BINGHAM: PLASTIC FLOW 



179 



>5 

 h 



h 



o 

 S3 



fluidity of viscous substances. The friction increases as a linear 

 function of the concentration of solid present (fig. 1). It is 

 independent of the length and diameter of the capillary and of 

 the viscosity of the medium. It is, however, affected by the 

 presence of alkalies or acids in the medium. The mobility 

 decreases rapidly as the concentration of the solid is increased, as 

 is seen in figure 3. The mobility is enormously sensitive to the 

 presence of alkalies or acids, 

 the mobility of a neutral clay 

 suspension being increased 330 

 per cent by the addition of 0.1 

 per cent of potassium carbon- 

 ate. At pressures little if any 

 greater than those necessary to 

 overcome the friction there was 

 detected a seepage of the me- 

 dium past the solid particles 

 (see capillary 6.1, fig. 2). At 

 high pressures there was some- 

 times a sudden increase in the 

 rate of flow, which is apparently 



due to slipping (see capillary 0*.3, fig. 2) . If the solid material con- 

 sisted of spheres of equal size, the pore space left when the spheres 

 were as closely packed as possible would amount to about 26 

 per cent, quite irrespective of the radius of the spheres. How- 

 ever, due to the friction of the spheres on each other, the pore 

 space may be larger than this, and this is particularly true if the 

 material is finely divided. As a matter of fact, it was found that 

 on shaking dry clay into a flask the pore space amounted to 

 81.6 per cent of the total volume. This corresponds closely 

 to the percentage of liquid present in the mixture having zero 

 fluidity, which is 80.5. It is upon this friction that plasticity 

 depends and the plasticity is thus closely related to the fineness 

 of subdivision of the material. 





Fig. 3. 



25 50 75 100 



VOLUME PERCENTAGE CLAY 



Relation of mobility to 

 concentration. 



