l. - s::- 100 8 6 4 2 10 4 2 1 FACIAL MASK OF DEAD SEA MUD 4 6 8 10 y (1/s) 4 6 8 100 Figure 6. Effect of temperature on the apparent viscosity of the facial mask (40°-60°C). 449 here that this type of starch was modified to start gelatinization at a low temperature, compared to natural starches. For example, the initial gelatinization temperature of wheat starch in water was found to be in the range of 5 5 ° to 66°C and for corn starch in water was found to be in the range of 65 ° to 76°C (17). This gives an advantage for the suspensions utilizing this polysaccharide: the system will be highly stable at room temperature. As the rheological parameters are concerned, the results of Table I demonstrate that the facial mask mud yield stress is strongly dependent on temperature. The values of the yield stress reflect the behavior of the apparent viscosity with temperature. The three stages of temperature effect on the yield stress can be distinguished easily in Table I. On the other hand, the shear-thinning behavior, which can be assessed by inspecting the values of the flow index, n, is the most pronounced at the end of the second stage and at the beginning of third stage (40 ° -45 ° C). SHEARING TIME EFFECT As mentioned above, the apparent viscosity of the facial mask was measured by increas­ ing (forward measurement) and decreasing (backward measurement) the shear rate in order to test the presence of a time-dependent behavior. The flow curves (T versus �) of the mud mask at different temperatures are shown in Figure 7. There are hysteresis loops between the forward and backward curves, indicating a time-dependent rheological

450 p 100 JOURNAL OF COSMETIC SCIENCE 4,-�===========�--------------------, 8 6 4 Facial Mask Forward measurement Backward measurement T= 5 ° C �-- -- --·-7 �,_:::.:..:::::::..�:__--_________________. T = 60 ° C 2 -+----......--------------------------------------- 4 1 6 8 10 y (1/s) 4 6 8 2 100 Figure 7. Temperature effect on the hysteresis loops of the flow curves of the facial mask. behavior. As shown in Figure 7, at low temperatures the direction of the hysteresis loops is counterclockwise, indicating an anti-thixotropic behavior, which means that there is an increase in the mud viscosity with shearing time. In some conditions, the right kind of attraction between particles of mud is given shearing can then promote temporary aggregation rather than breakdown, due to the collision of these attractive particles. This results in anti-thixotropy (18). Like other similar suspensions, there is a range of flow conditions under which shear-enhanced collisions make structure rather than break it (18). However, this anti-thixotropic behavior is relatively small (according to the size of the hysteresis loop) and disappears gradually with increasing temperature. Above 25°C, the facial mask shows hysteresis loops with a clockwise direction, indi­ cating a thixotropic behavior. The size of the hysteresis loops becomes wider as the temperature increases from 25° to 60 ° C (see Figure 7). It should be pointed out that the shear-thinning and thixotropic behaviors have indus­ trial and commercial significance. For example, since the viscosity decreases with shear rate and shearing time during the mixing process, this will lead to less power consump­ tion. Moreover, particle sedimentation, which in this case would negatively affect the consumer acceptance of the product, will occur slowly due to high viscosity at rest conditions. On the other hand, the shear-thinning and thixotropic behaviors have a significant importance in the ability of the facial mask to spread on the skin, where the Dead Sea mud mask can break down for easy spreading and the applied film can gain viscosity instantaneously to resist running. Newtonian materials do not behave in this way, because when spread on the skin they run very quickly, reducing the thickness of the required film.