Complex fluids characteristics

Third, a serious need exists for a data base containing transport properties of complex fluids, analogous to thermodynamic data for nonideal molecular systems. Most measurements of viscosities, pressure drops, etc. have little value beyond the specific conditions of the experiment because of inadequate characterization at the microscopic level. In fact, for many polydisperse or multicomponent systems sufficient characterization is not presently possible. Hence, the effort probably should begin with model materials, akin to the measurement of viscometric functions [27] and diffusion coefficients [28] for polymers of precisely tailored molecular structure. Then correlations between the transport and thermodynamic properties and key microstructural parameters, e.g., size, shape, concentration, and characteristics of interactions, could be developed through enlightened dimensional analysis or asymptotic solutions. These data would facilitate systematic... 

Perhaps the most important distinction between classical solids and classical liquids is that the latter quickly shape themselves to the container in which they reside, while the former maintain their shape indefinitely. Many complex fluids are intermediate between solid and liquid in that while they maintain their shape for a time, they eventually flowr They are solids at short times and liquids at long times hence, they are viscoelastic. The characteristic time required for them to change from solid to liquid varies from fractions of a second to days, or even years, depending on the fluid. Examples of complex fluids with long structural or molecular relaxation times include glass-forming liquids, polymer melts and solutions, and micellar solutions. 

Complex fluids are amazingly diverse, ranging from foodstuffs, to biological materials, to plastic coatings. Nevertheless, such substances have shared characteristics, including... 

The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

A critical review of emulsion flow in porous media has been presented. An attempt has been made to identify the various factors that affect the flow of OAV and W/O emulsions in the reservoir. The present methods of investigation are only the beginning of an effort to try to develop an understanding of the transport behavior of emulsions in porous media. The work toward this end has been difficult because of the complex nature of emulsions themselves and their flow in a complex medium. Presently there are only qualitative descriptions and hypotheses available as to the mechanisms involved. A comprehensive model that would describe the transport phenomenon of emulsions in porous media should take into account emulsion and porous medium characteristics, hydrodynamics, as well as the complex fluid-rock interactions. To implement such a study will require a number of experi-... 

As with other characteristic types of complex fluid flows, turbulent flows over and between different types or flexible obstacles above resistive surfaces have many features in common. This is why such flows can be studied in a similar conceptual framework and with similar techniques of analysis, computation and measurement. 

Rheological and elastic properties under flow and deformations are highly characteristic for many soft materials like complex fluids, pastes, sands, and gels, viz. soft (often metastable) solids of dissolved macromolecular constituents [1]. Shear deformations, which conserve volume but stretch material elements, often provide the simplest experimental route to investigate the materials. Moreover, solids and fluids respond in a characteristically different way to shear, the former elastically, the latter by flow. The former are characterized by a shear modulus Go, corresponding to a Hookian spring constant, the latter by a Newtonian viscosity r]o, which quantifies the dissipation. 

One of the central questions in the rheology of complex fluids is the molecular origin of mechanical propertie,s. Therefore, coupling of rheometry with techniques which are sensitive to molecular behaviour like molecular alignment, rotational reorientation, velocity distributions, and tramslational diffusion is required, A method which allow.s the detection of all these molecular characteristics is NMR imaging [Cal4J,... 

A typical result of a calculation [127] of the complex viscosity rf(co) is shown in Fig. 11. The real part of the viscosity, / (w), which describes the dissipation of energy when the fluid is sheared, is approximately frequency-independent for small cu, i.e., the fluid behaves as a Newtonian fluid. There is a characteristic frequency co where f/ (o>) drops rapidly. The imaginary part of the viscosity, rf"(o)), which describes the elastic response of the fluid to an external perturbation, increases linearly for small co and reaches a maximum at CO = CO. This behavior is not specific to microemuisions but has been observed in other complex fluids as well, such as in suspensions of spherical colloidal particles [128,129] and in dilute polymer solutions [130]. 

It is known that a viscoelastic fluid, e.g., a solution with a trace amount of highly deformable polymers, can lead to elastic flow instability at Reynolds number well below the transition number (Re 2,000) for turbulence flow. Such chaotic flow behavior has been referred to as elastic turbulence by Tordella [2]. Indeed, the proper characterization of viscoelastic flows requires an additional nondimensional parameter, namely, the Deborah number, De, which is the ratio of elastic to viscous forces. Viscoelastic fluids, which are non-Newtonian fluids, have a complex internal microstructure which can lead to counterintuitive flow and stress responses. The properties of these complex fluids can be varied through the length scales and timescales of the associated flows [3]. Typically the elastic stress, by shear and/or elongational strains, experienced by these fluids will not immediately become zero with the cessation of fluid motion and driving forces, but will decay with a characteristic time due to its elasticity. 

The DPD technique was first proposed by Hoogerbrugge and Koel-man [17] in the beginning of the 90 s with the intention of studying soft condensed matter, i.e. systems that have both solid and liquid behavior. It has been successfully used for the study of complex fluid systems, like pol3uneric [18-21] or colloidal [21-23] suspensions, micelles and immiscible mixtures, where one of the main characteristics is the presence of disparate time scales in the dynamics of the S3 tem. The technique uses... 

Monte Carlo methods offer a useful alternative to Molecular Dynamics techniques for the study of the equilibrium structure and properties, including phase behavior, of complex fluids. This is especially true of systems that exhibit a broad spectrum of characteristic relaxation times in such systems, the computational demands required to generate a long trajectory using Molecular Dynamics methods can be prohibitively large. In a fluid consisting of long chain molecules, for example, Monte Carlo techniques can now be used with confidence to determine thermodynamic properties, provided appropriate techniques are employed. 

The variation of the local Nu number along the circumference of a cylinder in cross flow of air (Pr= 0.7) for low and high Reynolds number is shown in Figures 3.2.10 and 3.2.11, respectively. The reason for the local variation of Nu is that the cross flow over a cylinder (and also over other bodies) exhibits complex flow characteristics. The fluid approaching the cylinder at the front stagnation point (angle y = 0) branches out and... 

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