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Ketchup is one of the most popular condiments (đồ gia vị) in the US, along with mayonnaise, but getting those few last dollops out of the bottle often results in a sudden splattering (bắn tóe). "It's annoying, potentially embarrassing, and can ruin clothes, but can we do anything about it?" Callum Cuttle, of the University of Oxford, said during a press conference earlier this week at an American Physical Society meeting on fluid dynamics in Indianapolis, Indiana. "And more importantly, can understanding this phenomenon (hiện tượng) help us with any other problems in life (vấn đề trong cuộc sống)?"

The answer to both questions, per Cuttle, is a resounding yes. Along with his Oxford colleague, Chris MacMinn, he conducted a series of experiments (thí nghiệm) to identify (xác định) the forces (lực) at play and develop a theoretical model (mô hình lý thuyết) for ketchup splatter. Among the most interesting findings: Squeezing the bottle more slowly and doubling the diameter of the nozzle helps prevent splatter. There is also a critical threshold where the flow of ketchup shifts suddenly from not splattering to splattering. A preprint paper has been posted to arXiv and is currently undergoing peer review.

Isaac Newton identified the properties of what he deemed an "ideal liquid." (chất lỏng lý tưởng) One of those properties is viscosity (tính sền sệt, tính nhầy), loosely defined as how much friction/resistance there is to flow in a given substance. The friction arises because a flowing liquid is essentially a series of layers sliding past one another. The faster one layer slides over another, the more resistance there is, and the slower one layer slides over another, the less resistance there is.

But not all liquids behave like Newton's ideal liquid. In Newton's ideal fluid, the viscosity is largely dependent on temperature and pressure: water will continue to flow—i.e., act like water—regardless of other forces acting upon it, such as being stirred or mixed. In a non-Newtonian fluid, the viscosity changes in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior. Physicists like to call this a "shearing force": Stirring a cup of water produces a shearing force, and the water shears to move out of the way. The viscosity remains unchanged. But the viscosity of non-Newtonian fluids changes when a shearing force is applied.

Ketchup is a non-Newtonian fluid. Blood, yogurt, gravy, mud, pudding, and thickened pie fillings are other examples, along with hagfish slime. They aren't all exactly alike in terms of their behavior, but none of them adheres to Newton's definition of an ideal liquid.

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