Some fluids exhibit a solid-like response to stress, suddenly thickening and becoming solids for a moment upon disturbance—and scientists have captured the exact moment it happens.
A team of researchers from Swansea University's College of Engineering used a high-speed camera to capture the moment fluids exhibited the solid-like response. Their experimental designs and findings are published in Communications Physics.
Discontinuous Shear Thickening (DST)
The phenomenon where a fluid exhibits a solid-like behavior against stress is called Discontinuous Shear Thickening—characterized by sudden thickening to a consistency like an actual solid. In the study, the research team used a cornstarch mixture—noting the use of cornstarch suspension mixtures with water for experiments that are looking to observe the said solid-like phenomenon.
"We used corn starch (as a model system for the wider class of shear thickening materials) as it is convenient, widely available and shows a dramatic shear thickening response," explained Dr. Deren Ozturk, author of the study from the Swansea University College of Engineering. He noted that the invasive nature of their experiment was not yet performed on a DST material, adding that they just wanted to try the experiment in hopes of finding "something interesting."
For the setup, the scientists used regular kitchen cornstarch and water. The suspension mixture is poured into a narrow cell, and pressurized air is burst into the mixture, with penetration of air being captured on camera. These invasion patterns take the form of either fluid-like fingers or solid-like fractures, dependent on the pressure exerted by the air and the concentration of the cornstarch.
The study was conducted from the Complex Flow Lab inside the IMPACT Lab, or the Institute for Innovative Materials, Processing, and Numerical Technologies, also headquartered at Swansea University. The laboratory focuses on the study of intricate patterns usually occurring among granular and porous materials, as well as complex fluids like gels, foams, and pastes.
Controlled Friction and DST Applications
Researchers first hypothesized that the mixture would "fracture" with sufficient stress applied to it. Dr. Bjornar Sandnes, Complex Flow Lab head and a co-author in the paper, said: "What is particularly interesting about the corn starch studied here is that friction can be turned on or off like a switch."
He explained that with slight disturbance, the cornstarch grains simply repel each other. Dr. Sandnes noted that the grains are not actually in contact, there is no friction, and that the material generally remains a liquid.
However, with the application of more force on the fluid, the grains are moved into contact with each other up to a point where friction prevents cornstarch grains from sliding. During this event, the mixture behaves more like a solid - exhibiting the hypothesized fractures.
"Our findings are of particular interest to the burgeoning DST field of research as it is a novel visual indication of DST behavior that could be used to calibrate future theoretical models," Dr. Ozturk commented.
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DST materials exhibit non-Newtonian behaviors, with applications in a variety of industries. In safety, military, or law enforcement applications, soft body armors or bulletproof vests use lightweight materials capable of stopping bullets from penetrating, despite being significantly lighter than metal plates previously used.
