes time, and it produces elongated and potentially troublesome strands of fluid. For example, in the process of gluing electronic components to printed circuit boards, a stray string of glue can ruin the device.
Now Amy Shen and Simon Haward, of Okinawa Institute of Science and Technology Graduate University in Japan, and their colleagues have developed a new torsion-based method to dispense viscoelastic fluids.
Shen and her group’s experimental setup is composed of two plates: an upper plate that can rotate and a lower plate that can move vertically to set the distance separating them. Silicone oil sandwiched between the plates forms the expected liquid bridge. Below some critical plate separation, the geometry of the bridge is stable. Above that separation, gravity gradually drains the liquid until the bridge breaks. With the introduction of torsion, the liquid divides in about one second, as shown in the video, no matter the plate separation. The twisting motion creates an indentation in the side of the fluid bridge that propagates toward the center and leaves behind no strings.
Because torsion splits an otherwise stable liquid bridge, gravity can’t be the source of the behavior. And although an ordinary Newtonian fluid in the same setup does separate, it does so without an indentation. The behavior must be related to an elastic effect. With complementary simulations performed by Patrick Anderson’s group at Eindhoven University of Technology in the Netherlands, the researchers confirmed that the indentation is related to a flow instability known as edge fracture. In the phenomenon, shearing induces a propagating indentation that localizes the shear stress.
The simplicity of the researchers’ torsion technique means it’s feasible in industrial settings, such as food engineering, 3D printing, and electronic packaging. (S. T. Chan et al., Proc. Natl. Acad. Sci. USA 118, e2104790118, 2021.)
