Sedimentation and creaming

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 

Here f is the frictional coefficient for the particle which is given by Stokes law as f = 6 jt r) a where r) is the fluid viscosity. Thus  

This equation shows why, for example, aggregated droplets in an emulsion cream faster than individual droplets, and why coagulated solids in a suspension usually sediment faster than individual particles (in both cases the effective radius is larger). 

At high enough electrolyte concentrations the electric potentials are quickly dissipated and this effect vanishes. Since droplets and bubbles are not rigid spheres, they may deform in shear flow. Also, with the presence of emulsifying agents at the interface, the drops will not be non-interacting, as is assumed in the theory. 

The sedimentation velocity can be used to yield a good estimate of the average dispersed species size as long as the particles are not too small. 

For spherical nonmteracting particles, the driving force for sedimentation or creaming is given by 

the direction ofis the same as that of the external force field and the particles sediment for pj pj, the directions are opposite and the particles move opposite to the direction of the external field, that is, the particles cream. 

The moving particles are retarded by friction with the continnons phase. According to Stokes, the friction force/f, is expressed as 

In equilibrium=ff, so that the rate of sedimentation or creaming can be derived as 

Because of their relatively large size, emulsion droplets and, even more so, foam bubbles sediment or cream with a noticeable velocity, provided that the densities between the dispersed and the continuous phases differ significantly. This last-mentioned condition is certainly the case for foams but for emulsions the densities could be rather similar. 

If the particles of a colloidal dispersion have a density different from that of the medium, they will be subjected to a gravitational force which will tend to cause the particles to sink (sediment), if they are denser than the medium, or rise to the surface (cream), if they are less dense. In either case the result will be to produce a concentration gradient of particles tending to restore a uniform concentration. Gravitational forces are thus opposed by diffusion, and a steady state is set up in which the two processes are in balance. As before, we can express the gravitational force as — (pp — pm)l g, where pp and pm are the densities of particle and medium, respectively, and v is the volume of a particle. The driving force for diffusion is (dp/dh). In the steady state 

Integrating this equation between a height h° and h° + Ah we obtain 

The experimental test of equation (6.55), and also of (6.3), was first carried out by Perrin (1908) in his classical work on the sedimentation of gamboge particles in the Earth s gravitational field. This relationship is now widely applied in two main areas. It may be used, first, to determine particle sizes of very small particles and, secondly, by increasing the gravitational force using a centrifuge (or ullracentrifugc), to study both the diffusion and molar mass of macromoleculcs. 

As we shall see, the idea that colloidal systems can be treated by statistical mechanics in just the same way as molecular systems has been revived in recent years, and it represents one of the important growing points in colloid science. 

In the Stokes model, the terminal settling velocity is proportional to gravity and the square of particle size and inversely proportional to the fluid viscosity [22,17, 

Equation 2.20 also assumes laminar flow (Reynolds numbers less than about 0.1), that is, low particle velocities, and a dilute suspension of particles that are large compared with the molecules of the fluid. For Reynolds numbers greater than about 0.1 but less than 1, Oseen s law is approximately 

For asymmetric particles, several other models are available [67, 68]. An example is the equation for the settling of a circular disk-settling broadside on 

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For CFC-based suspension formulations, a surfactant was typically included. A variety of surfactants were used in these systems, e.g., lecithin, oleic acid, sorbitan trioleate. " All these surfactants were freely soluble in the CFC propellants and allowed for a degree of control over the suspension characteristics. Rates of flocculation, sedimentation, and creaming could be controlled and deposition on the internal container components was minimized. The transition to HFA-based MDIs has created significant issues in that none of the surfactants, previously used with the CFC products are soluble in HFA propellants alone. Some formulations have still used these surfactants, but the addition of a cosolvent (ethanol) has been required to solubilize the surfactant. 

Sedimentation and Creaming. The creaming and sedimentation processes occur in emulsion systems mainly due to the density difference between the dispersed and continuous phases. Assuming a steady state,... 

The particle size was determined using dynamic light scattering (also termed photon correlation spectroscopy PCS), using a Malvern PCS instrument. The equilibrium sediment and cream volumes were recorded using measuring cylinders at room temperature, and viscoelastic measurements were made using a Bohlin VOR rheometer. 

First, the drops long distance approach, i.e., at several times their size, takes place according to different driving forces ranging from gravity pull, which produces sedimentation and creaming (94), to Brownian motion and drop collisions (95, 96). 

