FACIAL MASK OF DEAD SEA MUD 451 In order to evaluate the effect of shearing time on the rheological behavior of the mud mask, the viscosity-shear rate relationship was determined at different times of shearing. Dead Sea mud mask samples were sheared at different values of constant shearing rate and at different temperatures for 40 min. At 5°C the mud mask samples exhibit a time-independent behavior at low shear rate and a weak thixotropic behavior at high shear rate (see Figure 8). The weak bonding between particles could explain the strength drop observed when the temperature and shear rate increase. However, the rate and extent of viscosity decay depend on both the applied shear rate and the temperature. Typical thixotropic behavior obtained at different shear rates for the Dead Sea mud mask at 45°C is shown in Figure 9. The observed time-dependent flow behavior of the mud mask was modeled using the structural kinetics approach (9). This model postulates that the change in the rheological properties is associated with shear-induced breakdown of the internal fluid structure in the Dead Sea mud. Using the analogy with chemical reactions, the final form of the structural breakdown process can be expressed as in equation 2. For all mud mask samples investigated, it was found that their apparent viscosity data at constant shear rates could be correlated with equation 2, using a = 2, i.e., with a 2nd order irreversible kinetic model. A good comparison between the model fitted results (solid lines) and the experimental apparent viscosity/time data for the mud mask can be seen in Figures 8 and 9. The rate constant, k! is a measure of the rate of thixotropic breakdown. Meanwhile the ratio of the initial to equilibrium viscosity, 11 o /TJ00 , can be considered as a relative measure of the amount of structural breakdown, or in other words as a relative measure - UI � 100 8 6 4 2 10 1 0 Facial Mask T= 5 °C i--------------------------------------------------------+ : 2.2 0 1/s i----------------------------r-------------------·-------r- t1j+ 10.211/s 5-K �el. ! _ _____ . ----1- ___ ----:- -�----,-_:_:_::_:_: : ' ' ---------- - -- - - - - - - -- - --- - - -"' ------- --- -- - - - -- - - - -- -- - - - - -1- - - - ----------- -- -- - - - - - - - - - - .. - - - - - - - - - - - - - -- -------- ---- - 5 ' ' ' ' ' ' ' ' ' 10 15 25 20 Shearing time (min) 30 35 40 Figure 8. Dependence of the facial mask's apparent viscosity on shearing time at 5 ° C.

452 - UI JOURNAL OF COSMETIC SCIENCE 100 ----r-----�--------:-=====-�--,-----;----� ----------------------------- t -··------------------------- Facial Mask + 10 1 6 ------------------------·---·r----------------------------- T = 45 oc EE 2.2 0 1/s 10.211/S 28.38 1/s 0 ____ -----------�-----------------------------��---5-_K_m_o_d�el ___ , __ , ♦ 5 131.9 1/s ---------- ---------------------------"1------------------------------ ----------------------------- --------- ________________________ ,. _____________________________ ----------------------------- 10 15 25 20 Shearing time (min) 35 30 40 Figure 9. Dependence of the facial mask's apparent viscosity on shearing time at 45 ° C. of the extent of thixotropy. The values of k and r1olTJ00 as a function of the applied shear rate and the temperature are reported in Table II. As one expected for a thixotropic structured material, k generally increases with increasing shear rate and temperature. Thixotropy is the result of structural breakdown under shear and manifests itself as a decrease in the apparent viscosity with time. As time of shear elapses, the rate of breakdown will decrease, as a fewer structural bonds are available for breakdown. Struc­ tural reformation may take place and the rate of this process will increase with time of shear due to the increasing number of bonding sites available (15). Table II shows also that the amount of structural breakdown (TJ 0 ITJ00) increases also with temperature and shear rate. CONCLUSIONS The temperature and shearing conditions dependency of the apparent viscosity were investigated for a facial mask made mainly of Dead Sea mud. The mud facial mask behaved like a shear-thinning material with a yield stress and generally exhibited a thixotropic behavior in the temperature range of 5 ° to 60°C. This behavior has a practical significance that decelerates particle sedimentation due to high viscosity at rest conditions. In addition, the shear-thinning and thixotropic behaviors have a significant importance in the ability of the facial mask to spread on the skin with a controllable film thickness. The Herschel-Bulkley model fitted well the flow curves of the mud facial mask. The effect of temperature on the facial mask's apparent viscosity was divided into three stages. In the first stage, 5°-20°C, the viscosity decreased, as expected, with