Ketchup is classified as a non-Newtonian fluid. This means its resistance to flow, known as viscosity, is not constant. Unlike common liquids that flow predictably, ketchup’s viscosity changes depending on the force or stress applied. This unusual behavior explains why getting the condiment out of a glass bottle can be a frustrating experience.
Understanding Standard Fluid Behavior
The concept of viscosity, or a fluid’s internal friction, originates with the work of Sir Isaac Newton. He established a baseline for predictable fluid behavior, leading to the classification of Newtonian fluids. For these standard fluids, such as water or cooking oils, the viscosity remains consistent.
Whether stirred gently or shaken vigorously, the force applied does not change the fluid’s resistance to flow. The rate at which these fluids are sheared—the speed at which one layer moves past another—is directly proportional to the stress causing the movement. This predictable relationship means that doubling the force results in doubling the flow rate.
How Non-Newtonian Fluids Defy Expectations
Non-Newtonian fluids violate Newton’s linear relationship between applied force and flow rate. Their viscosity is variable and depends entirely on the amount of stress, or shear, they experience. When force is applied, their internal structure temporarily rearranges, causing the viscosity to either increase or decrease.
This category includes two main types of behavior. Shear-thickening fluids increase viscosity when stress is applied, making the fluid seem to solidify. For example, a mixture of cornstarch and water, often called Oobleck, becomes rigid when struck.
Other non-Newtonian fluids are shear-thinning, which is the opposite effect. When mechanical force is applied, the fluid’s viscosity decreases, causing it to flow more easily. Non-drip paints are formulated this way; they remain thick on the brush but thin out immediately when spread across a wall. This shear-thinning property is why ketchup behaves as it does.
Why Ketchup is Hard to Pour
Ketchup is a specific type of shear-thinning fluid, often referred to as a pseudoplastic substance. Its composition includes suspended solids, such as tomato pulp fibers, dispersed within a liquid base. In its resting state, these suspended particles form a weak, tangled network that gives the ketchup a high static viscosity.
This internal structure resists gentle tilting and requires a minimum amount of force, known as the yield stress, before it will flow. Once the yield stress is overcome, the physical force of shaking or tapping causes the tangled network of fibers to align in the direction of the flow. This alignment temporarily breaks down the structure and significantly reduces the viscosity.
The sudden drop in viscosity allows the ketchup to move from a near-solid state to a liquid state quickly. This explains why gently tapping a bottle yields no result, but a slightly harder tap causes the ketchup to pour out rapidly. The amount of force applied determines the degree to which the fluid thins and the speed at which it exits the bottle.