68 E. G. PICKELS 



A. FUNDAMENTAL PRINCIPLES 

 1. Sedimentation in Uniform Field of Force 



Under the influence of gravity, particles suspended in a liquid of 

 lower density tend to settle to the bottom of the containing vessel. 

 If there were no other disturbing influences, each particle would sedi- 

 ment at a rate governed primarily by its size, shape, and density. 

 If all the particles in a very dilute suspension were identical, they 

 would sediment at the same rate, and if in a vertical column of fluid 

 the starting concentration were uniform, those particles originally at 

 the surface would form a sharply defined "rear line of march" or 

 moving boundary, which would demarcate the supernatant fluid and 

 the sedimenting phase. Until the boundary has reached the bottom 

 of the vessel, the number of particles piling up there would increase 

 at a regular rate, while the concentration in the region between the 

 boundary and the bottom of the vessel would remain uniform and 

 constant. If a preparation under such conditions contained two 

 groups of particles of two different sedimentation rates, there would 

 be formed two different boundaries, the increase in the concentration 

 of the suspended phase at each boundary representing the concen- 

 tration of the respective component. In general, multiple boundaries 

 would represent discrete particulate components, and in typical cases 

 would probably be associated primarily with differences in particle 

 size. On the other hand, a group of particles having a continuous 

 gradation in size, shape, or density within a limited range would sedi- 

 ment at varying rates and thus exhibit a spreading or blurring of the 

 boundary about the mean position, the amount of spread increasing 

 in a regular manner with the displacement of the boundary from the 

 meniscus. 



In actual practice, a boundary representing a monodisperse system 

 is not infinitely sharp because of the thermal agitation or Brownian 

 movement of the particles. The boundary becomes progressively 

 less well defined with time because of this superimposed diffusion 

 across the boundary. Also, the particles approaching the bottom 

 of the vessel do not all proceed in a regular fashion to be incorporated 

 into a closely packed sediment. There is no sharp demarcation be- 

 tween the sediment and the sedimenting phase, but rather a transi- 

 tion zone in which the concentration is graded from that of the sedi- 

 menting to that of the sedimented phase. However, when the rate 

 of sedimentation is rapid enough in comparison to the diffusion proc- 



