Thixotropy

Thixotropy (Greek: ἡ θίξις, thixis " touching " and τρέπω, Trepo " I turn ", " I change " ) referred to in the rheology of a time dependence of the flow properties at non-Newtonian fluids, where the viscosity (thickness ) due to ongoing mechanical stress decreases and only after completion of stress increases again. The term was introduced by Tibor Péterfi (1883-1953) and Herbert Max Finlay Freundlich ( 1880-1941 ). The contrasting behavior of thixotropy is called rheopexy Antithixotropie or negative thixotropy.

Basics

Some non-Newtonian fluids build at a constant shear viscosity decreases with time. Following the suspension of the shear stress, the initial viscosity is rebuilt. Simplified, this means the thixotropic liquid with the period during deformation always thinner. After completion of the shear stress, the viscosity increases time-dependent again. No longer the liquid reaches its initial value, as they are called partially thixotropic.. Not to be confused with the thixotropic shear thinning, in which the viscosity decreases with increasing shear at a constant shear stress, but then remains constant over time.

The cause of both phenomena is similar: the structure in the fluid changes under shear force to produce smaller particle interactions occur. After the shear force applied to these structural changes form more or less back quickly.

Examples

• Trust is thixotropic behavior by kneading certain plasticine ( " work softening " )
• Ketchup usually has to be shaken before he runs out of the bottle and solidifies after a short rest period again
• Toothpaste intended, when it is pressed out of the tube when highly pasty material located on the toothbrush. The relaxation time is very short and transitions into the intrinsic viscosity.
• Margarine
• Shaving Cream
• Lipstick
• Wet coffee grounds
• Special drilling fluid ( DeHydril HT)
• Sand - water mixtures, near the shore ( approximately in the range of wet sand surf beach ), Quicksand, slurries and the like ( see below)
• Fluid mud deposits in the turbidity zone of certain Ästuartypen
• In the two blood - relics in the Cathedral of Naples, which liquefy twice a year for so-called blood miracle of Januarius, is allegedly also a thixotropic substance.

Yogurt, however, is not one of the thixotropic fluids, as its structure changes after crushing are irreversible, even condensed milk - but probably pure casein, the elastic protein of milk. Even honey is not thixotropic: He becomes thinner by mechanical but thermal action - the " solidification " is a sugar crystallization.

Synovial fluid in the joints is thixotropic by the hyaluronic acid contained in it, which is why one should " warm up " before sporting activities.

Technical Applications

• Paints and emulsion paints are thixotropic after preparation with specific additives of silica gels (Celite ®, etc.).
• Printing inks are converted by mechanical action such as stirring, shaking, or Umspachteln blades of a solid or paste-like consistency into a flowing consistency. In offset printing, this task is performed by the color divisions in the inking unit, reinforced by oscillating oscillating rollers. On the substrate, the tack should increase again.
• Spray paint is to be as fast as possible viscous after spraying, however, be very fluid during sputtering and impact.
• Dental impression materials, especially polyethers, are flowable under pressure at a position of the impression tray in the mouth and the associated compression of the mass. This is the representation of difficult details through a better Anfließen beneficial.
• Thixotropiebildner prevent corrosion protection oils in a runoff by changing its consistency after spraying on metal surfaces scherungsbedingt
• Drilling fluids for deep boreholes show, among other thixotropic behavior. Bohrsuspensionen promote the cuttings to the surface, during standstill of the mud pumps, the lowering of the drill cuttings back to the well bore is prevented by the thixotropy of the Bohrsuspensionen though the drilling fluid does not move upward. For the continental deep drilling an irrigation has been developed to keep the cuttings during the flushing process at position.
• Thixotropic gel is used for the sheathing of the optical fiber submarine cables in.
• To lubricate the connection between the escapement wheel to the armature in mechanical watches a thixotropic oil is used in order to prevent running on the components in general.
• For bonding of fiber reinforced polymer composite components, for example, in aircraft, among others, concentrated and thereby made ​​thixotropic resin used.

Thixotropy in soil science

In soil science thixotropy is the process in fine-grained, mostly clayey sediments occur in the reversible by mechanical stress viscosity differences. Typically, a change from solid to liquid is by vibration with subsequent return to the solid state. Thixotropic example, Quick earth nonswelling and quicksand.

Crucial to this property are grain size composition and type of sediment forming substances. Most are platy clay minerals, which - on a microscopic scale - initially support each other in all directions. In shock, the structure breaks down like a house of cards together, the mineral platelets parallelize and begin the absence of internal adhesive forces, and under the influence of gravity past one another.

Also in this area include mass movements aqueous substrates, ie, mudslides, mudflows, and the like which are highly flowable under the high mechanical load.

Thixotropy in shipping

As thixotropy in shipping an interfacial phenomenon of the system is solid-liquid of fine-grained Ladungsgut, eg Ore concentrates and clays referred to. The dry bulk material appearing can be mushy and liquid depending on its moisture content solely by mechanical vibration, such as vibration of the ship. The result can be charge displacements with ship losses. The acceptable moisture limit for sea transport is the highest water content of a concentrate. He is 90 percent of the liquefaction point and must be determined prior charge acquisition by an official expertise.