Fluid particle sedimentation

As long as there is a density difference between the phases, emulsion droplets, foam bubbles, and suspension particles will have some tendency to rise or settle, possibly according to Stokes law. Whereas a body in a fluid will sediment out if its density is greater than that of the fluid, it will rise if its density is lower (this is called... 

The disc centrifuge photosedimentometer (DCP) is an instrument used to determine the particle size and size distribution of latices, pigments and other particulates. Separation is based on Stokes Law for the sedimentation of particles in a centrifugal force field. Particles are assumed to be spherical. In the most common mode of operation the separation takes place in a cylindrical cavity of known dimensions and containing a known volume of fluid. A dilute suspension of the particles of Interest is injected onto the surface of the fluid while the disc is spinning and the particles sediment in the centrifugal force field radially outward past a detector system. The appearance time of the particle at the detector, t, in minutes is given by... 

Immobilization of Particles. If the particles involved are immobilized, sedimentation cannot occur. Neither can they encounter each other, whereby also aggregation is prevented. Since sedimentation or aggregation has generally to occur before fluid particles can coalesce, immobilization will also prevent coalescence. 

Settling of these fine tails suspensions (fluid fine tails that are not trapped or entrained in the beach sand) and the ultimate formation of MFT in the settling ponds can be divided into three stages Stokes law sedimentation, during which the particles sediment according to their... 

Although these PANI-coated monodispersed particles are ideal for use as model ER materials, the particle size is micrometer-size and large particle settling problem is needed to overcome. Recently, using nanoparticles as the dispersal phase or filler has attracted considerable interest in the development of a non-conventional ER fluids [ 12-15,48-52]. In particular, significant attention has been paid to one-dimensional (ID) nanofiber suspensions because it has been foxmd that the nanofiber suspensions exhibit not only higher ER or MR effect, but also reduced particle sedimentation... 

The primary electroviscous effect is the retardation of the particle velocity (or the fluid flow in pores) due the polarisation of the EDL, which is a consequence of the relative motion between diffuse layer and the particle surface (Fig. 3.4). The polarisation, in turn, causes a flux of ions and solvent that opposes and decelerates the relative motion. This kind of deceleration affects particle sedimentation (von Smoluchowski 1903 Ohshima et al. 1984 Keh and Ding 2000) capillary flow... 

In Figure 3, with the increasing of fluid density, the speed of particle sedimentation decreases gradually. This is good for the fluid to carry particles. But the amplitude of the particle settling... 

The power law fluid yield stress is zero, and the fluid is deformed as long as the effect of a small force on the fluid. Particle density is greater than that of the fluid. In addition there is a vertical downward force formed by particle gravity and buoyancy force of the particle fluid. Therefore, particles settle. When the particle diameter is small to a certain extent, it will not overcome the yield stress and get a suspension in the fluid. Then sedimentation does not occur, which is known as natural suspended state. When the fluid stops circulating, it can make the solid phase suspension in the annulus to prevent the deposition of the solid phase at the bottom of the borehole. In this case, accidents can be avoided. Conditions for particles sedimentation is shown as follows ... 

Germany. Interceram 27,1978, p 33) Elevator Kiln. A kiln into which a setting of ware is raised from below the ware is set (outside the kiln) on a refractory base which is subsequently elevated by jacks into the firing position. Kilns of this type have been used, for example, in the firing of abrasive wheels. EIntriation. The process of separation of particles, according to their size and/or density, by submitting them to an upward current of water, air or other fluid (Cf. SEDIMENTATION). 

Grbavcic ZB, Garic RV, Hadjismalovic DzE, Jovanovic S, Vukovic DV, Littman H, Morgan III MH. Variational model for prediction of the fluid-particle interphase drag coefficient and particulate expansion of fluidized and sedimenting beds. Powder Technology 68 199 211, 1991. 

Selim MS, Kothari AC, Turian RM. Sedimentation of binary suspensions. AIChE S5mp Series (Fluidization and Fluid Particle Systems Theories and Applications, Knowlton TM, ed.) 79(222) 103-108, 1983a. 

The velocity (rate) of particle sedimentation in a fluid medium is given by Stokes law. 

Aidun, C.K. and Ding, E.-J. (2003). Dynamics of Particle Sedimentation in a Vertical Channel Period-Doubling Bifurcation and Chaotic State. Phys. Fluids, Vol. 15, pp. 1612-1621. 

Note that anhydrous HR fluids encounter another big problem— particle sedimentation, which could cause ER fluids to malfunction totally and severely limit the practical applications. Efforts were then spent to develop homogeneous ER fluids with no particulate material dispersed inside. Liquid crystal polymer/oil and water/oil emulsions ER systems were thus developed, which will be discussed later in detail. 

ER fluids usually contain small amounts of water. In 1985 Block [1] developed a water-free acenequinoiie radical polymer ER fluid, which was termed anhydrous ER fluid. Anhydrous ER fluid was believed to be much more promising from the standpoint of industrial applications and thus received much attention. Many other kinds of water-free ER fluids were developed later in the 1980s [2, 3J. Since these very promising water-free ER fluids have a particle sedimentation problem, homogeneous ER fluids... 

Particle sedimentation is often a problem in ER fluids containing the solid particle. As mentioned above, additive and surfactant are frequently used for enhancing both the stability of the ER suspension and the ER effect linally. One way to resolve this problem is to make a polymer coated microballoon particle, matching the density between the particle and the carrier liquid and thus reducing the particle sedimentation. An example is the poly(vinyl alcohol) (PVA) coated silica microballoon dispersed in the mixture of heptane and toluene (63]. The shear stress against the electric field is shown in Figure 11 for such a system. In both particle concentrations, 10 wt% and 30 wt%, the coated samples show a much better sedimentation property and much stronger ER effect, also. However, it may be hard to solely attribute the enhanced ER performance to the improved sedimentation property, as the PVA may act as an additive to enhance the ER effect. 

Homogeneous ER fluids are very promising, as they do not have particle sedimentation problem in contrast with the heterogeneous ER fluids. However, they usually have a high viscosity under zero electric field and mostly they only show the ER effect in the nematic phase. Sometimes liquid-liquid segregation problem occurs. 

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