627 LINIMENT OF NAFTALAN OIL GELS The active properties of the gels are closely related to the structural and mechanical characteristics, so their study and optimization are of crucial technological and applied value. An analysis of the flow curves of gel systems with LNO (Figure 2) indicates that all compositions are non-Newtonian viscoplastic fluids (25). The shear stress during the period Figure 1. Dependence of the maximum height of the foam column and the viscosity on the concentration of LNO. Figure 2. Flow (A) and viscosity (B) curves of gel with the LNO: 1: without LNO 2: 0.2 wt% 3: 0.5 wt% 4: 0.7 wt% 5: 0.9 wt%.

628 JOURNAL OF COSMETIC SCIENCE of decreasing velocities is delayed in comparison with the period of their increase, as a result of which the ascending and descending flow curves form a hysteresis loop (Figure 2), which indicates good thixotropic properties (26). The following yield strengths are important parameters characterizing the structure and strength of the gels: (1) the static minimum (first) yield strength τ 0 characterizing the destruction of a structural network and (2) the dynamic yield strength (Bingham yield strength) τ 0D determined in the straight section of a flow curve. The obtained experimental data can be approximated with high accuracy by the Herschel-Balkley equation: τ τ γ = + ⋅ • 0 K n , where τ 0 is the static yield strength, Pa K is the consistency coefficient, Pa · s γ • is the shear rate, s–1 and n is the index of the flow of the system. The mechanical stability M of gels was determined by the ratio of the yield point after fracture (during the period of decreasing shear rate) to the ultimate strength before it. The strength of the spatial gel network in a series of liniment concentrations tends to decrease, expressed in a decrease in the values of the structural and mechanical characteristics of the compositions. This facilitates the application of the composition to the skin, as evidenced by the data on spreadability. The change in viscosity (Figure 2B) and other rheological parameters when the content of LNO is up to 0.5 wt% by weight is not significant the introduction of a larger amount of it affects significantly. So, for a concentration of 0.9 wt%, the values of the dynamic yield strength τ 0D and the consistency coefficient K decrease by 1.5 times and the viscosity η by 2.4 times, which is unacceptable in the technology for producing and using the gel (Table IV). Thus, cosmetic gels with LNO in the concentration range up to 0.5 wt% are characterized by high stability, structuredness, uniform distribution, and the ability to form long-acting films on the surface of the skin. CREAMS Microscopic analysis is an effective method for assessing the structures of emulsion systems and predicting their stability. Presented in Figure 3 is a micrograph of a cream with 0.2 wt%, 0.4 wt%, and 0.9 wt% of liniment oil 48 h after preparation, obtained using Olympus BX 51. All creams passed the centrifugal test and did not delaminate after 30 min. Of the compositions presented in Figure 3,onlythecreamwith0.4 wt% LNO is characterized as a multiple emulsion (27,28). Such a system has several advantages: it immobilizes the Table IV Rheological Parameters of Gels With LNO Rheological characteristics Concentration LNO, wt% 0 0.2 0.5 0.7 0.9 Dynamic yield strength τ0D (Pa) 182.0 165.0 150.0 135.0 120.0 Static yield strength τ0 (Pa) 19.2 17.5 16.0 14.5 13.0 Viscosity η (Pa·s) 25.7 22.3 17.7 14.7 10.9 Mechanical stability M 0.85 0.84 0.81 0.76 0.72 Consistency coefficient (Pa·s) 31.9 29.1 26.5 23.9 21.4 Index of the flow n 0.33 0.33 0.33 0.32 0.32 Spreading diameter (mm) 32.9 40.8 48.1 51.4 55.2