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CHAPTER VII

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Relation to Structural Yiscosity.

łts Appiicatfon to Dyestuffe, Cełlulose DerS**¹' Amalgams, fes, Solders, Soaps, Oils and Bitumer^& Cream, etc The Naturę of Yield-value L£

In industry, the problem often arises as to how to I treat data from materials that do not show straight- I linę flow cnrves in the Ringham treatment. It was I Reynolds who first płotted log(Flow)/log(Pressure); I but Ostwald and his pupOs have done the I bulk of the work, and the treatment is generally I associated with the łatter name. There is an I enormous number of materials which show good I straight-łine curves with this method. Rubber- I benzene sols are a case in point, as shown by the I detailed work on this system carried out by Dogadkin I and Pewsner of the Rubber Industrial Institute, I Moscow. Gering and Sauerwald, and alsojgjjlohler, I have worked on these lines with metal śćmalgams; I and McLean, Peek and Schumacher find that wiping I solders give good Ostwald curves. Nisizawa|.applies I the Ostwald treatment to vegetable oils and vatious I cełlulose compounds. Ostwald and Riedel fedeal I with metal soaps in benzene: iPhiHppoff |foitłi I viscose, developing the theory B^nsiderably: Rogovin and Ivanova with cełlulose esters :||and I Slansky and Kohler with vegetable oils.

Heymann describes an interesting system, narnely I aąueous methyl cełlulose. This is a Śbermo-

n^tion _Qf ffrg system from trały fluid behaviour w*eases as the temperaturę fałls towards the ^lation point, and at certain temperatures, Sotropy is shown. The thixotropic gel, but not tbe true gel, can be destroyed by ultrasonic vibration.

UCTURA

'ISCOSTT

The Ostwald method is espetially apphcable to those lyophflic materials, which have a (presumably) compressible dispersed phase, and to materials showing no definite yield-value. 11 may be that as pressure rises, dispersed particles undergo an elastic change of shape, which eases the flow and lowers the viscosity. Another possibility is Hatschek's suggestion that the soIvated envelopes are gradually sheared off as stress rises.

In the case of an orientated structure, work has to be done to disorientate (break up) the structure, and the polar mołecules reairange themselves when the stress is removed. If the rearranging process is slow enough to be followed, the materiał appears to be thixotropic. Ostwald’s explanation of the phenomenon does not go so far as this; but he shows that in systems which possess “ structure/’ the flow-rate/pressure curve curves upwards as stress increases, and he calls the phenomenon " Strukturviskositat,” which is best transłated,

" structural viscosity.” Some of the curves given by Ostwald and his pupils are sigmoid in shape (uicłe Fig. 15), and he explains the apparent rise in viscosity at high stresses by postulating a type of turbulence, due to the formation of eddies of the continuous phase round the dispersed particles.

Cunningham claims curves very much like this for highly concentrated clay suspensions. Mohr and