94 The Structure of Protoplasjn 



tion, which presumably has the same mechanism as the thixotropic 

 transformation and which is not changed essentially by a change of 

 temperature, and (2) a change in the solvation of the particles, 

 which depends pronouncedly on temperature.'^"' The latter factor 

 may secondarily influence the sol ^ gel change. Gelatin and agar 

 have the normal behavior — the gel is formed at low temperatures, 

 the sol at higher ones — whereas sols of methyl cellulose show the 

 so-called inverse, nonisothermal sol ^ gel transformation: The gel 

 is produced at higher temperatures and turns to a sol again on 

 being cooled. 



The problems of thixotropy are always those of a special kind of 

 coagulation. Dilatancy is observed when the particles have a very 

 small, or perhaps even the least possible, tendency to be coagulated, 

 i. e., they do not stick spontaneously to each other at all. As long as 

 the suspension is exposed to weak mechanical forces, the particles 

 glide easily past each other, thanks to the continuous layers of liquid 

 between them. But if the forces applied exceed a certain limit, the 

 particles are brought much closer together. This may lead to a 

 displacement of the layer causing the independence of the particles — 

 the layer of oleic acid in the case of the iron particles, or the electric 

 double layer in aqueous suspensions — and the particles may be 

 made to touch each other. This will cause a high resistance toward 

 the acting force. As soon as the force stops acting, the particles 

 return to their independent behavior, which corresponds to a state 

 of equilibrium, and the suspension again assumes its liquid state.^' 



We do not yet know whether thixotropy and dilatancy are 

 always produced by the same kind of mechanism or whether one will 

 have to distinguish different types of these phenomena in the future. 

 These do not represent the only characteristic features of anomalous 

 viscosity in colloidal solutions and in suspensions. One other phe- 

 nomenon may be mentioned, because it has been confused with 

 dilatancy. 



Gels of V-Or, and of bentonite, suspensions of clays, etc., are 

 liquefied by strong shaking and stirring, but a weak movement — a 

 tapping or rolling of the vessel containing the liquefied system — 

 may markedly increase the rate of setting to a solid gel or paste. 

 This phenomenon has been called rheopexy.^*^ Striking experiments 

 were done by Hauser and Reed ^^ with aqueous bentonite sols; these 

 sols were fairly homodisperse, the particle diameter of the finest 

 dispersion being about 15 mji (a) , that of the coarsest 35 mf^i (b) . The 

 times of normal thixotropic setting were 42 seconds and 70 minutes 



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