In physics, a fluid is defined as a substance that continuously deforms, or flows, under the application of a shear stress. Unlike a solid, which resists a change in shape, a fluid yields to even the smallest tangential force. This foundational characteristic is what unites a diverse group of matter, including liquids, gases, and plasma. The field of fluid mechanics is dedicated to understanding how these substances respond to forces and move through the world, governing everything from weather patterns to the flow of blood.
The Primary States of Fluid Matter
The most common types of fluids encountered on Earth are liquids and gases, but the definition extends to the much less common, high-energy state of plasma.
Liquids
Liquids are a type of fluid that possesses a definite volume but no fixed shape. They take the shape of their container but do not expand to fill it completely, instead forming a free surface under the influence of gravity. The particles in a liquid are closely packed, similar to a solid, but the intermolecular forces are weak enough to allow the particles to slide past each other easily. This free movement allows liquids to flow, pour, or drip. Everyday examples include water, milk, oil, and mercury. Liquids are generally considered to be nearly incompressible, meaning their volume does not significantly change when external pressure is applied. This property is central to their use in hydraulic systems.
Gases
Gases are fluids that have neither a definite shape nor a fixed volume. A gas will expand to completely fill any container it is placed in. The molecules within a gas are far apart from each other, and the intermolecular forces are very weak. These particles move about freely and randomly at high speeds, constantly colliding with each other and the walls of the container. Due to the large amount of space between molecules, gases are highly compressible, and their density is variable and changes significantly with pressure. The air we breathe is a gaseous fluid.
Plasma
Plasma is often called the "fourth state of matter" and is considered a fluid. It is an ionized gas that consists of a mixture of neutral atoms, free electrons, and positively charged ions. Because of these charged particles, plasma is an excellent conductor of electricity and is influenced by electromagnetic forces. Plasma is the most abundant state of visible matter in the universe and is found in high-energy environments like stars, lightning, and the auroras. Though less common in our immediate environment, it is also used in technology like neon lights and plasma televisions.
Classifications of Fluids
Beyond the basic states of matter, fluids can be classified in more specific ways based on their behavior under certain conditions.
Newtonian vs. Non-Newtonian Fluids
This classification depends on the relationship between a fluid's shear stress and the rate of shear strain. A Newtonian fluid is a real fluid in which the shear stress is directly proportional to the rate of shear strain. This means its viscosity remains constant regardless of the force applied. Examples include water and air. A non-Newtonian fluid, however, does not follow this rule, and its viscosity changes depending on the applied stress.
Common non-Newtonian fluid behaviors include:
- Shear-thickening (dilatant) fluids: Viscosity increases with increased stress. A cornstarch and water mixture (oobleck) is a classic example; punching it makes it feel solid.
- Shear-thinning (pseudoplastic) fluids: Viscosity decreases with increased stress. Ketchup becomes less viscous when you shake the bottle.
Compressible vs. Incompressible Fluids
- Incompressible fluids: A fluid is considered incompressible if its density does not change significantly with the application of force. Liquids are largely treated as incompressible fluids for most engineering purposes.
- Compressible fluids: The density of a compressible fluid, such as a gas or vapor, changes with applied force. This is a crucial consideration in aerodynamics and other areas of fluid dynamics.
Key Properties of Fluids
Several physical properties define the behavior of fluids:
- Viscosity: A measure of a fluid's resistance to flow. For example, honey has a high viscosity, while water has a low viscosity.
- Density: The mass per unit volume of a fluid. The density of a fluid can vary with temperature and pressure.
- Pressure: The force a fluid exerts per unit area, acting equally in all directions at a given depth.
- Surface Tension: A property of liquids that causes the surface to behave like an elastic membrane, minimizing the surface area. This is why insects can walk on water.
- Buoyancy: The upward force exerted by a fluid on an object immersed in it. This principle explains why objects float or sink.
Comparison of Major Fluid Types
| Characteristic | Liquid | Gas | Plasma |
|---|---|---|---|
| Fixed Shape | No | No | No |
| Fixed Volume | Yes | No | No |
| Compressibility | Nearly Incompressible | Highly Compressible | Highly Compressible |
| Particle Spacing | Close | Far Apart | Very Far Apart |
| Particle Interaction | Can slide past one another | Move freely and randomly | Move freely, influenced by electromagnetic forces |
| Electrical Conductivity | Poor (non-ionized liquids) | Poor | Excellent |
Conclusion
In summary, the question of what do fluids include can be answered by looking at substances that flow. The definition covers liquids, gases, and plasma, each with distinct properties determined by molecular arrangement and energy state. While liquids are defined by a fixed volume, gases by their compressibility, and plasma by its electrical conductivity, all share the fundamental trait of flowing under stress. The study of these different fluid types and their behaviors forms a critical branch of physics, impacting everything from industrial applications to the understanding of our universe. The rich field of fluid mechanics delves into the response of fluids to forces and is a central topic in physical science.
