Creaming of emulsions

J. Bibette, D. Roux, andB. Pouligny Creaming of Emulsions The Role of Depletion Forces Induced by Surfactant. J. Phys. II France 2, 401 (1992). 

This is the result of gravity, when the density of the droplets and the medium are not equal. When the density of the disperse phase is lower than that of the medium, creaming occurs, whereas if the density of the disperse phase is higher than that of the medium, sedimentation occurs. A schematic representation for the creaming of emulsions in three cases is shown in Figure 10.23 [1-3]. 

Figure 10.23 Representation of creaming of emulsions. See text for details.

Creaming of emulsion n. Separation of an emulsion into two layers of different concentration. The more concentrated top layer, i.e., the layer containing the greatest number of dispersed droplets per volume, has a creamy appearance. Gentle agitation of the two layers often effects uniformity of the emulsion. 

III. Gradual phase separation leading to "creaming" of emulsion. 

Figure 4.8 Creaming of emulsion without added polymer, (a) Volume fraction profiles measured using the ultrasonic creaming monitor showing the progression from uniform (horizontal line) to fully creamed state over 37 days. (Inset) Droplet size distribution inferred from the creaming data, (b) Height of eight ordinates y]-y8 with time. The gradient of each ordinate is related to the Stokes velocity of the size fraction represented by each volume fraction...

Creaming of Emulsions with Low Concentrations of Nonadsorbing Polymer... 

Figure 4.12 Creaming of emulsion with 0.95% hydroxyethylcellulose. (a) Volume fraction profiles showing a flocculated network, (b) Height of the base of the network with time, showing delay period...

Emulsifiers are classified by the hydrophilic—lipophilic balance (HLB) system. This system indicates whether an emulsifier is more soluble in water or oil, and for which type of emulsion (water-in-oil or oil-in-water) it is best suited. Emulsifiers having alow HLB value are more oil soluble, and are better suited for water-in-oil appHcations such as margarine. Conversely, emulsifiers having a high HLB value are more water soluble, and function more effectively in oil-in-water emulsions such as ice cream (34). The use of this system is somewhat limited because the properties of emulsifiers are modified by the presence of other ingredients and different combinations of emulsifiers are needed to achieve a desired effect. The HLB values of some common emulsifiers are given (35). 

Butter. In the United States about 10 wt % of edible fats used are butter. Butter is defined as a product that contains 80% milk fat with not more than 16% moisture. It is made of cream with 25—40% milk fat. The process is primarily a mechanical one in which the cream, an emulsion of fat-in-semm, is changed to butter, an emulsion of semm-in-fat. The process is accompHshed by churning or by a continuous operation with automatic controls. Some physical properties are given in Table 16 (see Emulsions). 

When the problem is to disrupt Ughtly bonded clusters or agglomerates, a new aspect of fine grinding enters. This may be iUustrated by the breakdown of pigments to incorporate them in liquid vehicles in the making of paints, and the disruption of biological cells to release soluble produces. Purees, food pastes, pulps, and the like are processed by this type of mill. Dispersion is also associated with the formation of emulsions which are basically two-fluid systems. Syrups, sauces, milk, ointments, creams, lotions, and asphalt and water-paint emulsions are in this categoiy. 

Some detergents and surfactants are used as emulsifying agents. An emulsifier keeps oil droplets and water droplets from joining together, so a thick mixture of oil and water will not separate. Examples of emulsions are mayonnaise, butter, cream, homogenized milk, and salad dressings. 

Altvater, M., Rietz, R., Neubeet, R., Micromixer based formation of emulsions and creams for pharmaceutical applications, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, pp. 467 77 (5-9 March 2000), AIChE Topical Conf Proc., Atlanta, USA. 

Y. Hemar, D. N. Pinder, R. J. Hunter, H. Singh, P. Hebraud, D. S. Home 2003, (Monitoring of flocculation and creaming of sodium-caseinate-stabilized emulsions using diffusing-wave spectroscopy), /. Colloid Interface Sd. 264, 502. 

Four major phenomena are associated with the-physical instability of emulsions flocculation, creaming, coalescence, and breaking (Fig. 8) [144]. 

Phase Separation. An approximate estimation of phase separation may be obtained visually. In general, creaming, flocculation, and coalescence have occurred before phase separation is visible, thus sometimes making quantitative evaluations more difficult. Accelerating the separation by centrifugation followed by appropriate analysis of the specimens may be useful to quantitatively determine the phase separation. Details on mechanisms of creaming and phase separation as well as some advances in the monitoring techniques of emulsion stability have been reviewed by Robins [146]. 

Mechanical agitation of the cream - a process called whipping - creates a metastable foam (i.e. it contains much air). Further whipping causes this foam to collapse some water separates out, and the major product is yellow butter. Incidentally, butter is a different form of colloid from milk, since its dispersed medium is water droplets and its dispersal phase is oil (milk is an oil-in-water colloid). Forming butter from milk is a simple example of emulsion inversion. 

Chicken broth is cloudy because it is colloidal, containing microscopic particles of chicken fat suspended in the water-based soup. Like milk, cream or emulsion paint, the cloudy aspect of the soup is a manifestation of the Tyndall effect. Adding the eggshells to the colloidal solution removes these particles of fat, thereby removing the dispersed medium. And without the dispersed medium, the colloid is lost, and the soup no longer shows its cloudy appearance. We say we have broken the colloid. 

---