Differential viscosity, definition

Because of the frequent use of personal care products by the consumer for hair, body, and skin care, rheological properties are designed to achieve product differentiation. Products are formulated to achieve efficacy within a definite matrix of rheological properties. Certain manufacturers lean toward lower viscosity systems, while others focus on a thicker, richer composition. Regardless of the rheology preference, formulators need to overcome obstacles to achieve robust product design, including variables such as pH and electrolyte concentration. 

The viscosity of a material suddenly changes and loses fluidity at the gel point. Techniques to follow this phenomenon as a function of temperature are called thermal analysis techniques. According to the definition of the International Confederation of Thermal Analysis and Calorimetry, thermal analysis is a series of collective techniques to measure the physical properties of a material (or a reaction product) by changing the temperature according to a certain program [212, 213]. There are various thermal analyses depending on the physical properties to be measured. In this section, differential scanning calorimetry (DSC), which is the technique to measure heat capacity of the sample, and thermomechanical analysis (TMA), which measures the viscosity or modulus, will be discussed. 

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