SOME MECHANICAL PROPERTIES OF SOLS AND GELS AND 

 THEIR RELATION TO PROTOPLASMIC STRUCTURE 



Herbert Freundlich 



Editor's Note: This article was found among the papers of the late Professor 

 H. Freundlich in an unfinished state. The first part, dealing with the mechanical 

 properties of sols and gels, seems to be nearly complete. No notes concerning the 

 latter part were found. Except for minor styhstic alterations and correction of 

 typographical errors, the manuscript is printed as found. The references and 

 figures were added by L. Moyer and K. Sollner. 



In older textbooks of physiology, let us say of about 1900, the 

 state of aggregation of protoplasm is left very unclear.^ It is fre- 

 quently considered to be fluid. On the other hand, it is emphasized 

 that life processes seem to require a well-defined "organization" 

 and that such an organization can hardly be imagined without some 

 kind of a more or less solid structure. There is practically no indi- 

 cation of the fact that the mechanical properties of protoplasm differ 

 essentially from those of normal liquids or solids and that they may 

 be correlated with its colloidal nature. References to colloids are 

 found, if at all, when the impermeability of membranes to proteins, 

 or swelling, or the scarcity of distinct crystals are discussed. 



This outlook has completely changed during the last twenty 

 years. It has been found that there may exist a number of inter- 

 mediate stages between the normal liquid state and the state of a 

 crystalline solid. These are frequently observed in colloidal sys- 

 tems, particularly if they contain a sufficiently high amount of dis- 

 perse phase. Consequently, concentrated colloidal solutions may 

 differ essentially in their mechanical properties (viscosity, elas- 

 ticity, etc.) from normal, so-called Newtonian liquids. These differ- 

 ences may be manifold. Two limiting cases are of outstanding im- 

 portance; the first is that of thixotropy. Whereas the viscosity of a 

 Newtonian liquid is not changed by mechanical means, such as flow- 

 ing or stirring, a thixotropic system becomes less viscous while 

 flowing or when being stirred. This phenomenon is particularly ob- 

 vious if we have a thixotropic gel: It is liquefied by shaking and sets 

 again to a gel when at rest. We thus have an isothermal, reversible, 

 sol-gel transformation. - 



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