Newtonian fluid

Newtonian fluid ( by Isaac Newton ) is a fluid (ie, a liquid or a gas) with ideal - elastic, that is linear, non-elastic flow properties. These fluids, that is, the shear rate is proportional to the shear stress. Such fluids, such as water and air, are characterized by a load- independent viscosity. Their movement obeys the equations of Navier -Stokes.

Different behavior is called nichtnewtonsch or anomalviskos and is the subject of rheology. Examples of non-Newtonian fluids are blood, cement pastes, quicksand and ketchup. The deformation behavior of such materials can not be more simply by Newton's law (see below) describe.

Definition

Newtonian fluid is a liquid or a gas, the shear stress is proportional to the shear rate in a laminar flow process. This linear relationship is known as Newton 's law:

Wherein the proportionality constant is referred to as dynamic viscosity. Typical examples of Newtonian fluids are water, many oils and gases. The movement of the Newtonian fluid is described by the equation of Navier Stokes and.

Many materials do not behave ideally - elastic and therefore can not be correctly described by Newton's law. This applies, for example, for viscoelastic materials. These substances are called non-Newtonian fluids. The viscosity of non-Newtonian fluids varies in particular with the shear rate and / or the load duration.

Classification of the nonlinear behavior

Shear rate dependence

Materials, their viscosities decrease with increasing shear rates, are generally referred to as pseudoplastic or shear thinning. Examples are polymer melts or dispersions. Conversely, there is also the ( much less frequent ) case that the viscosity increases with increasing shear rate, which is known as dilatancy. A familiar example is starch suspensions.

Intrinsic viscosity can be manifested in many different ways. Many pseudoplastic materials exhibit a yield stress below which they behave a Scherpannung elastic. The zero shear viscosity, thus the viscosity at a shear velocity of zero, is infinitely high for these materials. The simplest representative of this category are Bingham fluids, for which there is to elastic behavior and about Newtonian behavior.

Time dependence

Many materials also show a temporal change in viscosity of the viscosity at a constant shear stress. The viscosity decreases with the loading period, as is the case with yogurt, it is called thixotropy, takes them from rheopexy. Strictly speaking, these terms are, however, only correct if the substances relax in a subsequent rest period again and with time, the initial viscosity is achieved. If this only partially restored, it is called partial thixotropy or rheopexy.

Examples

A common example of a non-Newtonian fluid is obtained by mixing water and starch. These corn starch is stirred into small portions in a cup of water. If the suspension approaches the critical concentration, the dilatancy is emerging. Force, for example by poking with a finger or quickly turning the cup, resulting in that the mixture behaves as a solid rather than a liquid. Slower and gentler action, eg the quiet insertion of a spoon, leaving the substance in the liquid state. It is even possible to run on these liquid or bounce. Fisticuffs are completely absorbed and there are no time splashing.

Blood miracles such as the of St. Januarius of Naples, where solid blood turns into liquid, according to experts based on the physical properties of a non-Newtonian fluid. A recipe for making a blood-like liquid with non-Newtonian properties of iron (III ) chloride hexahydrate and calcium carbonate in water was already known in the Middle Ages.