Many people have experienced the frustration of trying to get ketchup from its bottle, only for it to remain still before suddenly gushing out. This common issue raises questions about how fluids behave. The way substances flow, or resist flow, is a field of physics that explains these everyday phenomena. Understanding fluid properties reveals why some liquids pour smoothly while others require effort.
Understanding Fluid Behavior
Fluids are substances that continuously deform under applied shear stress. Their behavior is categorized into two types: Newtonian and non-Newtonian. A Newtonian fluid has constant viscosity, meaning its resistance to flow remains the same regardless of applied shear stress. This consistent behavior makes them predictable; for example, water maintains its flow rate whether gently stirred or vigorously agitated. Gasoline and most simple oils are also Newtonian fluids.
Non-Newtonian fluids, in contrast, have viscosity that changes with applied shear stress. Their resistance to flow can increase or decrease when stirred, shaken, or squeezed. A common example is “oobleck,” a cornstarch and water mixture. When gently handled, it flows like a liquid, but applying sudden force makes it feel solid. This variable viscosity allows for unique behaviors.
Ketchup’s Distinctive Flow
Ketchup demonstrates a specific type of non-Newtonian behavior: shear-thinning, or thixotropy. This explains why ketchup is initially difficult to dispense from a bottle. At rest, ketchup appears thick and resistant to movement, clinging to the container. However, applying shear stress—like shaking, tapping, or squeezing—causes its viscosity to decrease significantly.
Once ketchup begins to flow, it becomes thinner and pours more easily. This change is temporary; when stress is removed, the ketchup gradually thickens again. This property of becoming less viscous under shear and then regaining thickness at rest classifies ketchup as a non-Newtonian fluid.
The Science Behind Ketchup’s Behavior
Ketchup’s non-Newtonian nature stems from its composition and internal structure. It is primarily a suspension of solid particles in a liquid medium. These include tomato pulp, spices, and sugar, suspended in water and vinegar. At rest, these particles form a weak, interconnected network throughout the liquid. This network traps some liquid, contributing to ketchup’s thick, gel-like consistency.
When shear stress is applied, such as by shaking or squeezing, this particle network breaks down. Particles align with the flow, reducing internal friction and allowing free flow. This breakdown causes a decrease in viscosity. Once stress is removed, particles slowly re-form their network, and the ketchup gradually regains its original, thicker consistency.
Everyday Implications of Ketchup’s Fluidity
Understanding ketchup’s shear-thinning property provides practical insights. To dispense ketchup, apply sufficient shear stress, like a firm shake or tap to the bottle’s neck, to break down its internal structure. This temporarily reduces viscosity, allowing smooth flow instead of an unpredictable gush. Tapping the “57” on a glass Heinz ketchup bottle is an effective way to apply this shear force.
Ketchup’s fluid principles extend to many other common substances. Paint, for example, exhibits shear-thinning behavior, allowing it to spread easily with a brush but then thicken quickly on the wall to prevent drips. Other examples include toothpaste, which is solid at rest but flows when squeezed, and blood, which flows through vessels and clots when necessary. These applications highlight the significance of understanding non-Newtonian fluid dynamics.