41994 str (91)

ELASTIC AFTER-_En_EC7

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-NH

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CH. CH,. S. S. CH,. CH —CO

strong Chemical

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lik:

thereby forming a As Astbury puts it :•

If we may think of twó netghbouring poW, chains as foiming the sides of a ladder, then we ^e think of cystine as contributing the rungs of the ladder^

It is found that cystine, ądded to flour dougł, increases its “ strength ”ip japacity to a good loaf), and ^diminishes the,. property ₀j “ shortness,” which will t^_: discussed in the next chapter.

Work on materials differing as ^widely as wool, hairs, flour dough, rubberkinuscles and, morę recently, cheese and butter,Bpgj|0nstrates mjięh similarity in their stracture, a fact that serves well to show the unity of rheological problems, and the importance to every practical rheologist of being well informed about every branch of the subject.

When a body is stretched and allowed jj&ipover, truły elastic materials recover immediately; ^but many substances, in practice, contract slowly. This may be pictured mechanically in terms of the model described on p. 94; but in this case we wali suppose that the whole system is encased in vaseline.j^; When the springs are extended they will not complete their whole extension instantaneously, but will „cxtend slowly. On release, they will also recover slowly. The latter phenomenon is known as “ elastic after-effect/’ and the former/although really a |nąnifesta-tion of the same property, is sometimes distinguished from it under the name of " fore-effect."

Filon and Jessop have studied these phenomena for glass and nitrocellulose.

Kohlrauch describes an amusing experiment to show the after-effect properties of a rubber thread. The thread is fastened at one end, and twisted through two complete circles to the right, and held in this position for eighteen hours. It is then released and twisted past the zero point 45⁰ to the left, and held for thirty seconds. On release it recovers first in one direction, and then it reverses

and recovers in the opposite sense. This pheno-menon can be morę ąuickly demonstrated with some of the polymerised acrylic acid compounds.

Suppose a spring is extended by a series of extensions, given say every minutę, and the load on the spring and the deformation are measured, the deformation will increase proportionately with the load, the proportionality constant being the modułus of the spring. Suppose the spring immersed in vaseline, the deformation will, as explained above, be incomplete when each reading is taken after the

INDUS. RHEO.