456 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Equation (XVII) can be written in the form -t_ -t _ln IJ(t)_Co_C_t ) (XlX) -- '•'R CRTJR O' If the right-hand side of equation (XIX) is plotted against t, the slope of the line represents- 1/C R • R, so that •R and zR can be calculated since CR is known. In this way all parameters associated with the near-stationary structure of a solid material, and the effect of recipe modifications, processing, etc., can be studied. This technique has proved suitable for investigating soap texture. MODULUS OF RIGIDITY The main advantage of this test (20) for materials showing a high degree of elasticity is simplicity of the apparatus required. It consists of a glass capillary U-tube, one arm of which opens up into a wider bore tube, while the other capillary arm is graduated in mm (Fig. 7a). The tube is filled with mercury to the base of the wide-bore tube, and a fixed volume of sample is then introduced into the latter. In the case of a gel 20 ml of the warm solution is pipetted through the opening of the wide bore arm, and allowed to set overnight under careful temperature control. The sample is subjected to variable shear conditions by using air pressure, and the resultant displacements cause the mercury to move up the graduated capillary arm. Air pressures are registered with a mercury manometer. Provided L is very much larger than R pR 4 Ci ---- 8LB•H maximum strain at the tube wall maximum stress (xx) 4Q (xxI) r•R a PR (xxn) 2L where G is modulus of rigidity, P is the net applied pressure, i.e. corrected for the back pressure of mercury, L and R are the length and radius respectively of the cylindrical column of sample, B is radius of the capillary tube along which the mercury moves, H is displacement of the mercury meniscus, and Q is the volume displaced. The above apparatus is suitable for measuring high values of G.

TECHNIQUES FOR ASSESSING RHEOLOGICAL PROPERTIES 457 cop/llory orro Sounders and Word's method for determining_ r._/gidity of hard gels. •0 mortomelet •l'i i,.•.' i i i '. ,'. ', I • Cop// Ior y Wide bore gloss 't/' tube Modified technique for determining. the r__/g/d/ty of soft gels. Figure 7