Theories Of Homogenization

Because of the dynamic forces occurring in the homogenizing valve, there has been some interest in determining the mechanism of homogenization. Many researchers have suggested theories to account for homogenization by relating the presumed flow condition in the homogenizing valve to known concepts of fluid dynamics. Homogenization includes the formation of emulsions (one immiscible liquid uniformly distributed into another) or dispersions (solid particles dispersed throughout a liquid), but most theories of homogenization only consider the formation of emulsions. Therefore, these theories usually relate to the mechanism by which dispersed oil droplets are disrupted into smaller droplets and distributed throughout a continuous water phase. This approach is a consequence of the fact that most researchers were investigating the homogenization of milk when they developed their theories.

Theories of homogenization include shear, impact, wire drawing, acceleration and deceleration, homogenizing valve vibration, turbulence, and cavitation (1,11-13). Many of these theories have been discounted over the years. For example, impact of the dispersed droplets on the impact ring is not the cause of homogenization because the required velocities at impact are not large enough to disrupt the droplet, and increasing the distance from the valve seat to the impact ring does not significantly affect homogenization of an emulsion. Wire drawing is the elongation and subsequent disruption of the thinned droplet, but this is unlikely to occur due to the flow conditions in the valve. Shear is commonly mentioned as a mechanism, but the velocity gradients in the valve gap boundary layer do not appear to be adequate for emulsification. Also, the successful homogenization of oils with viscosities greater than that allowed for shearing action to occur indicate a mechanism other than shear (14).

Two other prominent mechanisms of homogenization involve turbulence and cavitation (15,16). The intense turbulent eddies generated in the liquid at the instant of energy conversion (potential to kinetic energy) produce significant local velocity gradients that disrupt the droplets. Cavitation theory suggests that the extreme pressure drop in the homogenizing valve generates cavitation bubbles, and when these bubbles collapse, the shock waves in the fluid cause the droplets to break apart. Actual measurements of cavitation noise in the homogenizing valve have suggested that the greater the intensity of cavitation, the greater is the homogenizing effect (16). However, the fact that cavitation is present does not prove that it is the actual mechanism for homogenization. Published research has shown that if cavitation is dampened or suppressed, homogenization still occurs, suggesting that turbulence may be the predominant mechanism (17). Actual visualization of the emulsification process in a homogenizing valve using a Nd:YAG laser with a pulse duration of 10 ns has shown that homogenization occurs after discharge from the gap of the valve and that emulsification is produced by the intense turbulent mixing zone in the exit region. No homogenization occurs in the region between the valve and the seat (18).

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