Rheological properties of confined fluids

An equally remarkable feature to whidi we shall turn now is the fact that confined fluids may sustain a certain shear stress without exhibiting structural features normally pertaining to solid-like phases that is, they do not necessarily assume any long-range periodic order. We tacitly assumed this from the very beginning of this book in our development of a thermodynamic description of cuiifiiied fluids, wliich closely resembles that appropriate for solid-like bulk phases (see Section 1) (12). 

In addition we pointed out in Section 5.3.1 that the shear deformation can be measured experimentally in one mode of operation of the SFA. Hence, this section will be devoted to an analysis of these experiments in the framework of various computer simulation approaches. 

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After illustrating the rather fascinating structural and rheological properties of confined fluids we conclude our discussion of MC simulations of continuous model systems (i.e., models in which fluid molecules move along continuous trajectories in space) with yet another example of the imique behavior of confined fluids. For pedagogic reasons we selected a topic that is standard in physical chemistry textbooks [26, 199-203] as far as bulk fluids are concerned, namely the Joule-Thomson effect. 

The surface forces apparatus (SFA) measures forces between atomically flat surfaces of mica. Mica is the only material that can be prepared with surfaces that are atomically flat across square-millimetre areas. The SFA confines liquid films of a few molecular layers thickness between two mica surfaces and then measures shear and normal forces between them (figure C2.9.3)b)). In essence, it measures the rheological properties of confined, ultra-thin fluid films. The SFA is limited to the use of mica or modified mica surfaces but can be used to study the properties of a wide range of fluids. It has provided experimental evidence for the formation oflayered structures in fluids confined between surfaces and evidence for shear-induced freezing of confined liquids at temperatures far higher than their bulk freezing temperatures [14. 15]. 

The second part covers the structural aspects of different colloidal systems. Chapters 3 and 4, by Martin-Molina et al. and Haro-Pdrez et al., deal with electric double layers and effective interactions. Chapters 5 and 6, by Delgado et al. and Martinez-Pedrero et al., explore the structure of extremely bimodal suspensions and fllaments made up of miaosized magnetic particles. Chapters 7 and 8, by Puertas and Fuchs, and Hynninen et al., analyze the role played by the attractive interactions, confinement, and external fields on the structure of colloidal systems. Chapters 9 and 10, by Tromp and Maldonado-Valderrama et al., cover some structural aspects in food emulsions. This second part of the book finishes with Chapter 11, by de Vicente, which analyzes the rheological properties of structured fluids in order to establish a connection between measured material rheological functions and structural properties. 

The Couette cell is a classical geometry used to study the rheological properties of the complex fluids (see Figure 8.Id). One typically imposes a shear rate and measures the resulting shear stress and velocity profile [23]. In a 3D Couette cell, the granular material is confined between two coaxial cylinders. The wall friction can be controlled by coating the surface of each cylinder with a layer of randomly... 

Computer simulations of nanoscopic confined fluids have revealed many details of the dynamics under confinement. The nature of the confined fluids - especially in the immediate vicinity of attractive surface - has been shown to be strongly altered by the confining surfaces, and this is manifested by a behavior dramatically different from the bulk fluids in the local relaxation [38a], the mobility [38c] and rheological properties [39] of molecules near adsorbing surfaces. For monomeric systems many computer simulation studies [40] provide a clear enough picture for the dynamics of confined films of small spherical molecules. On the other hand, for confined oligomers and polymers less has been done, especially towards the understanding of the dynamics of nanoscopic films [41]. 

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