Читать книгу The Rheology Handbook - Thomas Mezger - Страница 42

Deformation energy acting on a fluid leads to relative motion between the molecules. As a consequence, in flowing fluids frictional forces are occurring between the molecules, causing frictional heating, also called viscous heating. For fluids showing ideal-

viscous flow behavior, the applied deformation energy is completely used up and can be imagined as deformation work. A part of this thermal energy may heat up the fluid itself and another part may be released as heat to the surrounding environment. During a flow process, the applied deformation energy is used up completely by the fluid, and therefore, it is no longer available for the fluid afterwards, i. e., it is lost. Scientists explain this process as energy dissipation : Here, all the applied deformation energy is lost (dissipated), as it is completely transformed into heat energy.

When the load is removed, the state of deformation which was reached finally by the fluid is remaining to the full extent. Not even a partial elastic re-formation effect can be observed. Therefore here, an irreversible process has taken place since the shape of the sample remains permanently changed finally, after the load is released from the fluid.

If fluids are showing ideal-viscous flow behavior, there are absolutely no or at least no significant interactions between their mostly small molecules or particles. Examples are pure solvents, oils and water; and there might be also some diluted polymer solutions and dispersions, however, only if they show a really very low concentration. Since this kind of fluids does not show any visco-elastic gel-like structure, they may tend to separation, and therefore, effects like sedimentation or flotation may occur in mixtures of fluids and in dispersions.

End of the Cleverly section