Rheology of Solutions and Suspensions

The study of the flow properties of liquids is called rheology. The thickness or runniness of liquids is characterized by their viscosity. Consider a liquid between two parallel surfaces (with area A). The bottom surface is fixed and the top one moves at a constant velocity (v). A force (F) is applied to the top plate to keep it moving. The liquid is dragged along with the moving plate due to its viscosity with a velocity that is largest close to the top plate and decreases with the distance from it. This 

Newton suggested that the velocity gradient is proportional to the shear stress a (ie. the force applied to keep the plate moving per unit area). 

The constant of proportionality in equation 2.10 is the viscosity of the liquid tf). Some fluids, such as water, olive oil and sucrose solutions obey this equation and are said to be Newtonian. Their viscosity does not depend on the velocity gradient, i.e. how fast the liquid is sheared - known as the shear rate, More complex fluids (e.g. solutions of polymers) have a viscosity that does depend on the shear rate. Such fluids are called non-Newtonian . Many complex fluids, for example tomato ketchup and ice cream mix, become less viscous when they are sheared and are described as shear-thinning . Tapping the bottom of the bottle applies shear to the ketchup, which becomes less viscous and flows more easily onto your plate. Other fluids, such as a concentrated solution of cornstarch or quicksand, become more viscous (i.e. they are shear-thickening ). Experiment 7 in Chapter 8 gives some examples of non-Newtonian fluids. A single viscosity is not sufficient to describe the flow properties of non-Newtonian liquids and if a viscosity is stated, the shear rate at which it was measured must also be given. 

The rheology of ice cream is much more complex than that of a simple liquid. The matrix is a solution of small (sugar) and large (stabilizer) molecules, in which particles of other phases (ice crystals, fat droplets and air bubbles) are suspended. We must first look at the effects of each of these, and temperature, in order to understand the rheology of ice cream. 

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