Time-dependent viscosity

Some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity and a non-linear stress-strain behavior in which the longer the fluid undergoes shear stress, the lower its viscosity becomes. A thixotropic fluid is one that takes time to attain viscosity equilibrium when introduced to a step change in shear rate. When shearing in a thixotropic fluid exceeds a certain threshold, it results in a breakdown of the fluid's microstructure and the exhibition of a shear thinning property. Some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity and a non-linear stress-strain behavior in which the longer the fluid undergoes shear stress, the lower its viscosity becomes. A thixotropic fluid is one that takes time to attain viscosity equilibrium when introduced to a step change in shear rate. When shearing in a thixotropic fluid exceeds a certain threshold, it results in a breakdown of the fluid's microstructure and the exhibition of a shear thinning property. Certain gels or fluids that are thick (viscous) under static conditions will begin to thin and flow as they are shaken, agitated, or otherwise stressed. When stress ceases, they regress to their more viscous state after a passage of time. 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 increasingly fluid when agitated. Cytoplasm, synovial fluid (found in joints between some bones), and the ground substance in the human body are all thixotropic, as is semen. Some varieties of honey (e.g.heather honey)can exhibit thixotropy under certain conditions. Some clays (including bentonite and montmorillonite) exhibit thixotropy, as do certain clay deposits found in caves (slow flowing underground streams tend to layer fine-grained sediment into mudbanks that initially appear dry and solid but then become moist and soupy when dug into or otherwise disturbed). Drilling muds used in geotechnical applications can be thixotropic. Semi-solid casting processes such as thixomoulding use the thixotropic property of some alloys (mostly light metals, e.g. bismuth) to great advantage. Within certain temperature ranges and with appropriate preparation, these alloys can be injected into molds in a semi-solid state, resulting in a cast with less shrinkage and other superior properties than those cast in normal injection molding processes. Solder pastes used in electronics manufacturing printing processes are thixotropic. Many kinds of paints and inks (e.g. the plastisols used in silkscreen textile printing) exhibit thixotropic qualities. In many cases it is desirable for an ink or paint to flow sufficiently fast to form a uniform layer, but then resist further flow (which on vertical surfaces can result in sagging). Thixotropic inks that quickly regain a high viscosity are used in CMYK-type printing processes; this is necessary to protect the structure of the dots for accurate color reproduction. 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.