VISCOSITY 225 



v = !silk 



The value of k(ir/S) is inserted in the more usual form of 

 Poiseuille's formula: 



irprH 



Pure liquids and true solutions all obey Poiseuille's law and are 

 said to be Newtonian, that is to say, to exhibit true viscous flow. 

 But many solutions do not obey Poiseuille's formula; they are 

 non-Newtonian and exhibit anomalous flow. They possess not 

 one viscosity value but an infinite number. Why this is true is 

 not known. The fact and its causes have been the center of a 

 lively and informative discussion which led to the founding of 

 the Society of Rheology through the activity of the American 

 chemist Eugene Bingham. We cannot hope here to consider 

 the technical and mathematical features of this difficult problem, 

 but we can learn something of the nature of non-Newtonian 

 substances and consider a possible cause of their anomalous 

 behavior. 



Practically all of the lyophilic colloids — the organic gel-forming 

 elastic systems (e.g., proteins) — exhibit variable viscosity; they 

 deviate from Poiseuille's formula. If a pure liquid such as water 

 or glycerin is run through a capillary viscometer at different 

 pressures, the calculated viscosity value remains the same. 

 The rate of flow v/t will change with pressure, but the viscosity rj 

 remains constant. All pure liquids have a definite viscosity 

 value which is characteristic of them. But if a solution of 

 gelatin, albumin, or soap is run through the capillary at different 

 pressures, each experiment yields a different viscosity value until 

 a certain maximum pressure is reached. Which of these variable 

 viscosities is the correct one? The only one that has something 

 in its favor is that obtained at high pressure, as it remains fairly 

 constant with increased pressure. 



The difference in behavior between a pure liquid or true solu- 

 tion exhibiting true viscous flow and a colloidal or other non- 

 Newtonian fluid exhibiting anomalous flow is seen in a graph. 

 The line OA (Fig. 110) is the graph of a Newtonian liquid where 

 rate of flow is plotted as ordinates, and the pressure or shearing 



