Time-dependent viscosity

Time-dependent viscosity is a property of one class of non-Newtonian fluids in which the apparent viscosity of the fluid changes with time as the fluid continues to undergo shear.

Most commonly, in a non-Newtonian fluid, the viscosity (the measure of a fluid's ability to resist gradual deformation by shear or tensile stresses) of the fluids is dependent on shear rate or shear rate history (time). These shear-thickening fluids are divided into two groups: time dependent and time-independent viscosity. In the case of time dependent viscosity, the apparent viscosity of a fluid changes with time as the fluid is continuously sheared. They can be termed as memory materials. If the apparent viscosity decreases with time, the fluid is called thixotropic and if it increases with time, it is called rheopectic. Thixotropic behaviour is the result of a break down in the microstructure of the material as shearing continues. This happens when the shear is exceeded of a limit. It leads to non-linear stress-strain behaviour. Thixotropy can be associated with the effect of bubbles. Examples of these types of fluids are gelatine, shortening, cream, paints, yogurt, xanthan gum solutions, aqueous iron oxide gels, gelatine gels, pectin gels, synovial fluid, hydrogenated castor oil, some clays (including bentonite, and montmorillonite), carbon black suspension in molten tire rubber, some drilling muds, many paints, many floc suspensions, and many colloidal suspensions. In the case of rheopatic fluids, the structure builds as shearing continues. Rheopectic behaviour may be described as time-dependent dilatant behaviour. This type of behaviour is much less common but can occur in highly concentrated starch solutions over long periods of time. Shear induced crystallization may be responsible for rheopatic behaviour. Other examples are gypsum pastes and printer inks.

Thixotropic fluids

Thixotropic fluids are fluids that show a shear thinning property. Certain gels or fluids that are thick (viscous) under static conditions will flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. They then take a fixed time to return to a more viscous state. In more technical language: some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a step change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

Applications

Drilling muds used in geotechnical applications can be thixotropic. Honey from honey bees may also exhibit this property under certain conditions.(heather honey).

Synovial fluid found in joints between some bones. Both cytoplasm and the ground substance in the human body is thixotropic, as is semen.[1] Some clay deposits found in the process of exploring caves exhibit thixotropism: an initially solid-seeming mudbank will turn soupy and yield up moisture when dug into or otherwise disturbed. These clays were deposited in the past by low-velocity streams which tend to deposit fine-grained sediment.

Thread-locking fluid is a thixotropic adhesive that cures anaerobically.

Thixotropy has been proposed as a scientific explanation of blood liquefaction miracles such as that of Saint Januarius in Naples.[2] Semi-solid casting processes such as thixomoulding use the thixotropic property of some alloys (mostly light metals) (bismuth). Within certain temperature ranges, with appropriate preparation, an alloy can be put into a semi-solid state, which can be injected with less shrinkage and better overall properties than by normal injection molding.

Solder pastes used in electronics manufacturing printing processes are thixotropic.

Many kinds of paints and inks— e.g.plastisols used in silkscreen textile printing— exhibit thixotropic qualities. In many cases it is desirable for the fluid to flow sufficiently to form a uniform layer, then to resist further flow (which may become a sagging problem on a vertical surface). Some other inks, such as those used in CMYK-type process printing, are designed to regain viscosity even faster, once they are applied, in order to protect the structure of the dots for accurate color reproduction.

Rheopectic fluids

Rheopectic fluids are a rare class of non-Newtonian fluids. These show a time-dependent increase in viscosity; the longer the fluid undergoes shearing force, the higher its viscosity.[3] Rheopectic fluids, such as some lubricants, thicken or solidify when shaken. Examples of rheopectic fluids include gypsum pastes and printer inks.

Applications

There is ongoing aggressive research into new ways to make and use rheopectic materials. There is great interest in possible military uses of this technology. Moreover, the high end of the sports market has also begun to respond to it. Body armor and combat vehicle armor are key areas where efforts are being made to use rheopectic materials. Work is also being done to use these materials in other kinds of protective equipment, which is seen as potentially useful to reduce apparent impact stress in athletics, motor sports, transportation accidents, and all forms of parachuting. In particular, footwear with rheopectic shock absorption is being pursued as a dual-use technology that can provide better support to those who must frequently run, leap, climb, or descend.

Hysteresis

After the gradual attrition of the microstructure of the fluid, the reverse situation may take place. There is no reason why the forward and the backward processes take place in the same manner. In reality, some hysteresis takes place. That is the stress-strain relation is not identical when measured with increasing and decreasing strain rates.

Blue: With increasing shear rate the system is breaking downGreen: With decreasing shear rate the system is building up

The marker and cell method

A technique is described for the numerical investigation of time-dependent flow of an incompressible fluid, of which is partially confined and partially free. The full Navier–Stokes equations are written in finite-difference form, and the solution is accomplished by finite-time-step advancement. The primary dependent variables are the pressure and the velocity components. Also used is a set of marker particles which move with the fluid.[4]

X Axis: Viscosity Y Axis: Shear Force

See also

Notes

  1. ↑ Hendrickson, T: "Massage for Orthopedic Conditions", page 9. Lippincott Williams & Wilkins, 2003.
  2. ↑ Garlaschelli, Ramaccini, Della the swagg fights of air forces Sala, "The Blood of St. Januarius", Chemistry in Britain 30.2, (1994:123)
  3. ↑ "BBC Science - How to: make a liquid that's also a solid". Bbc.co.uk. 2013-08-05. Retrieved 2015-03-08.
  4. ↑ "Numerical Calculation of Time‐Dependent Viscous Incompressible Flow of Fluid with Free Surface". Retrieved 2014-05-25.

References

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