It is the property exhibited by some gels or fluids that are generally viscous or thick under normal conditions, but turn to a less viscous state when shaken, stirred or agitated. These gels later take a certain period of time to return to their original state when allowed to stand without being disturbed.
Viscosity of a non-Newtonian pseudoplastic fluid is inversely proportional to time; longer the fluid is prone to shear stress, lower its viscosity. The fluids have flow properties that are not described by a single constant value viscosity. These are dependent only on the shear rate. Thixotropic fluids generally take a specific amount of time to attain equilibrium viscosity when the value of the shear rate changes. The term “thixotropy” is derived from the Greek word “thixis” that refers to touch and “tropy” meaning, of turning, or to turn.
Some of the common examples of thixotropic fluids include:
These are generally synthetic fluids whose apparent viscosity or consistency decreases rapidly with an increase in shear rate, resulting in thixotropy. Ketchup, nail polish, and whipped cream are the regularly used thixotropic fluids. This property also exists in many artificial fluids like yoghurt that usually takes longer time to become nearly solid.
Some civil engineering fields like structural and geotechnical engineering use clays that display thixotropy. Many clay deposits that are found in mountain caves do undergo thixotropy when being explored. In this process, continuous stress or disturbance on the pseudosolid mudbank causes liquefaction, resulting in a semi-solid condition. In the past, these clays would have slowly got deposited by low-velocity streams that had a tendency to accumulate fine-grained sediment.
Landslides that have occurred in the cliffs around Lyme Regis, Dorset as well as in the Aberfan spoil tip disaster in Wales prominently describe this geological phenomenon. Thixotropy is also observed in a type of mudflow called lahar during a volcanic eruption.
Drilling muds that are used for drilling of boreholes into the earth for extracting oil and natural gas are thixotropic in nature. Heather honey extracted from honey bees also imbibes this property.
Synovial fluid found in the joints of some bones to lubricate the articulating surfaces is a good example of a thixotropic fluid. The amorphous, gel-like, non-cellular component of the extracellular matrix called ground substance is also thixotropic. The property is also demonstrated by semen- an organic fluid secreted by the sexual organs of adult human males and hermaphrodite animals.
A wide range of thixotropic fluids are used in innumerable industries like:
Thread-locking fluid is commonly used as a thin, single-component thixotropic adhesive. It is generally applied to the threads of fasteners such as screws and bolts to prevent loosening, leakage, and corrosion. The adhesive undergoes time-dependent decrease in viscosity when subjected to shear stress to show thixotropic behavior.
In semi-solid casting processes like thixomoulding, few alloys, primarily comprising of light metals like bismuth are majorly used, owing to their thixotropic property. Unlike injection molding, here, an alloy can easily attain semi-solid form within a limited temperature range. Any form of alloy can be induced with less shrinkage and improved properties using this thermal preparation.
It is an alloy or pure metal that liquefies on application of heat followed by melting. It later flows onto the space between two close fitting parts, creating a soldered joint. The solder cream is generally applicable in electronics manufacturing printing processes that connect the leads of integrated chip to attachment points on a printed circuit board.
Varieties of inks known for their thixotropic property are used in a number of printing processes such as silkscreen textile printing and CMYK-type process printing. Thixotropic ink is used in Fisher space pen that claims to write in zero gravity, underwater, over greasy and wet paper, and at any temperature. The consistency of the ink is quite identical to thick rubber cement. It flows smoothly due to the shearing action of the rolling ball in socket of the pen. The shearing effect causes the solid gel thixotropic ink to undergo liquefaction that enables the pen to write on the surface of various mediums.