Consequently, from a purely practical point of view, the ratio of the relaxation time ts that a phenomenon (e.g., the translocation of a quantity of liquid along the length of a channel) takes to the time to for which it is observed is important. This ratio is known as the Deborah number (De).*

Here we attempt a first approach to the issue of what a “fluid” is, as opposed to what a “solid” is. Further on we will see that these terms are both inadequate; for a correct description of the material world, it would be better to use the terms “viscous” and “elastic.” The aim of the following

passage is to help the reader make this transition as easily as possible. The simplest way is the gradual introduction of the concept of elasticity of solids, in juxtaposition with the Newtonian flow and viscosity of liquids and gases. To continue, the states of plastic or nonplastic flow will be described, finishing with the hybrid term viscoelasticity, which as a rule covers the majority of edible and cosmetic preparations.*

Here we briefly discuss the basic theme of the paragraphs to follow. Let us imagine a material body comprised of molecules organized in layers (Figure 7.1). If one applies a force to the top layer, with a direction parallel to the surface (this will subsequently be called a shear force), three basic scenarios can occur, depending on the magnitude of the forces existing between the molecules comprising this body. The first scenario assumes that the attractive forces between the component molecules range from very weak to negligible. In that case, the top layer, to which the force is applied, will accelerate and move, leaving behind the layers underneath it. The careful reader may link this behavior to that of

ideal gases discussed in Chapters 1 and 2 and in reality it characterizes the so-called superfluids. The second scenario assumes that attractive forces do exist between the molecules, linking to some extent the molecules of two adjacent layers. In that case, momentum will be transferred to the layer under the first one, which, in turn, will transfer part of its momentum to the layer underneath it, and so on. This will result in a collective movement of these layers, albeit with decreasing velocity as we move from the top to the lower layers. The gradient of velocity with distance from the top layer (to which the driving force is applied) will be correlated to viscosity. This is a typical behavior of liquids; the reader might consider here correlating the development of intermolecular forces and viscosity to the transition from gases to liquids discussed in Chapter 2. The third scenario assumes very strong bonding between the subsequent layers of molecules. This means that the applied force will be dissipated on equal terms to all layers underneath the first layer, resulting in an equal velocity in every layer. This way, the form of the material body will be preserved, as all of its components maintain their relative positions in the structure while it is moved under the influence of the force applied to the top layer. This, of course, is a solid body. Here, the reader might consider the effect of strengthening the intermolecular forces in the transition from liquid to solid, as discussed in Chapter 2.