Gravitational separation

Atmospheric gases trapped in polar ice are enriched in the heavy species due to gravitational separation. This fractionation takes place in the thick fim layer (up to around 100 m) where the pore space is still connected. Below this layer, air bubbles are closed off in the ice and henceforth essentially preserved. The change in the ratio of any two gases from the initial atmospheric value fo to the value f in the trapped gas depends on the absolute mass difference AM between the two gases, the depth Z of the fim layer, and temperature T, as follows (Craig and Wiens 1996)  

However, elemental ratios can be fractionated due to gas loss from the samples through microfractures or crystal imperfections, which is governed by differences in molecular volumes (Craig et al. 1988 Bender et al. 1995). This process does not significantly affect isotope ratios. For this reason, much of the work on fractionation of trapped gases in ice has focused on high-precision isotope ratio measurements. 

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Aerosols are solid or liquid particles, suspended in the liquid state, that have stability to gravitational separation over a period of observation. Slow coagulation by Brownian motion is implied. 

Craig, H., Horibe, Y. and Sowers, T. 1988. Gravitational separation of gases and isotopes in polar ice caps. Science 242,1675-1678. 

The Earth s crust, mantle and core are strongly influenced by differentiation processes which could have resulted from gravitational separation ( smelting ) in an early molten phase of the planet, or from the sequence in which different chemical species condensed from the primitive solar nebula and were subsequently accreted. Seismology indicates that there is a liquid core (with a solid inner core) with radius 3500 km consisting mainly of iron (with some Ni and FeS) surrounded by a plastic (Fe, Mg silicate) mantle of thickness 2900 km. 

In addition, oily sludge from a wastewater treatment facility that results from treating sour wastewaters may be a hazardous waste (unless recycled in the refining process). These include API separator sludge, primary treatment sludge, sludge from various gravitational separation units, and float from dissolved air flotation units. 

At present, gravitation separation and photometric sorting are mainly used for the treatment of phosphates with high dolomite concentration (Salter and Wyatt 1991). However, their effectiveness in many cases is Umited due to minimal density and reflectance differences between dolomite containing lumps... 

The gravitational separation of water fh oil is controlled by the well known Stokes equation, which is as follows -... 

The precise determination of the location of the visible boundary layer is often difficult, particularly in the initial phase of the gravitational separation. 

Basaran, T.K., Demetriades, K.., et al. 1998. Ultrasonic imaging of gravitational separation in emulsions. Colloids Surfaces, A Physicochem. Eng. Aspects 136 169-181. 

Instability of an emulsion may be physical or chemical in nature. Chemical instability, which results in an alteration in the chemical structure of the lipid molecules due to oxidation or hydrolysis (McClements, 1999), will not be considered in this chapter for more information, the reader is referred to Chapters 11 and 12. Physical instability results in an alteration in the spatial distribution or structural organization of the globules (i.e., the dispersed phase of the emulsion). A number of important mechanisms responsible for the physical instability of emulsions, as depicted in Figure 5.1, can be divided into two categories gravitational separation and droplet aggregation. 

The size of the droplets in an emulsion has a strong influence on many of its physicochemical and sensory properties, e.g., shelf life, appearance, texture, and flavor (1,2, 4). For example, the stability of an emulsion to gravitational separation or droplet aggregation can be greatly improved by decreasing the droplet size. This is because the velocity of sedimentation is proportional to the square of the droplet size. The size of the droplets in an emulsion is largely determined by the emulsifier type and concentration, the physicochemical properties of the component phases, and the homogenization conditions (4). A food manufacturer normally specifies a preestablished desirable droplet size distribution for a particular product. If the product does not meet this specification, it typically must be reprocessed or even discarded. 

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