MadSci Network: Engineering

Re: what kind of non-newtonian fluid is not viscoelastic?

Date: Tue May 11 00:58:30 1999
Posted By: Werner Sieber, Research Scientist, Pigment Division, Ciba Specialty Chemicals Corp.
Area of science: Engineering
ID: 925367876.Eg

Dear Lina,

Liquids generally differ from solids in that they are deformed permanently 
even by small (shear-)forces, e.g. their own weight under gravity makes 
them FLOW. If a small force is applied to a typical SOLID, it deforms 
elastically, i.e. after cessation of the force, it assumes again its 
original shape. Viscoelastic fluids (like jelly or certain polymer 
solutions) have something of both these classes: after being slightly 
deformed, they return to their original shape, but larger deformation makes 
them flow. It is thus the AMPLITUDE of stress that makes them cross the 
border between "solidlike" and "liquidlike" behaviour. Fluids that have no 
such "restoring force" are not visco-elastic. They can still be 
non-newtonian in various ways.
 All fluids whose viscosity depends either on the applied SHEAR RATE (speed 
divided by distance) or on HISTORY (what has been done to them before the 
moment of measuring) are non-newtonian. The first category includes 
shear-thinning and shear-thickening fluids (mud, paint..) the second mainly 
the THIXOTROPIC ones (this is probably the word you were looking for). It 
is important to distinguish these 2 categories, even though a given fluid 
can have properties belonging to both categories.

Best Regards
Werner Sieber  

Mukund Karwe adds the following examples:

Examples of non-Newtonian fluids which are inelastic are:

Orange juice, dilute solutions of Xanthan gum and carboxymethylcellulose (CMC) in water, salad dressings, tomato juice.

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