Rheology

The rheology (from Greek ῥεῖ Rhei, flow ' and Greek λόγος logos, teaching ', see Panta Rhei ) is the science that deals with the deformation and flow behavior of matter. The rheology therefore includes branches of the theory of elasticity, plasticity theory and fluid dynamics. It deals both with continuum mechanical problems as well as the derivation of the required material laws of the micro-and nanostructure of different classes of condensed matter (eg macromolecular systems, suspensions ). As a typical interdisciplinary subject is in contact with the rheology of physics, physical chemistry, engineering and materials science and in the last decades with the life sciences.

The rheology can be divided into four areas:

• 3.1 Plastics Processing
• 3.2 Medicine
• 3.3 Printing Techniques

Genesis

Already the Egyptians Amenemhet took into account in the 16th century BC, the temperature dependence of the viscosity of water at the construction of his water meter. However, a scientific study of rheological problems only took place in the early modern period, in particular by Isaac Newton, who defined the viscosity of Newtonian liquids, and Robert Hooke, who figured the law named after him for elasticity ( Hooke's law ).

In the 19th century carried out further work to Fluid Mechanics by George Gabriel Stokes, Claude Louis Marie Henri Navier, God Help Hagen and Jean Léonard Marie Poiseuille. End of the century explored James Clerk Maxwell, William Thomson, 1st Baron Kelvin, John Henry Poynting and other viscoelasticity. The work of eg Barré de Saint- Venant and Ludwig Prandtl contributed to a deeper understanding of plasticity.

The term rheology for the science that deals with the flow and deformation behavior of matter, but was, coined in the late 1920s by Eugene Cook Bingham, who was active even in the field of plasticity theory together with Markus Reiner. The name was Heraclitus' aphorism of Panta Rhei (Eng. everything flows ) inspired. The chemist Bingham, was convinced of the necessity of a branch of physics that deals with such issues and thus has certain sectional areas with the chemistry and the engineering sciences. Opposite Reiner he said:

"Here you are, a civil engineer, and I, a chemist, collaborate on common problems. With the development of colloid chemistry is so often a situation occur rapidly. We must, therefore, a branch of physics establish that deals with such problems. "

On August 29, 1929, he and others founded the Society of Rheology in Columbus, Ohio.

Basics

Properties of non- Newtonian liquids

Many substances unite in itself properties of a solid (elastic ) and a liquid ( viscosity), which is described by the rheological models. Depending on the experimental conditions, they can absorb elastically on short time scales external influences, on long time scales but flow away. Such behavior is called viscoelastic; It can be described by a frequency-dependent, but still linear grouping of elasticity and Newtonian hydrodynamics. Other typical rheological effects, however, are based on non-linear relationships such as the normal stress effect and the Weissenberg effect.

Application Examples

The ability of an adhesive to wet a substrate surface is characterized by its rheological properties. Of importance are the viscosity, thixotropy, shear thinning, rheopexy and dilatancy.

In mills and bakery laboratories Teigprüfgeräte as Aleurometer, falling number device, farinograph, Extensograph or Amylograph be used to examine grain and flour quality. The resistance of a dough is always measured against a constant mechanical load and recorded in the rheometer. This can kneading, stretching or Verkleisterungsprüfungen ( with increasing temperature ) be.

In the dairy industry typical product properties, eg, cream, puddings or desserts can be destroyed if you do not know their flow behavior.

Rocks in the earth's interior may behave brittle, elastic and stretchable. Is critical for the prevailing temperature and the size and duration of the voltage. This results in the lithosphere breaks under conditions of compression and expansion, caused among other things, by the movement of continental and oceanic plates.

Plastics Processing

The so-called melt rheology is an important branch of rheology dar. The shear viscosity and sometimes, the extensional viscosity be measured as a function of shear or elongation rate by various methods. The most common capillary rheometer and rotational rheometer come for the shear viscosity is used, while only special devices, called extensional rheometer, used in the extensional viscosity. The goal is a better understanding of the processing of various plastics. In an extruder shear- dominated flows are found, while in many processes ( fiber spinning, Folien-/Hohlkörperblasen, deep drawing or foaming) after extrusion strong dehndominierte processes take place, which are the most technically demanding and therefore most in need understanding of the material.

Medicine

In medicine, the flow Eigenschaftes of the blood ( hemorheology ) play a very important role for the microcirculation and thus for the supply of all organs with nutrients and oxygen. In particular, the blood flow in the smallest vessels, the capillaries with a diameter of 4 to 10 microns, is decisively influenced by the rheological properties of blood. These parameters are controlled mainly by the deformability and aggregation tendency ( rouleaux formation ) of erythrocytes (mean diameter 7.6 microns ), platelet aggregation, temperature, hematocrit and viscosity of the blood plasma.

Within the treatment of various circulatory disorders ( and putative circulatory disorders ) is often desired to improve the hemorheology, to improve the microcirculation. These include, inter alia, the hemodilution and the administration of antiplatelet agents in stroke, at the AVK and ISSNHL. However, is well documented, only the positive effect of antiplatelet agents in stroke and at the AVK, this can be quite independent of the rheological effects. No sufficient scientific evidence there is, however, of the effect for acute hearing loss and other inner ear disorders. A rather rare name for circulation-enhancing drugs is rheological agents ( singular: Rheologikum ).

Printing Techniques

The rheology of inks also plays an important role in the printing technology. The physical characteristics of viscosity and yield value, as well as the device size adhesiveness ( " tack " ) largely determine the behavior of the ink in the printing press ( color separation ), transfer to the substrate and the quality of the printed product.

Printing inks are thixotropic, because it is colloidal systems, ie dispersions is. The thixotropy is undesirable in printing inks, as a rule.