Surface rheology bulk fluids

To address these challenges, chemical engineers will need state-of-the-art analytical instruments, particularly those that can provide information about microstmctures for sizes down to atomic dimensions, surface properties in the presence of bulk fluids, and dynamic processes with time constants of less than a nanosecond. It will also be essential that chemical engineers become familiar with modem theoretical concepts of surface physics and chemistry, colloid physical chemistry, and rheology, particrrlarly as it apphes to free surface flow and flow near solid bormdaries. The application of theoretical concepts to rmderstanding the factors controlling surface properties and the evaluation of complex process models will require access to supercomputers. 

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]. 

The answer is yes. In fact, in many colloidal systems containing large liquid-fluid Interfaces, such as foams and emulsions, there is abundemt evidence for the action of surface rheology in addition to the omnipresent rheology of the adjoining bulk phases. Let us give three illustrations. 

M. van den Tempel. Surface Rheology, in Journal of Non-Newtonian Fluid Mechanics 2 (1977) 205-19. (Review with a phenomenological emphasis operational definitions and coupling to bulk rheology.)... 

Before going on to the main principles of surface rheology (2D-rheology), we have to deal with the problem of visco-elasticity studied by relaxation (or retardation) phenomena in bulk phases of fluids. 

Basic aspects of rheology are discussed in Sections 5.1.1 and 2. This concerns bulk rheology. Rheological theory can also and usefully be applied to the deformation of fluid interfaces. A main problem is that an interface cannot exist by itself it is the boundary between two phases and these phases must be deformed with the interface. Surface or interfacial... 

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]. 

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