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CHAPTER X

Fibrosity and Work - hardenlng. Adhesion “ Stickiness.” “ Olliness." Shortness and TensI'¹ Strength. Their Relationship with Structural Vlscoslty Extensimeters. Applications to Egg-white, H'oney, Flour Fats and Glasses. Viscosity of non-Newtonian Materials ’

In making artificial textiles, ldąuids are sometimes drawn out into threads which " 'solidify f as they are drawn out.. Long threads can be drawn from such adhesives as seccotine, and such threads have been suggested for use as crosswires in microscopic work. Egg-white shows this property to a marked extent. Erbring, studying this property in^jdetail, calls it I Spinnbarkeit,” which perhaps is best translated as “ fibrosity.” He takes ą narrow capillary tubę, with a conical mouth into which he sucks the materialsthen he withdraws the tubę at a constant ratę, and measures the tńaximum length of the fibrę which can be produced, before rupture occurs. Over a considerable rangę, this length is proportional to the ratę of extension. He finds that there is no universal connection between A (the maximum length of the fibrę under standard conditions) and either viscosity or surface tension. Some of the oils have much highef yiscosities than egg-white, and yet are not fibrous. In some cases, Erbring has observed parallelism between A and structural viscosity; but this is not generał. Similar substances often show a parallelism between I and A.

The author has tested a number of honeys, the ereater majority of which show a smali A, pro-portional to j; but tarę exceptions show A’s perhaps fifty times as great, having no direct bearing on the viscosity. The surprising thing is that such honeys appear to obey Poiseuille’s law in the ordinary capillary tubę techniąue. Experiments deseribed by Schofield and Scott Blair give some indication of what is taking place in these materials. In their experiments, a blob of honey is anchored at one end on a smali piece of cork, and floats on mercury. This blob is then extended and, in the caseof fibrous {“ stringy ”) honey, a long, stabłe fibrę is formed. After holding for some minutes and then releasing, the fibrę gradually retracts till the blob is re-formed. The deformation is completely fecoverable. The last part of the recovery is naturally influenced by surface tension; but no rod of a length greater than tt times its diameter is stable under simple surface tension forces. Fibrous honeys also show the phenomenon of 'dłow-elasticity. There is evidently a kind of work-hardening, analogous to that found in metals. When a rod of metal is extended, it would neck and break, but for the fact that the consistency increases enormously under straining. This means that the sample hardens where necking has started, and is strengthened where it most needs it. Some kind of elastic structure is set up in fibrous materials under the particular strain condifions of fibrę formation; but such conditions are not present in ordinary shear in a tubę.

In the case of a viscous, or partly viscous materiał, which can be cast into cylinders, a viscosity can be measured by loading. If W is the load on a cyhnder of radius r, and if the height is permanently reduced