Basic characteristics of semi-solids

Perhaps the best way to define semi-solids is based on their rheological characteristics. They show flow behaviour intermediate between liquids and solids, i.e. they are viscoelastic systems. To understand the flow behaviour of semi-solids, let us first consider the characteristics of elastic solids and viscous liquids. Elastic solids follow Hooke s law, which states that the relative strain (y, dimensionless) increases linearly with the applied stress (r/Pa), 

J4 Topical Formulations and Semi-solid Systems 149S 

Viscous liquids follow Newton s law, which states that the shear rate (y/s ) varies hnearly with the applied stress r, 

Systems that obey Newton s law (such as simple liquids) are defined as Newtonian. 

With semi-solids that are viscoelastic, they neither obey Hooke s law or Newton s law. This can be illustrated if one plots the shear stress z or viscosity tj versus shear rate y. The flow curve (sometimes referred to as pseudo-plastic or shear thinning) can be analysed using, for example, the Herschel-Buckley equation as discussed above. 

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