Dilatant fluids seem very useful, but relatively uncommon, when compared with shear-thinning fluids. Why is this so?
Imagine a relatively dense suspension of microscopic particles, like paint or blood. The stronger the particles interact, the higher the viscosity of the fluid at the macro scale. Under high shear stress, the particles in the fluid will tend to organize in lubricated layers which will lower the overall viscosity. So for suspensions, shear thinning is a very common behavior. The exception is when particles somehow interact more under high shear stress, for example by forming clusters through hydrodynamic interaction.
Now imagine a polymer, with long, coiled chains. Under high shear stress, the chains will orient, again lowering the viscosity. This happens with most polymers. A shear thickening polymer would typically have very long, very entangled chains. Think about combing tangled hair slowly vs fast. Again, shear thinning seems to be the rule and thickening the exception.
Dilatant fluids are generally used for making protection guard for armored and wearing clothes for safety of vehicles and personnel of army/firefighting personnel/sportsmen. However, these require multi layers of high density/concentration dilatant(40 to 70% solid). This increases weight of wearing clothes or protection layer and dilatant fluid volume requirement is more. Also, this protection will be useful only against high velocity object piercing but will fail to protect against slow piercing. Thus, its use is uncommon. Shear-thinning fluids are used for lubrication and paints and require less volume. Due to wide application, these are more common.
the shear thickening fluids are uncommon because of the ingredients to form these fluids. the problem is that, the application of these fluids in the industry is limited and expensive. most of the commercially feasible products are shear thinning. you can form shear thickening liquids through the formulation of polymers-surfactants complexes that have multiple identity that depends totally on the amount of shear applied.
Application of increasing shear or stress to a non-Newtonian fluid changes the rheological behavior of the fluid based on two competing mechanisms. One is the structural breakdown and the other is the increased probability of inter-molecular or inter-particulate contact leading to higher opportunities for physicochemical interactions. When physicochemical interactions create bonds that are strong enough to resist breakage during shear, this mechanism would dominate and dilatant (shear-thickening) behavior is observed. In majority of the cases, the bonds thus formed are not strong enough and hence structural breakdown is what dominates, thereby depicting pseudoplastic (shear-thinning) behavior. This is the prime reason why dilatant (shear-thickening) fluids are so uncommon as compared to pseudoplastic (shear-thinning) fluids. A better description of such and other types of non-Newtonian fluids is covered in the chapter on fundamental/basic rheological concepts in the following two books.
Aroon V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Netherlands (1999).
A. V. Shenoy and D. R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New York (1996).
Besides, the high sensitivity of their properties to environment parameters (temperature, for instance) restricted their application in many cases.
Additionally, limitations associated with simulation tools for this type of materials can be another possibility.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Plating