Ketchup is a well-known example of a shear-thinning fluid. This means the condiment’s viscosity, or resistance to flow, is not constant and changes dramatically when a mechanical force is applied to it. When at rest, ketchup maintains a thick, almost solid-like consistency, making it difficult to pour. Applying stress, such as shaking or squeezing the container, causes the fluid to thin out instantly, allowing it to flow freely.
Understanding Non-Newtonian Fluids
Viscosity is the measure of a fluid’s internal resistance to flow. Fluids like water or light oils are classified as Newtonian fluids because their viscosity remains constant regardless of the amount of force, or shear rate, applied to them. For these materials, the relationship between the force applied (shear stress) and the resulting speed of flow (shear rate) is directly proportional and linear.
Non-Newtonian fluids, in contrast, do not follow this linear relationship. This class of materials includes both shear-thickening fluids, which become more viscous when force is applied, and shear-thinning fluids, which become less viscous. Shear thinning is the most common type of non-Newtonian behavior and describes a material that flows more easily the faster it is forced to move.
The study of flow behaviors is called rheology, which analyzes how materials deform and flow. In a shear-thinning fluid, the apparent viscosity decreases as the shear rate increases. This property explains the typical experience with ketchup, where a small force results in no flow, but a sharp tap or squeeze yields a sudden rush of the liquid.
Ketchup’s Colloidal Structure
Ketchup behaves as a non-Newtonian fluid because it is a type of colloidal suspension. A colloidal suspension is a mixture where microscopic solid particles are dispersed evenly throughout a continuous liquid medium. In the case of ketchup, this liquid medium is primarily water, vinegar, and sweeteners, while the dispersed solids are fine particles derived from tomato pulp.
These tomato solids, along with long-chain polymers like pectin, form a temporary, loose, three-dimensional network when the ketchup is at rest. This entangled internal structure is what gives the ketchup its high viscosity, preventing it from flowing easily under gravity. The network acts like a weak gel, requiring a certain minimum amount of force, known as the yield stress, before it begins to deform and flow.
The concentration of these suspended particles and polymers dictates the degree of the fluid’s non-Newtonian behavior. The higher the tomato solid content, the more pronounced the resistance to flow will be when the ketchup is stationary.
The Mechanism of the Pour
When the ketchup is resting, the entangled particles create maximum internal friction, resulting in its high viscosity. Applying shear stress, such as a sharp tap on the bottle or a forceful squeeze, introduces a velocity gradient, causing the fluid to deform.
This applied force provides the energy necessary to temporarily break apart the weak internal network. The suspended tomato particles and polymer chains then align themselves in the direction of the flow. This alignment reduces the interactions and friction between the internal components, causing the apparent viscosity to drop dramatically. The material transforms from a thick, quasi-solid state to a thinner state, enabling it to pour easily.
Once the applied stress is removed, the flow stops, and the particles lose their alignment. The internal components then quickly return to their original tangled state, causing the viscosity to increase again. This rapid, reversible change in flow resistance is the fundamental principle behind the “all-or-nothing” behavior experienced when pouring ketchup.
Shear Thinning in Other Common Materials
A prime example of shear thinning is house paint, which must flow smoothly off a brush when being applied to a wall but immediately recover its high viscosity to prevent drips and runs. Yogurt and salad dressings are also formulated to be shear-thinning. They maintain a thick consistency in the container but thin out enough to be poured or spooned easily when stirred.
Human blood exhibits shear-thinning characteristics. Blood must flow easily through narrow capillaries but maintain higher viscosity when at rest. The red blood cells align under the shear stress of flow, reducing resistance and ensuring efficient circulation throughout the body. Toothpaste and certain cosmetics are also designed with this property, allowing them to be extruded easily from a tube before regaining firmness immediately upon application.