str (94)

~n&PEXY AND PASSIVE £>_Il    ^1

beaker is rotatcd, the ratę of setting is m    j I

Ihe rotation. Freundlich and    I

discussed this m connection with gyng firi- II v.madium pentoxide sols, but it is now    11

present in other systems, such as bentonite _a ^to ^ 11 Thixotropy is clearly due to an orientaf ^ell>l 1 suspended particles or molecules, which. b°^{n of} disorientated in shear, and re-orientate sW^COllle rcsting. If the re-orientation takes pl_ace ^ ⁰¹¹ rapidly, the phenomenon appears only as a st^ 1 tural viscosity. It is natural that any influen i such as flow, tending to alter the orientation, shoulj j alter the ratę of setting. Sols in which the molecular orientations are affected by electric and magnetic fields are of a similar naturę. Electric currents can be madę to liąuefy and solidify thixotropic bentonite pastes.

Freundlich and Jones have recently described a modification of dilatancy, which they cali ” passive dilatancy/’ In this phenomenon, materials not showing ordinary dilatancy “ exude liąuid when gently tapped.” This phenomenon sometimes goes hand in hand with thixotropy. Freundlich puts forw^?ard the idea that close packing is associated with smali sediment volume and dilatancy, and open packing with thixotropy and plasticity; but to the last part of the hypothesis there are exceptions. He points out that materials having anisotropic particles tend to give thixotropic pastes.

An interesting case of thixotropy, ąuoted by Freundlich, is that of ocean-bed mud and its capacity for preserving fossils. Animals are deposited on a shallow beach which is often flooded by sea water. binc dust blown from neighbouring downs covers them and, when mixed with sea water, forms I ^This Phenomenon is called Rheopexy.

plastic sludge thixotropic*

This sludge, which takes the impression a    forms, must solidify very ąuickly, a

of _which cannot be accounted for by drying. ^c°^ncH _re(j _slate mixed with sea water gives a highly P°^w. -irather like putty: but 1

it is qmte

Harvey suggests that thixotropy plays an impor-tant part in the behaviour of protoplasm.

It will be shown later that some emulsions show a type of dilatancy (inyerse thixotropy).

BIBLIOGRAPHY

Evans and Reid. Trans, of Mining and Geol. Inst. of India,

1936,    XXXII., 1.

Freundlich. " Thixotropy,” 1935. Hermann et Cie, Paris, etc.

Freundlich and Jones. J. Phys. Chem., 1937, XL., 1217. Freundlich and Juliusburger. Trans. Parad. Soc., 193?. XXXI., 920.

Fulmer and Williams. J. Phys. Chern., 1936, XL., 143. Gamble. J. Ind. Eng. Chem., 1936, XXVIII., 1204.

Goodeve and Whitfield. Trans. Farad. Soc., 1938, XXXIV., 5”-

Harvey. J. Appl. Phys., 1938, IX., 68.

Kandelaki, Kikodze and Dolitze. Russ. J. Phys. Chem.;

1937.    X., 521.

Nieuwenburg, van. "First Report on Viscos. and Plastic.,” Dutch Roy. Acad Set., 1935.

Pryce-Jones. J. Oil and Col. Chem. Assoc., 1934, XVII., 305 ; 1936. XIX., 295- .

Scott Blair. J. Phys. Chem., 1935, XXXIX., 213, Williamson. J. Ind. Eng. Chem., 1929, XXI., 1108. Williamson and Heckert. J. Ind. Eng. Chem., 1931. XXIII., 667.

Williamson, Patterson and Hunt. /. Ind. Eng. Chem., 1929, XXI., im-