Thixotropy

Thixotropy ( ancient Greek ἡ θίξις , TiX | is "touching" + joints According to - o - + τροπή, trop | É "turn; change") referred to in the rheology of a time dependence of flow properties in non-Newtonian fluids in which the viscosity as a result of prolonged external Influences decreases and only returns to the original viscosity after the end of the exposure. Simplified, this means that the thixotropic liquid with the duration of its deformation becomes thinner.

The term was introduced by Tibor Peterfi (1883–1953) and Herbert Max Finlay Freundlich (1880–1941).

The opposite behavior to thixotropy is called rheopexy , antithixotropy or negative thixotropy.

Thixotropic behavior is not to be confused with the structural viscosity, in which the viscosity decreases due to increasing shear , but remains constant over time with constant shear stress.

Basics

Viscosity curve of thixotropic and rheopex fluids (schematic)

Some non-Newtonian fluids degrade viscosity over time with constant shear; after the end of the shear stress, the viscosity increases again as a function of time. If the viscosity no longer reaches its initial value, the liquid is referred to as partially thixotropic.

The cause of thixotropy and intrinsic viscosity is similar: the structure in the fluid changes under the action of shear forces, so that smaller interactions occur between the particles. After the action of the shear force, these structural changes recede more or less quickly.

Examples

• Familiar is thixotropic behavior by kneading certain plasticine ( "work softening")
• Ketchup usually has to be shaken before it runs out of the bottle and solidifies again after a short period of rest
• Toothpaste , when pressed out of the tube, should lie on the toothbrush as a highly pasty mass. The relaxation time is very short here and changes into the structural viscosity.
• margarine
• Shaving cream
• lipstick
• Sand-water mixtures, near the shore (for example in the area of ​​the sandy beach that is wet from the surf), quicksand, sludge and the like such as moist coffee grounds . (see below )
• Fluid mud deposits in the turbidity zones of certain types of estates
• The two blood relics in the cathedral of Naples , which liquefy twice a year during the blood miracle of Januarius , are presumably also a thixotropic substance.
• Synovial fluid in the joints is by it contains hyaluronic acid thixotropic, is why before sporting activities to warm up to.

Solid yoghurt, on the other hand, is not a thixotropic fluid, as its structural changes after being crushed are irreversible , not even condensed milk - but pure casein , the gluten in milk. Even honey is not thixotropic: it is not mechanical, but by thermal thinner action - the "solidification" is a sugar crystallization .

Technical applications

• Thixoforming is a manufacturing process that is a mixture of casting and forging , allows very precise workpieces and requires little force for processing.
• Lacquers and emulsion paints can be thixotroped after production with specific additions of silica gels ( Celite, etc.).
• Printing inks are converted from a solid or pasty consistency to a flowing consistency by mechanical action such as stirring, shaking, filling or squeegeeing . In offset printing, this task is done by the color splitting in the inking unit , reinforced by oscillating distribution rollers. On the substrate which is to tack again increase.
• Spray paint should be very thin when atomized and hit, but should become viscous as quickly as possible after spraying.
• Dental impression materials , in particular polyether , are more flowable under pressure when the impression tray is placed in the mouth and the resulting compression of the mass is achieved. This is conducive to the representation of difficult details through a better flow.
• Thixotropic agents prevent anti-corrosion oils from running off by changing their consistency after spraying on metal surfaces as a result of shear
• Drilling fluids for deep drilling show, among other things, thixotropic behavior. They bring the cuttings to the surface; at standstill of the mud pumps the fall of the cuttings will return to by their thixotropic wellbore , preventing even though they do not move upwards. A flush was developed for the continental deep drilling project to hold the cuttings in place during the flushing process.
• Special drilling fluid (DeHydril HT)
• Thixotropic gel is used to coat the optical waveguide in submarine cables .
• A thixotropic oil is generally used to lubricate the connection between escape wheel and armature in mechanical watches to prevent it from running on the components.
• For bonding fiber-plastic composite components, e.g. B. in aircraft construction is u. a. Thickened and thereby made thixotropic synthetic resin is used.

Thixotropy in soil science

In soil science, thixotropy describes a process in fine-grained , mostly silty or clayey sediments , in which reversible viscosity differences occur due to mechanical stress . A change from solid to liquid due to vibration and subsequent return to the solid state is typical. Thixotropic example, loess , Quick Earth , nonswelling and quicksand .

The decisive factors for this property are the composition of the grain size and the type of substances that make up the sediment. It is mostly platelet-shaped clay minerals that - on a microscopic scale - initially support each other in all spatial directions. In the event of a shock, the structure collapses like a house of cards, the mineral platelets become parallel and, since there are no longer any internal adhesive forces, begin to slide past each other under the action of gravity .

This area also includes mass movements of aqueous substrates , i.e. mudslides (mudslides) and the like, which become highly fluid under the high mechanical load.

Thixotropy in shipping

In shipping is a thixotropic an interfacial phenomenon of the system solid-liquid of fine-grained Ladungsgut such. B. ore concentrates and clays . The dry appearing bulk material can, depending on its moisture content , become pulpy and liquid solely through mechanical shocks such as vibration of the ship. This can lead to uncontrollable shifting of cargo with a corresponding critical heeling of the ship and even capsizing .

The permissible moisture limit for sea transport is the maximum water content of a concentrate. It is 90 percent of the liquefaction point and must be determined by an official expert before the load is accepted.

Individual evidence

1. ↑ See DIN 1342-3: 2003-11, chap. 4.2.3, point a).
2. ^ L. Gehm: Rheology - Practice-oriented Basics and Glossary . Vincentz Verlag, 1998, ISBN 3-87870-449-6 .
3. ↑ Christian Ucke: Physical internship for doctors . 14th edition. Technical University of Munich, Munich 1999, viscosity, p. 84 ( users.physik.tu-muenchen.de [PDF; 161 kB ]).
4. ^ Günter Vollmer and Manfred Franz: Chemical products in everyday life . Georg Thieme Verlag Stuttgart, 1985, p. 161, ISBN 3-13-670201-8 .
5. ^ U. Scharnow: Lexicon of seafaring. Various volumes, transpress VEB Verlag für Verkehrwesen Berlin, ISBN 3-344-00190-6 , p. 605.