str (95)

of sbear ; but it is admitted that thia treat only applicable for COmparatively simple sys?¹*¹¹⁴ U Pryce-Jon es maintains that thixotropic rn⁶?¹³- I should be tested at a constant ratę of shea ^c'^ah 1 lias deyised an interesting aipparatus for do®¹’

A cylinder is rotated in the j4SJdxotropic    1

the movements being recorded by means of a 1 of light reflected from a tnirror, attaohed tofB torsion wire. The rotation is produced by the acti I of electromagnets on a mpgnetised rod,.which af*¹ 1 is attached to the torsion wire. The light spot cf° I draw its own curve on a photographic piąte. Try^ Jones has studied all kinds of rnaterials . . . paints, creams, bentonite clay and heather honey. The curves show elastic recoil in those colloidal Systems in which it occurs, and also distinguish between the two types of thixotropic behaviour . . . thixotropy (a xeversible gel-sol tfeasformation) and false body, which Pryce-Jones describes as a | partial dispersion of a gel under the influence o{ shear and the re-formation of the gel on the yemoval of shear." Since no system adheres strictly to the form er definition, it is doubtful if the pjistinction has any great significance. Gamble uses an apparatus rather similar to    Piyce-Jones’ to

study the thixotropy of paints in relation to

" broshabiiity.”

The thixotropy of heather honey is important in connection with botind water, bubble^ size and spreading capacity. The consumer likes heather honey to have large air bubbles, and in order to have this, the viscosity at rest must be high.?! If the honey is to spread without breaking the bread|the viscosity, after working with a knife, should net be too great; hence the advantage of 'Considerable false body. Heather honeys often contain morę

mmmm ^

than could exist in other honeys without v/*^tetr 0f fermentation; but if the honey is ^tropie, some of the water appears to exist in ^tb?und cońdition, not available for ferments. The 9H matter is still rather uncertain.

^W The importance of thixotropy for drilling muds has iready heen mentioned. Evans and Reid have used ?he Scott Blair-Crowther plastometer to make a complete study of the flow properties of drilling jauds. Thixotropy can be both advantageous and (jisadvantageous in this materiał. If the mud is not sufficiently thixotropic, settling takes place as soon as agitation of the mud ceases, and this gives an uneven concentration when the agitation is resumed; but too much thixotropy is disadvantageous. Turbulence comes in at a surprisingly Iow ratę of shear, in fact pług flow conditions and turbulent conditions almost overlap. Evans and Reid point out the commercial importance of good quantitative measurements of the rheological properties of drilling muds, in view of the large sums of money invołved in procuring and utilising the mud.

Kandelału, Kikodze and Dolitze claim that there

is thixotropy in what are generally considered to be ordinary gels, such as gelatine and agar; but the grounds for the condusions are doubtful.

Hamaker attempts to explain thixotropic behaviour in terms of potential-energy/distance curyes.¹ When such curves show a Iow maximum at a considerable distance, particles can form a loose association, which is easily destroyed by shaking, but re-establishes ltself on standing.

There is another phenomenon associated with thixotropy. When a thixotropic materiał in a

1

Obtained by plotting the potential energy against the dis-tance from the surface of the molecule.