Emulsification droplet disruption

The kind and concentration of emulsifier molecules applied have a big influence on the coalescence process during the emulsification process itself and during prolonged shelf-life. In emulsification (droplet disruption processes), the droplets are disrupted into smaller ones. New interfacial area develops, being insufficiently covered by emulsifier molecules. Interfacial active molecules (emulsifiers) are transported by laminar and turbulent flow to the droplet subsurface, diffuse to the interface and adsorb and re-orientate at it. This stabilizes the new droplets formed. However, this process takes some time (milliseconds to minutes), depending on the emulsifier molecular stmcture. Droplets colliding with each other in the meantime will coalesce [2], A detailed study about droplet coalescence... 

In the premix emulsification the basic mechanism for the droplet formation is different from the direct emulsification. In fact, in this case the predominant formation mechanism is the droplet disruption within the pore. 

Several possible mechanisms have been proposed to explain the influence of US energy on droplet formation and disruption. One assumes the formation of droplets as a consequence of unstable oscillations at the liquid-liquid interface. Such oscillations contribute to droplet disruption only if the diameter of droplets is considerably larger than the oscillation wavelength, which is about 10 xm for oil-water systems. Therefore, this mechanism, which is known as the capillary waves mechanism and rarely used to explain US-assisted emulsification, is only valid for droplets larger than 10 (xm (first steps of the process). 

Further research is needed on the influence of the viscosity of the continuous phase on US emulsification with a view to assess the significance of cavitation as a droplet disruption mechanism. 

In continuous industrial emulsification, the residence time tres in the zones ofhigh power dissipation is in the order of milliseconds to tenths of a second, also influencing droplet disruption, as determined empirically [15] ... 

Figure 20.14 Droplet disruption and droplet formation in emulsification devices different energy input, different efficiency.

Membrane emulsification processes can be directly visualized by microscope as vell as by the use of high-speed cameras. In this case, information can be obtained about droplet disruption [10, 11] and fouling of the membrane. An indirect characterization method is the (inline) measurement of the emulsion characteristics. The emulsion is mainly characterized by its droplet size and droplet size distribution [2]. These infiuence important product characteristics like structure, mouthfeel, color and appearance, texture and viscosity [12, 13]. 

Compared to other mechanical emulsification apparatus as stirred vessels, colloid mills, or gear rim devices, the mechanical stresses are significanfly increased. This is important for effective droplet disruption, especially if the submicrometer- and nanometer-sized range is targeted for low viscous products. 

Similar results are found in literature for the emulsification of particle loaded droplets by ultrasound [22, 66, 68]. It is questionable if the bulk viscosity inaease is the only reason for the hindered droplet breakup. The complex rheological behavior of suspensions and the possible interactions of the nanoparlicles at the liquid-liquid-interface have to be taken into consideration in understanding these effects. Numerical simulation of the influence of nanoparticles on droplet deformation yielded a decreased deformation for an increased particle load due to viscous and interfacial effects [73]. Depending on particle-particle- and particle-fluid-interactions, suspensions show an increase in shear thinning or viscoelastic behavior with increasing particle load [65]. Investigations with non-Newtonian fluids without particles showed that droplet deformation and droplet disruption is reduced by... 

It was previously shown that the formation of a stable emulsion of methylene chloride in water was vital for the successful formation of individual microspheres [4,9]. Two main factors played an important role in the emulsification of methylene chloride in water and influenced the microsphere size the interfacial tension of the methylene chloride droplets in the surrounding aqueous phase and the forces of shear within the fluid mass. The former tends to resist the distortion of droplet shape necessary for fragmentation into smaller droplets whereas the latter forces act to distort and ultimately to disrupt the droplets. The relationship between these forces largely determines the final size distribution of the methylene chloride in water emulsion which in turn controls the final size distribution of the solid microspheres formed. 

In order to obtain emulsification, a premix of the fluid phases containing surface-active agents and further additives is subjected to high energy for homogenization. Independent of the technique used, the emulsification includes first deformation and disruption of droplets, which increase the specific surface area of the emulsion, and second, the stabilization of this newly formed interface by surfactants. 

One mechanism similar to that of capillary waves is based on the oscillation and subsequent disruption of droplets under US action. The corresponding resonance radius at a frequency of 20 kHz (common for ultrasonic sources) is about 10 xm. This mechanism must be considered as one source of US-assisted emulsification, but can only be applied to immiscible liquid-liquid systems with a diameter within the established range for most of the droplets. In fact, most immiscible liquid-liquid systems are formed by droplets with... 

The various methods of agitation to produce emulsions have been described recently (18). In addition, the emulsions of smaller droplets can be produced by applying more intense agitation to disrupt the larger droplets. Therefore, the liquid motion during the process of emulsification is generally turbulent (9) except for high viscosity liquids. 

Many types of emulsification equipment are widely appUed in industry, such as high pressure homogenizers and rotor-stator systems. In these machines the premix droplets are deformed and disrupted in the flow field of the emulsification device [1]. In addition to these techniques, alternative methods for the production of emulsions using microporous devices have been developed since the early 1990s. 

In membrane emulsification processes, one phase (future disperse phase) is usually pressed through the pores of a membrane into the other phase (continuous phase) [3]. A different approach is the disruption of large droplets by pushing an emulsion premix through the pores of a membrane [4]. Furthermore, special shapes of the pore outlet of microcharmel modules allow the production of small droplets due to a special detachment mechanism [5-7]. 

Emulsification in the HPPF geometry is possible due to the forced passage of the droplet containing phase through the turbulence field after the orifice valve [83, 84]. However, the droplets do not get elongated at the valve inlet, which is reported to be essential for the disruption of high-viscous droplets [47, 85, 86]. 

---