As a rule of thumb, if is above 1 pm, sedimentation and creaming may occur for Pd > Pc and Pd < Pc, respectively. If the droplet size is smaller than 1 pm and if water is the continuous phase and a common monomer being the oil, then Brownian motion is able to keep the droplets almost evenly distributed throughout the emulsion. 

Reddy et al. [4] developed a general equation describing the behavior of a polydis-persed emulsion in which Brownian flocculation, sedimentation, and creaming take place simultaneously. The observation is made at various abscissas of the sample. 

The processing operations for fluid or manufactured milk products include cooling, centrifugal sediment removal and cream (a mixture of fat and milk semm) separation, standardization, homogenization, pasteurization or sterilization, and packaging, handling, and storing. 

Centrifugation. Centrifugal devices include clarifiers for removal of sediment and extraneous particulates, and separators for removal of fat (cream) from milk (see SEPARATION, CENTRIFUGAL). 

Sedimentation, or creaming, results from a density difference between the dispersed and continuous phases. This is not yet a destabilization of the dispersion, but produces two separate layers of dispersion that have different dispersed phase concentrations. One of the layers will contain an enhanced concentration of dispersed phase, which may promote aggregation. (The term creaming comes from the familiar separation of cream from raw milk.)... 

Small density difference between the phases - this reduces the rate of cream-ing/sedimenting and therefore collisions and aggregation. 

The pressure drop and pumping requirements are functions of the type of flow and of the rheological properties of the dispersion. If the flow rate in a pipeline falls below the critical deposit velocity then particles or emulsion droplets will either sediment or cream to form a layer on the bottom or top wall, respectively, of the pipe. Some correlations that have been developed for the prediction of critical deposit velocity are discussed by Nasr-El-Din [86] and Shook et al. [90]. 

A beverage emulsion is a concentrate added to sugar and carbonated water to make soda and fruit drinks. The oil-in-water emulsion provides flavor as well as opacity in products such as orange soda. Traditionally, gum arabic has been used to stabilize these emulsions. Interfacial starch derivatives (Section 20.4.2) are used to prevent creaming (phase separation), sedimentation, and loss in flavor and opacity, where desired, both in the concentrate and in the finished beverage. The concentrate is made by homogenizing the oils with an equal amount of the solubilized lipophillic starch, citric acid, sodium benzoate and color. A fine emulsion, typically 1 micrometer or less, is required for stability and for opacity, where desired. 

Rather than using a thermal quench, a more common way to make an emulsion is by mechanical mixing, or agitation, of two or more liquid components, such as occurs in an old fashioned butter chum. Unless surfactants, or emulsifiers, are present, however, when agitation ceases, interfacial tension will drive the two phases back toward separation. This separation occurs by droplet-droplet collision and fusion, if the droplets are Brownian by sedimentation or creaming, if the droplets are non-Brownian or by Ostwald ripening, if the droplet phase is soluble in the continuous phase. 

In spite of the droplets being destabilized electrostatically, no evidence of droplet coalescence is seen. By the same token, an electrostatically stabilized emulsion might still coalesce and separate, sediment, or cream if other destabilizing forces overbalance the electrostatic component. Creaming refers to concentration of the dispersed phase without completely separating the oil and water phases. 

In solid particulate systems, direct observation is justifiably the last word. In emulsions where creaming, sedimentation, and coalescence can change the nature of the sample, microscopic observation has unique sample handling problems. If these special sampling problems are addressed, then microscopy can indeed provide the benchmark for the physical characterization of the dispersed phase in emulsion systems. 

The rate of creaming depends on the difference in density between the dispersed particles and the dispersion medium, the particle radius, a, and the viscosity of the dispersion medium rj. According to Stokes law the rate of sedimentation (or creaming) of a spherical particle, V, in a fluid medium is given by... 

Gum arabic (acacia) has been used in pharmacy as an emulsifier. It is a polyelectrolyte whose solutions are highly viscous owing to the branched stmcture of the macromolecular chains its adhesive properties are also believed to be due to, or in some way related to, this branched stmcture. Molecular weights of between 200 000 and 250 000 (MJ have been determined by osmotic pressure, values between 250 000 and 3 x 10 by sedimentation and diffusion, and values of 10 by light scattering, which also points to the shape of the molecules as short stiff spirals with numerous side-chains. Arabic acid prepared from commercial gum arabic by precipitation is a moderately strong acid whose aqueous solutions have a pH of 2.2-2.7. It has a higher viscosity than its salts, but emulsions prepared with arabic acid cream are not as stable as those made with its salts. 

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