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Preparation and Assessment of Chitosan Encapsulated Menthol Microcapsules for Leave-on Conditioners 100 The Journal of the Royal Institute of Thailand Vol. 37 No. 4 Oct-Dec 2012 Flow properties The flow behavior of the chitosan/menthol emulsion, conditioner base, and leave-on conditioner showed that the shear stress linearly increased with shear rate and vice versa for the chitosan/menthol emulsion, conditioner base and leave-on conditioner (Figure 2). They are all non-Newtonian fluid in that the viscosity of non-Newtonian fluids changes with the rate of shear and do not have a constant viscosity value. 14 The thixotropic property was revealed in the chitosan/ menthol emulsion that the emulsion exhibited the pseudoplastic propertywith yield stress [Figure 2 (a)] when one interpolated the shear stress-shear rate curve to the zero shear rate which is attri- buted to its three-dimensional network structure caused by TPP crosslinking reaction that resisted to flow. 15 This shear thinning behavior is a very valuable role in determining storage stability and a sensory feature of cosmetic product quality in which chitosan is used as a microcapsule shell. Similarly, the conditioner base and leave-on conditioner can also be observed as the non-Newtonian materials as shown in Figure 2 (b) to (c). Figure 2 exhibits the relationship of shear rate vs. viscosity in that the viscosity of the microcapsules in the emulsion was influenced by its formulation from which the large molecular sized material produced higher viscosity. The viscosity decreased as the shear rate increased and vice versa. Chitosan/menthol microcapsules exhibited a thixotropic behavior of the non-Newtonian fluid when they were subjected to shear rate on the forward direction and decreasing shear rates on the backward direction. It is observed that the flow behavior is influenced by the structure of the microcapsules and conformation of polymer. The structures of the microcapsules in the emulsion and the conditioner base were elongated under high shear and they rebuilt under Brownian motion behavior. The microcapsules were flown and moved thus the viscosity decreased with decreasing shear stress or increasing shear rate. The reversible property occurred as observed in a hysteresis loop created by the area enclosed by the upward and downward curves of shear stress or viscosity. The area within the hysteresis loop from viscosity vs. shear rate curves can be considered as the thixotropic behavior of fluids. The thixotropic property revealed in the chitosan/menthol microcapsules and conditioner base is the pseudoplastic prop - erty. 15 The area within the hysteresis loop of chitosan/menthol microcapsules in Figure 2 (a) and Table II was found much smaller (0.15 Pa) than that of conditioner base (17.11 Pa) in Figure 2 (b). The o/w emulsion of chitosan/menthol microcapsules contains a large amount of water than that of the conditioner base which comprises cetearyl alcohol, dipalmitoylethyl hydroxyethylmonium