Effects Fluid Particles

The effects of surface-active agents on the motion of and transfer from bubbles and drops have been discussed in earlier chapters. The main effect is to reduce the mobility of all or part of the interface. In this section we consider briefly two other interfacial phenomena interfacial convection during mass transfer and interfacial barriers to mass transfer. 

Movements in the plane of the interface result from local variations of interfacial tension during the course of mass transfer. These variations may be produced by local variations of any quantity which affects the interfacial tension. Interfaeial motions have been ascribed to variations in interfacial concentration (H6, P6, S33), temperature (A9, P6), and electrical properties (AlO, B19). In ternary systems, variations in concentration are the major factor causing interfacial motion in partially miscible binary systems, interfacial temperature variations due to heat of solution effects are usually the cause. 

On the interface between quiescent fluids, interfacial motions may take the form of ripples (E4, 02) or of ordered cells (B5, L5, 02, S22). Slowly growing cells may exist for long periods of time (B5, 02), or the cells may oscillate and drift over the surface (L6, L7). When the phases are in relative motion, interfacial disturbances usually take the form of localized eruptions, often called interfacial turbulence (M3). This form of disturbance can also be observed at the interface of a drop (S8). A thorough review of interfacial phenomena, including a number of striking photographs, has been presented by Sawistowski (S7). 

The shape of a drop moving under the influence of gravity may be affected by interfacial motions the drop may also wobble and move sideways (S27, W3). In one system (S22) the terminal velocity was reduced yielding a drag coefficient nearly equal to that of a solid particle. Interfacial convection tends to increase the rate of mass transfer above that which would occur in the absence of interfacial motion. The interaction between mass transfer and interfacial convection has been treated by Sawistowski (S7) and Davies (D4, D5). 

The factors determining the appearance of ordered cell-like motions were first investigated by Sternling and Scriven (S33) who considered the two-dimensional stability of a plane interface separating two immiscible semi-infinite fluid phases with mass transfer occurring between the phases. This system was shown to be unstable for mass transfer in one direction, but stable for transfer in the opposite direction. For an interfacial tension-lowering solute, instability 

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Some pioneering work has been done on the effect of particle size on mouthfeel and texture perception (31). When particles of food materials are smaller than 0.1 ]lni they impart no sense of substance and the consumer calls the product watery. Particles of 0.1—3.0 ]lni are sensed as a smooth rich fluid, but when the particles exceed 3 ]lni the food is perceived as chalky or powdery. By controlling particle size, deskable creaminess can be obtained (32). 

In view of the facts that three-dimensional coUoids are common and that Brownian motion and gravity nearly always operate on them and the dispersiag medium, a comparison of the effects of particle size on the distance over which a particle translationaUy diffuses and that over which it settles elucidates the coUoidal size range. The distances traversed ia 1 h by spherical particles with specific gravity 2.0, and suspended ia a fluid with specific gravity 1.0, each at 293 K, are given ia Table 1. The dashed lines are arbitrary boundaries between which the particles are usuaUy deemed coUoidal because the... 

Clearly, the inclusion of the liquid phase (xi) will tend to reduce the number of fines in the system. Thus, a diminution in particle size must be effected to provide an equivalent particle population in the fluid-particle assembly. The particle size ratio normally used in practical systems tends to be somewhat lower than the one computed from Eq. (8). 

When the length scale approaches molecular dimensions, the inner spinning" of molecules will contribute to the lubrication performance. It should be borne in mind that it is not considered in the conventional theory of lubrication. The continuum fluid theories with microstructure were studied in the early 1960s by Stokes [22]. Two concepts were introduced couple stress and microstructure. The notion of couple stress stems from the assumption that the mechanical interaction between two parts of one body is composed of a force distribution and a moment distribution. And the microstructure is a kinematic one. The velocity field is no longer sufficient to determine the kinematic parameters the spin tensor and vorticity will appear. One simplified model of polar fluids is the micropolar theory, which assumes that the fluid particles are rigid and randomly ordered in viscous media. Thus, the viscous action, the effect of couple stress, and... 

Figure 3.8 Ln(m/m0) vs. scaled time tt(= 7t2Dt/r2) for the hot-ball model, including the effect of particle shape. After Bartle et al. [286]. Reproduced from Journal of Supercritical Fluids, 3, K.D. Bartle et al., 143-149, Copyright (1990), with...

Palakodaty, S. and York, P. (1999) Phase behavioral effects on particle formation processes using supercritical fluids. Pharmacological Research, 16 (7), 976-985. 

The wall effect for particles settling in non-Newtonian fluids appears to be significantly smaller than for Newtonian fluids. For power law fluids, the wall correction factor in creeping flow, as well as for very high Reynolds... 

Either a liquid or a gas can be used as the carrier fluid, depending on the size and properties of the particles, but there are important differences between hydraulic (liquid) and pneumatic (gas) transport. For example, in liquid (hydraulic) transport the fluid-particle and particle-particle interactions dominate over the particle-wall interactions, whereas in gas (pneumatic) transport the particle-particle and particle-wall interactions tend to dominate over the fluid-particle interactions. A typical practical approach, which gives reasonable results for a wide variety of flow conditions in both cases, is to determine the fluid only pressure drop and then apply a correction to account for the effect of the particles from the fluid-particle, particle-particle, and/or particle-wall interactions. A great number of publications have been devoted to this subject, and summaries of much of this work are given by Darby (1986), Govier and Aziz (1972), Klinzing et al. (1997), Molerus (1993), and Wasp et al. (1977). This approach will be addressed shortly. 

Pressure and Temperature Effects in Fluid-Particle Systems... 

Temperature and pressure affect the operation of fluid-particle systems because they affect gas density and gas viscosity. It is the variation in these two parameters that determine the effects of temperature and pressure on fluid-particle systems. Increasing system temperature causes gas density to decrease and gas viscosity to increase. Therefore, it is not possible to determine only the effect of gas viscosity on a system by changing system temperature because gas density is also changed and the resulting information is confused. Very few research facilities have the capability to change system pressure to maintain gas density constant while the temperature is being changed to vary gas viscosity. 

In between these two extremes, the effective hydrodynamic boundary layer depends on the combined effects of particle size and hydrodynamics. Talking about borderline particle sizes is meaningful only if all other relevant data, such as the fluid viscosity,... 

The effect of particle shape on the forces acting when the particle is moving in a shear-thinning fluid has been investigated by Tripathi et alP7>, and by Venumadhav and Ciiiiaisra 41 1. In addition, some information is available on the effects of viscoelasticity of the fl uid135. ... 

Coulson, J. M. Trans. Inst. Chem. Eng. 27 (1949) 237-57. The flow of fluids through granular beds effect of particle shape and voids in streamline flow. 

The increase in velocity seen as part of the Venturi effect simply demonstrates that a given number of fluid particles have to move faster through a narrower section of tube in order to keep the total flow the same. This means an increase in velocity and, as predicted, a reduction in pressure. The resultant drop in pressure can be used to entrain gases or liquids, which allows for applications such as nebulizers and Venturi masks. 

The pressure that would be required to prevent the movement of water across a semipermeable membrane owing to the osmotic effect of interstitial fluid particles (jtj mmHg). 

As for other types of fluid particle, the internal circulation of water drops in air depends on the accumulation of surface-active impurities at the interface (H9). Observed internal velocities are of order 1% of the terminal velocity (G4, P5), too small to affect drag detectably. Ryan (R6) examined the effect of surface tension reduction by surface-active agents on falling water drops. 

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