DIFFUSION IN DEFORMED GELS 



By EMIL HATSCHEK, F.Inst.P. 



Elastic gels, e.g. gelatin gels, are optically isotropic when 

 free from strain and exhibit accidental birefringence when 

 deformed. This behaviour has been known for a considerable 

 time ; as the refractive indices of the ordinary and extraordinary 

 ray differ very slightly, specimens of some thickness can be 

 examined in polarised light, and this method has been used 

 in several investigations on the elastic properties of gelatin 

 gels.^ Reiger employed it for determining the relaxation time, 

 with a view to testing Maxwell's relation between modulus, 

 viscosity, and relaxation time, and found the latter to be of 

 the order of minutes at 29° C. — a temperature which is very 

 near the " melting point " of gels with the concentrations 

 used by him. 



No other property of gels has been studied from this point 

 of view. One such propert}'', which is of considerable importance 

 in the economy of nature, is the permeability of gels to sub- 

 stances in true solution in the liquid constituent of the gel. 

 The diffusion of true solutes into gels has been studied by 

 Thomas Graham and several later investigators, but the ques- 

 tion whether elastic gels under stress become anisotropic or 

 remain isotropic as regards diffusion velocity does not appear 

 to have been raised. Assuming gels to be heterogeneous 

 systems, anisotropy seems at least possible, and the experi- 

 mental investigation of the question appeared to me of interest 

 from two reasons : on the one hand, because the results might 

 throw some light on this very question of gel structure, and 

 on the other, because a considerable number of processes in 

 organisms, which depend on diffusion, take place in gel-like 

 media subject to continuous or periodic deformations. 



Reference has been made above to Reiger's results, viz. 

 complete relaxation in 10 to 40 minutes. If these held good 

 at ordinary temperatures, investigation of the diffusion velocity 

 would have been hopeless, as diffusion experiments in gels of 



^ E. Hatschek, " The Properties of Elastic Gels." General Discussion 

 on " The Physics and Chemistry of Colloids, etc," October 25, 1920. H.M. 

 Stationery Ofi&ce. 



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