Fluids

Fluids The states of matter are solid, liquid, gas and plasma. A fluid is a subset of the states of matter, consisting of liquids, gases and plasmas. This is because they have common properties that are distinct from solids. A fluid does not have a specific shape as does a solid. Instead, fluids take the shape of their containers. They also will flow or pour when under the influence of a force such as gravity. Questions you may have include: What is the natural shape of fluids? How do fluids take the shape of their containers? How do fluids flow? Natural shape Solids have specific shapes because the molecular forces holding particles in place are stronger than the kinetic energy of the molecules. Usually, the molecules just vibrate in place, with little or no other movement. On the other hand, fluids exist at higher temperatures and thus their particles have greater kinetic energy. The shape of a fluid adapts to its environment or container. Liquids A liquid in space will form the natural shape of a sphere. This is because the attraction between its atoms or molecules is greater than the forces from their kinetic energy moving outward. A sphere is a shape with the smallest surface area for a given volume of material. A liquid sphere or drop of liquid—such as water—that is falling toward the Earth through the atmosphere will be a slightly flattened sphere, due to the air resistance. If you spill some water on the floor, it will splash and spread out on the floor. Liquids like thin oil will spread out even more than water on the floor. Gases The molecules in a gas have more energy than when the material is in the liquid state, such that they overcome the molecular forces. A gas in space or in the atmosphere will continually spread in a shapeless form. A gas that is heavier than air may gravitate toward the floor, where it then spreads out. The rate that the gas expands is a function of its temperature or kinetic energy of its particles. Plasmas A plasma is an ionized gas, usually at extremely high temperatures. That means some of its electrons have been stripped off. Plasmas have most of the same properties as gases. Shape in container Under the influence of gravity, a fluid will take the shape of its container, provided the volume of the container is greater than or equal to the volume of the fluid. Liquids If you pour a liquid into a container, it will take the shape of the container, provided none overflows. Under the influence of gravity, a liquid will stay in an open container, such as a cup. If the container is filled to the top, the volume of the liquid will equal the volume of the container. This fact has been used to measure the volume of an irregularly shaped container or flask. Gases A gas will take the shape of its container too. If the container is open and the gas is heavier than air, it will stay in the container for a while. For example, Chlorine gas (Cl2) is one of the few gases that is colored. If you pour it into a container, you will see the light green gas take the shape of the container. But the high energy of the gas molecules will result in it slowly dissipating into the air. Usually, gases are put in closed containers. Since gases tend to spread, and since the rate of spreading is proportional to the temperature of the gas or the kinetic energy of its particles, there is a constant pressure on the walls of the container in all directions. This pressure increases with increased temperature or reduced volume of the container. Plasmas Because of their high temperature, plasmas are seldom placed where they could take the shape of the container. Flowing The major feature of a fluid is that it flows when acted upon by some force. This makes a fluid different than a solid, which may be distorted by a force but will not start to flow. Typically, the force is that of gravity, but other forces can also apply. Gravity Fluids under the influence of gravity will flow or can be poured. You certainly have poured liquids from one container to another. Gases also can be poured. Since plasmas are typically very hot, they are seldom poured. Although, you cannot see carbon dioxide (CO2), you can demonstrate pouring it from one jar to another. This is shown by using dry ice to fill a jar with CO2 and then pouring it into a jar containing a burning candle. The candle flame will be snuffed out as the invisible CO2 is poured into the jar. Other forces The forces caused by the acceleration, deceleration or change in direction of a moving container can cause the fluid to flow or change its shape. The force of wind on a body of water will cause the water to flow, as well as to create surface waves. Summary A fluid is a subset of the states of matter, consisting of liquids, gases and plasmas. They have common properties that are distinct from solids. Fluids do not have a specific shape as do solids. Instead, fluids take the shape of their containers. They also will pour when under the influence of a force such as gravity. Fluid Continuum mechanics Laws Conservation of mass Conservation of momentum Conservation of energy Entropy inequality Solid mechanics Solids Stress • Deformation Compatibility Finite strain • Infinitesimal strain Elasticity (linear) • Plasticity Bending • Hooke s law Failure theory Fracture mechanics Frictionless/Frictional Contact mechanics Fluid mechanics Fluids Fluid statics • Fluid dynamics Surface tension Navier–Stokes equations ________________________________________ Viscosity: Newtonian, Non-Newtonian Rheology Viscoelasticity ________________________________________ Smart fluids: Magnetorheological Electrorheological Ferrofluids ________________________________________ Rheometry • Rheometer Scientists Bernoulli • Cauchy • Hooke Navier • Newton • Stokes v • d • e In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress, no matter how small. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. In common usage, "fluid" is often used as a synonym for "liquid", with no implication that gas could also be present. For example, "brake fluid" is hydraulic oil and will not perform its required function if there is gas in it. This colloquial usage of the term is also common in medicine and in nutrition ("take plenty of fluids"). Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and fluid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered to behave like a solid or a fluid, depending on the time period over which it is observed. It is best described as a viscoelastic fluid. There are many examples of substances proving difficult to classify. A particularly interesting one is pitch, as demonstrated in the pitch drop experiment currently running at the University of Queensland. Physics Fluids display such properties as: not resisting deformation, or resisting it only lightly (viscosity), and the ability to flow (also described as the ability to take on the shape of the container).This also means that all fluids have the property of fluidity. These properties are typically a function of their inability to support a shear stress in static equilibrium. Solids can be subjected to shear stresses, and to normal stresses—both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress. Modelling Main article: Fluid mechanics In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of strain rate. A consequence of this behavior is Pascal s law which describes the role of pressure in characterizing a fluid s state. Depending on the relationship between shear stress, and the rate of strain and its derivatives, fluids can be characterized as one of the following: Newtonian fluids : where stress is directly proportional to rate of strain Non-Newtonian fluids : where stress is not proportional to rate of strain, its higher powers and derivatives. The behavior of fluids can be described by the Navier–Stokes equations—a set of partial differential equations which are based on: continuity (conservation of mass), conservation of linear momentum, conservation of angular momentum, conservation of energy. The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.