"Oobleck" still holds some surprises

New research reveals the complex, two-stage spreading behavior of dense oobleck droplets, challenging simple liquid-solid classifications. | Contesto: cronaca

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• "Oobleck" still holds some surprises

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A dense droplet of the common non-Newtonian fluid known as oobleck has been observed to spread out like a liquid before abruptly stiffening into a solid, according to new research. The phenomenon, driven by high shear rates within the droplet itself, provides a clearer window into the complex transitional behavior of shear-thickening materials that defy conventional classification. The experiment focused on the precise moment a dense suspension of cornstarch and water—the classic kitchen recipe for oobleck—impacts a surface. High-speed imaging revealed a two-stage process previously obscured in less dense mixtures. Initially, the droplet spreads radially outward in a manner consistent with a liquid. This liquid-like phase, however, is fleeting. As the internal shear rate—the speed at which layers of the fluid slide past each other—crosses a critical threshold, the suspension undergoes a rapid jamming transition. The spreading halts abruptly as the droplet's outer edges and structure solidify, locking the shape in place. This behavior underscores the fundamental challenge in describing materials like oobleck, which are categorized as shear-thickening fluids. Their viscosity, or resistance to flow, is not constant but increases dramatically with applied stress. Under gentle pressure, they flow easily; under sudden force, they behave like a solid. The new findings detail how this transition is not instantaneous across the entire droplet but a dynamic front that propagates, allowing for a brief period of liquid spreading even as solidification begins. The implications of this research extend far beyond a kitchen science experiment. Shear-thickening fluids are critical in the design of advanced materials, most notably in liquid body armor and adaptive damping systems. Understanding the precise mechanics of the jamming transition under impact is essential for engineering materials that can reliably dissipate energy. A clearer model of how and when the solid state emerges allows for better predictions of a material's performance under extreme conditions, such as the strike of a bullet or a sudden shock. For decades, oobleck has served as a simple, accessible...

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Categoria: cronaca