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Recoil (fluid behavior)
Recoil is a rheological phenomenon observed only in non-Newtonian fluids that is characterized by a moving fluid’s ability to snap back to a previous position when external forces are removed. Recoil is an observable result of a fluid’s elasticity and memory where the speed and acceleration by which the fluid moves is dependent on molecular structure and the location to which it returns is dependent on conformational entropy. This effect is observed in numerous non-Newtonian liquids to a small degree, but is exceptionally prominent in some materials such as molten polymers.
==Memory==
The degree to which a fluid will “remember” where it came from is dependent on entropy. Viscoelastic properties in fluids cause them to snap back to entropically favorable conformations.〔Rudin, Alfred, and Phillip Choi. The Elements of Polymer Science and Engineering. 3rd. Oxford: Academic Press, 2013. Print.〕 Recoil is observed when a favorable conformation is in the fluid’s recent past. However, the fluid cannot fully return to its original position due to energy losses stemming from less than perfect elasticity.
Recoiling fluids display fading memory meaning the longer a fluid is elongated, the less it will recover. Recoil is related to characteristic time, an estimate of the order of magnitude of reaction for the system. Fluids that are described as recoiling generally have characteristic times on the order of a few seconds.〔Bird, R.B. Armstrong, R.C. and Hassager, O. Dynamics of Polymeric Liquids, Vol. 1, Fluid Mechanics, Wiley, New York (1977), p. 90-91.〕 Although recoiling fluids usually recover relatively small distances, some molten polymers can recover back to 1/10 of the total elongation.〔J. Meissner, Rheol. Acta, 10, (1971), p. 230-242.〕 This property of polymers must be accounted for in polymer processing.
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