Sunday, 28 April 2013

NEWTON’S LAW OF VISCOSITY / NEWTONIAN FLUIDS / NON NEWTONIAN FLUIDS


NEWTON’S LAW OF VISCOSITY :-
NEWTON’S LAW OF VISCOSITY STATES THAT “ SHEAR STRESS IS DIRECTLY PROPORTIONAL TO VELOCITY GRADIENT ”. 
THAT IS THE SHEAR STRESS BETWEEN THE TWO ADJACENT LAYERS OF FLUID IS DIRECTLY PROPORTIONAL TO THE NEGATIVE VALUE OF THE VELOCITY GRADIENT BETWEEN THE SAME TWO ADJACENT FLUID LAYERS.
MATHEMATICALLY THE LAW CAN BE STATED AS :-
    
     τyx   = - µ dVx / dy   
 
WHERE  µ IS THE CONSTANT OF PROPORTIONALITY KNOWN AS DYNAMIC VISCOSITY HAVING UNITS N.s / m2 .

NEWTONIAN FLUIDS
THE FLUIDS THAT OBEY THE NEWTON’S LAW OF VISCOSITY ARE KNOWN AS NEWTONIAN FLUIDS.

NON NEWTONIAN FLUIDS
THE FLUIDS THAT  DO NOT OBEY NEWTON’S LAW OF VISCOSITY ARE CALLED AS NON-NEWTONIAN FLUIDS.

TYPES OF NON NEWTONIAN FLUIDS
•      NON-NEWTONIAN FLUIDS ARE DIVIDED INTO 2 BROAD CATEGORIES ON THE BASIS OF THEIR SHEAR STRESS/SHEAR RATE BEHAVIOUR AS :-
1)    THOSE WHOSE SHEAR STRESS IS DEPENDENT ON TIME.
2)    THOSE WHOSE SHEAR STRESS IS INDEPENDENT OF TIME.

SHEAR DIAGRAM FOR NEWTONIAN AND TIME-INDEPENDENT NON-NEWTONIAN FLUIDS


  1) NON-NEWTONIAN FLUIDS WHOSE SHEAR STRESS IS INDEPENDENT OF TIME
      ARE :-

 BINGHAM PLASTIC FLUIDS
•      THESE ARE THE SIMPLEST TYPES OF NON-NEWTONIAN FLUIDS. THEY DIFFER FROM NEWTONIAN FLUIDS ONLY IN THE LINEAR RELATIONSHIP THAT THEY DO NOT PASS THROUGH ORIGIN.
•      EXAMPLES :- SOAP, TOOTHPASTE, PAPER PULP, SEWAGE SLUDGE, CHOCOLATE MIXTURES ETC.
 

PSEUDOPLASTIC FLUIDS
•      THE MAJORITY OF NON-NEWTONIAN FLUIDS ARE INCLUDED IN YHIS CATEGORY.
•      FLOW BEHAVIOUR OF PSEUDOPLASTIC FLUIDS SHOW AN DECREASE IN APPARANT VISCOSITY  WITH INCREASE IN SHEAR RATE.
•      EXAMPLES :- POLYMER SOLUTIONS, GREASES, STARCH SUSPENSIONS, BIOLOGICAL FLUIDS, DETERGENT SLURRIES ETC.
 
DILATENT FLUIDS
•      THESE FLUIDS ARE LESS COMMON THAN PSEUDOPLASTIC FLUIDS.
•      FLOW BEHAVIOUR OF DILATENT FLUIDS SHOW AN INCREASE IN APPARANT VISCOSITY  WITH INCREASE IN SHEAR RATE.
•      EXAMPLES :- CORN FLOUR –SUGAR SOLUTIONS, WET BEACH SAND, STARCH IN WATER, POTASSIUM SILICATE IN WATER ETC.

VISCOSITY SHEAR OVERTIME DIAGRAM FOR TIME-DEPENDENT NON-NEWTONIAN FLUIDS

  
2) NON-NEWTONIAN FLUIDS WHOSE SHEAR STRESS IS DEPENDENT OF TIME 
    ARE :-

THIXOTROPIC FLUIDS
•      THESE FLUIDS EXHIBIT A REVERSIBLE DECREASE IN SHEAR STRESS WITH TIME AT A CONSTANT SHEAR RATE.
•      EXAMPLES :- SOME POLYMER SOLUTIONS, SOME FOOD MATERIALS AND PAINTS.

RHEOPECTIC FLUIDS
•      THESE FLUIDS ARE QUITE RARE IN OCCURANCE.
•      THESE FLUIDS EXHIBIT A REVERSIBLE INCREASE IN SHEAR STRESS WITH TIME AT A CONSTANT SHEAR RATE.
•      EXAMPLES :- BENTONITE CLAY SUSPENSIONS, CERTAIN SOLS AND CLAY SUSPENSIONS.

     3) VISCOELASTIC FLUIDS
•      IN ADDITION TO THESE NON-NEWTONIAN FLUIDS  SOME NON-NEWTONIAN FLUIDS EXHIBIT ELASTIC BEHAVIOUR AND ARE CALLED AS VISCOELASTIC FLUIDS.
•      EXAMPLES :- FLOUR DOUGH, POLYMER MELTS ETC.




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