Pastes often exhibit behaviors that blur the lines between traditional states of matter. Understanding their true nature requires delving into scientific definitions of fluids and how they respond to forces.
Understanding What Defines a Liquid
A liquid is a state of matter characterized by a definite volume but no fixed shape, conforming to its container. Liquids are nearly incompressible, maintaining their volume even under pressure. Particles within a liquid are held together by intermediate intermolecular forces, allowing them to move around one another while remaining closely packed. This arrangement enables liquids to flow freely, a distinguishing property that differentiates them from solids, where particles are tightly bound in fixed positions.
The Unique Characteristics of Pastes
A paste is typically a concentrated suspension of solid particles dispersed within a liquid medium. Unlike simple liquids, pastes possess high viscosity, resisting flow. A defining feature of pastes is yield stress: they behave like a solid until a certain force is applied, then begin to flow. Solid particles within the paste can become “jammed,” contributing to its solid-like nature until sufficient force overcomes this.
Pastes as Non-Newtonian Fluids
Defining Non-Newtonian Fluids
Pastes are a specific type of non-Newtonian fluid. A non-Newtonian fluid is one whose viscosity, or resistance to flow, changes when subjected to shear stress. This contrasts with Newtonian fluids, like water, which maintain constant viscosity regardless of applied force. Non-Newtonian fluid viscosity can either decrease (shear-thinning) or increase (shear-thickening) with increasing shear stress.
Shear-Thinning and Shear-Thickening Pastes
Many pastes exhibit shear-thinning behavior, becoming less viscous and flowing more easily when force is applied, allowing them to be dispensed or spread. Conversely, some non-Newtonian fluids are shear-thickening, becoming more viscous under increased stress. The rheological behavior of pastes, influenced by particle concentration and size, is important in various applications.
Everyday Examples of Paste Behavior
Common household items demonstrate the non-Newtonian properties of pastes. Toothpaste is a shear-thinning fluid; it remains semi-solid in the tube but flows easily when squeezed, becoming solid-like again on the toothbrush. Ketchup is another shear-thinning paste that becomes runnier when shaken or squeezed from its bottle. Conversely, a mixture of cornstarch and water (oobleck) exemplifies a shear-thickening fluid, feeling like a liquid when gently handled but becoming rigid when struck quickly. These examples illustrate how pastes defy simple liquid classification.