![]() Other examples of emulsions where Pickering particles may be the stabilizing species are for example detergents, low-fat chocolates, mayonnaises and margarines. The casein replaces the milkfat globule membrane, which is damaged during homogenization. The milk protein ( casein) units are adsorbed at the surface of the milk fat globules and act as surfactants. One example of a Pickering-stabilized emulsion is homogenized milk. The most common type of Pickering emulsions are oil-in-water emulsions due to the hydrophilicity of most organic particles. Generally, the phase that preferentially wets the particle will be the continuous phase in the emulsion system. When the contact angle is approximately 90°, the energy required to stabilize the system is at its minimum. Δ E = π r 2 γ O W ( 1 − | cos θ O W | ) 2 is the contact angle of the particle with the interface. The optimal contact angle for a stable emulsion is achieved when the particle is equally wetted by the two phases (i.e. Particles that are partially hydrophobic are better stabilizers because they are partially wettable by both liquids and therefore bind better to the surface of the droplets. If the contact angle of the particle to the interface is low, the particle will be mostly wetted by the droplet and therefore will not be likely to prevent coalescence of the droplets. The particle’s contact angle to the surface of the droplet is a characteristic of the hydrophobicity of the particle. Particle properties such as hydrophobicity, shape, and size, as well as the electrolyte concentration of the continuous phase and the volume ratio of the two phases can have an effect on the stability of the emulsion. However, if solid particles are added to the mixture, they will bind to the surface of the interface and prevent the droplets from coalescing, making the emulsion more stable. If oil and water are mixed and small oil droplets are formed and dispersed throughout the water (oil-in-water emulsion), eventually the droplets will coalesce to decrease the amount of energy in the system. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903. Pickering emulsions were named after S.U. Typically, the emulsions are either water-in-oil or oil-in-water emulsions, but other more complex systems such as water-in-water, oil-in-oil, water-in-oil-in-water, and oil-in-water-in-oil also do exist. The homogenization phenomenon is completed before the fluid leaves the area between the valve and the seat.A Ramsden emulsion, sometimes named Pickering emulsion, is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the water and oil phases. ![]() The liquid then moves across the face of the valve seat and exits in about 50 microseconds. It then moves into the gap between the valve and the valve seat and its velocity increases to 120 meters/sec in about 0.2 milliseconds. ![]() As it first enters the valve, liquid velocity is about 4 to 6 m/s. To understand the mechanism, consider a conventional homogenizing valve (Figure 1) processing an emulsion such as milk at a flow rate of 20,000 l/hr. The homogenization process reduces the globule size by passing milk under high pressure through a tiny orifice. The net result of this process is a decreased creaming rate according to Stokes’ Law, reduced clustering during creaming, and better density matching with the continuous phase. Homogenization is a mechanical process used to reduce the size of the fat globules in the milk. If raw milk were left to stand the fat globules would form a cream layer and rise to the surface. Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous phase of skim milk.
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