Microstructured fluids

The special property of surfactants in solution is that they associate into a monolayer or sheetlike structure with the water-soluble moieties (hydrophilic heads) on one side of the sheet and water-insoluble moieties on the other side [8], These sheetlike structures provide the building blocks for a rich variety of fluid microstructures, which, depending on thermodynamic... 

The fluid microstructure is of central interest in the theoretical study of supercritical solutions for several reasons. Bulk fluid properties can be obtained from knowledge of the fluid structure and the intermolecular potential through distribution function theory. In addition, it has recently become clear, both from experimental evidence and theoretical analyses (1-5) that solvation structure changes rapidly near a CP. 

Fluid microstructure may be characterized in terms of molecular distribution functions. The local number of molecules of type a at a distance between r and r-l-dr from a molecule of type P is Pa T 9afi(r)dr where Pa/j(r) is the spatial pair correlation function. In principle, flr (r) may be determined experimentally by scattering experiments however, results to date are limited to either pure fluids of small molecules or binary mixtures of monatomic species, and no mixture studies have been conducted near a CP. The molecular distribution functions may also be obtained, for molecules interacting by idealized potentials, from molecular simulations and from integral equation theories. 

Flow field/fluid microstructure Flow induced phase inversion (FIPI) Agglomeration,... 

The above-described mixers are essentially low-viscosity devices. In many operations where the viscosity is high, when dealing with concentrated multiphase gas-liquid-solid binary or tertiary systems, or when liquid-to-solid phase transformation occurs during mixing, novel equipment designs are needed to intensify the heat/mass transfer processes. The multiphase fluids also represent an important class of materials that have microstructure developed during processing and subsequently frozen-in, ready for use as a product. To deliver certain desired functions, the control of microstructure in the product is important. This microstructure is developed in most cases by the interaction between the fluid flow and the fluid microstructure hence, uniformity of the flow field is important. 

Bodet, J.F., Bellare, J.R., Davis, H.T., Scriven, L.E. and Miller, W.G. (1988) Fluid microstructure transition from globular to bicontinuous in midrange microemulsion. /. Phys. Chem., 92, 1898-1902. 

Smalley MV (1996) Long range attractions in charged colloids. In Arora AK, Tata BVR (eds) Ordering and phase transitions in charged colloids. Complex fluids and fluid microstructure series. VCH, New York, chap 12... 

The characteristics of the microstructure formed (such as emulsion droplet size) are dependent on the type of microstructure, type of deformation (shear, extension, or combined), and deformation rate as well as the TSVs. In order to maximize the fluid microstructure/flow field interactions, the flow field must be uniform which requires the generation of the flow field over a small processing volume. There are several types of equipment such as multiple expansion contraction static mixer (MECSM) or its dynamic... 

D. Fluid Microstructural Transition Studies from Globular to Bicontinuous Morphologies in Midrange Microemulsions... 

X. Zhu, Surfactant Fluid Microstructure and Surfactant Aided Spreading , Ph. D. Thesis, University of Mitmesota, Minneapolis, 1992. 

Aifantis, E.C. and J. Serrin Equilibrium solutions in the mechanical theory of fluid microstructures. J. Colloidal Interface Science 96 (1983) 530-547. 

In this chapter, fluid-fluid flow patterns and mass transfer in microstructured devices are discussed. The first part is a brief discussion of conventionai fluid-fluid reactors with their advantages and disadvantages. Further, the ciassi-flcation of fluid-fluid microstructured reactors is presented. In order to obtain generic understanding of hydrodynamics, mass transfer, and chemical reaction, dimensionless parameters and design criteria are proposed. The conventional mass transfer models such as penetration and film theory as well as empirical correlations are then discussed. Finally, literature data on mass transfer efficiency at different flow regimes and proposed empirical correlations as well as important hydrodynamic design parameters are presented. 

Figure 7.2 Schematic representation of fluid-fluid microstructured reactors ...

Pressure Drop in Fluid-Fluid Microstructured Channels... 

This type of technique is faced by freewill isotropic fluid microstructures in a colloidal equilibrium dispersion form. Microemulsions with immiscible liquids, especially short-chain alcohols such as methanol, ethanol, 1 -butanol, and ionic or nonionic am-phiphiles, have been studied to reduce the high viscosity of vegetable oils (Demirbas, 2003). Through this method, the problem of the high viscosity of vegetable oils would be solved (Ma Hanna, 1999). Regarding this subject, Ziejewski, Kaufman, Schwab, and Pryde (1984) prepared an emulsion of 53% (vol) alkah-refined and winterized sunflower oil, 13.3% (vol) ethanol, and 33.4% (vol) 1-butanol. Lower viscosities and better spray patterns (more even) were observed with an increase of 1-butanol. 

Adolf D, Tirrell M, Davis HT (1985) Molecular theory of transport in fluid microstructures diffusion in interfaces and thin films. AlChE J 31 1178-1186... 

Furthermore, NEMD enables the fluid microstructure to be studied in nonequilibrium steady states and to compare this structure with experiment (Hess Hanley 1982). The nonequilibrium distributions of particle positions and momenta are reflected in the thermophysical properties of viscosity, dilatancy and normal pressure differences. In molecular fluids such as lubricants, nonlinear fluid behavior is brought about in part by shear induced changes in molecular conformation. ... 

In the linear regime, changes in fluid microstructure are in principle calculable using G-K methods but, in practice, such calculations are so difficult that no one has yet attempted them. 

The pressure tensor can be written as the sum of kinetic and potential terms. The potential part arises from the direct intermolecular forces and is therefore a function of the local fluid microstructure. In a dense liquid the shear viscosity is overwhelmingly dominated by the potential contribution. ... 

The discussion on the distorted fluid microstructure demonstrates the utility of NEMD as a practical tool and as a means to get an insight into the physics of viscous phenomena. The shear-rate dependent non-Newtonian properties of a liquid can be calculated from the appropriate steady-state averages, but the mechanical transport properties can be obtained indirectly from the nonequilibrium pair distribution function using equation (9.49), and a more physical picture of the behavior of a nonequilibrium fluid is obtained. As an example, the phenomenological theory of Hanley et al. (1987,1988) is outlined where the functionals are evaluated from a relaxation time approximation. The... 

Figure 2.11 Schematic representation of fluid-fluid microstructured reactors (a micromixer settler b cyclone mixer c interdigital mixer d microchannel with partial overlap e microchannel with membrane or metal contactor f microchannels...

The flow in fluid-fluid microstructured channels is characterized using dimensionless numbers. The most important dimensionless number for characterization of all types of flows is the Re number that relates inertial force to viscous force. Due to low flow velocities and characteristic dimension in the micrometer range, Re is often less than 1 meaning that viscous force is dominant over inertial force. The capillary number Ca is the ratio of viscous to interfacial forces. The range of Ca in a typical microchannel is lO " to 10 . Multiplying both numbers. Re and Ca, results in the Weber number We, which represents the ratio between inertial and interfacial forces. The importance of gravity vhth respect to interfacial forces is characterized by the Bond number Bo. The definitions of the dimensionless numbers are summarized in Table 2.2. 

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