Water dynamic viscosity
One of the most common instruments for measuring kinematic viscosity is the glass capillary viscometer. Non-Newtonian fluids exhibit a variety of different correlations between shear stress and shear rate. Their viscosity cannot be described by a single number.
The fluids without a constant viscosity are called non-Newtonian fluids. For some fluids, it is a constant over a wide range of shear rates. Close temperature control of the fluid is essential to accurate measurements, particularly in materials like lubricants, whose viscosity can double with a change of only 5 ☌. Viscosity measurementĭynamic viscosity is measured with various types of rheometer. James Clerk Maxwell called viscosity fugitive elasticity because of the analogy that elastic deformation opposes shear stress in solids, while in viscous fluids, shear stress is opposed by rate of deformation. Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained. The rate of shear deformation is u / y and can be also written as a shear velocity, du/dy. The equation can be expressed in terms of shear stress τ = F/ A = μ ( u / y). Combining these three relations results in the equation F = μ ( Au/ y), where μ is the proportionality factor called the dynamic viscosity (also called absolute viscosity, or simply viscosity). The applied force is proportional to the area and velocity of the plate and inversely proportional to the distance between the plates. If this force causes the substance between the plates to undergo shear flow (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be applied to the upper plate. The relationship between the shear stress and the velocity gradient can also be obtained by considering two plates closely spaced apart at a distance y, and separated by a homogeneous substance. Non-Newtonian fluids exhibit a more complicated relationship between shear stress and velocity gradient than simple linearity. Many fluids, such as water and most gases, satisfy Newton's criterion and are known as Newtonian fluids. This means: it is not a fundamental law of nature, but a reasonable first approximation that holds in some materials and fails in others. This is a constitutive equation (like Hooke's law, Fick's law, Ohm's law). Here, the constant μ is known as the coefficient of viscosity, the viscosity, the dynamic viscosity, or the Newtonian viscosity. Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂ u /∂ y, in the direction perpendicular to the layers. In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force. Laminar shear, the non-constant gradient, is a result of the geometry the fluid is flowing through (e.g.
Extensional viscosity, a linear combination of shear and bulk viscosity, describes the reaction to elangation, widely used for characterizing polymers.įor example, at room temperature, water has a dynamic shear viscosity of about 1.0 × 10 −3 Pa∙s and motor oil of about 250 × 10 −3 Pa∙s.Volume viscosity or bulk viscosity, describes the reaction to compression, essential for acoustics in fluids, see Stokes' law (sound attenuation).
Shear viscosity, the most important one, often referred to as simply viscosity, describing the reaction to applied shear stress simply put, it is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this is what is referred to as a velocity gradient).The most usual decomposition yields the following viscosity coefficients: Viscosity is a tensorial quantity that can be decomposed in different ways into two independent components. Kinematic viscosity is the dynamic viscosity divided by the density.Dynamic viscosity, also absolute viscosity, the more usual one.Viscosity coefficients can be defined in two ways: A viscous glue was made from mistletoe berries and used for lime-twigs to catch birds. The word "viscosity" derives from the Latin word " viscum" for mistletoe.