Emulsification devices

Emulsification devices where the membrane is immersed in a stirred vessel containing the continuous phase, so as to obtain a batch emulsification device operating in deadend emulsification mode, have also been developed (Figure 21.13). Both flat-sheet and tubular membranes are used. In this membrane emulsification device, the continuous phase kept in motion creates the shear stress at the membrane surface that detaches the forming droplets. In a different operation mode, that is, when the continuous phase is not stirred, droplet formation in quiescent conditions is obtained. 

Figure 21.13 Emulsification devices where the membrane is immersed in a stirred vessel containing the continuous phase. Transmembrane pressure applied from (a) external or shell side, and (b) internal or lumen side.

This concept of energy density allows one to compare different types of continuous or semi-continuous mechanical emulsification devices in terms of efficiency and is a helpful tool for scaling-up purposes in practical applications. 

The T-shaped junction is the most studied shear-based emulsification device. In T-shaped junctions as shown in Fig. 4, the continuous phase flows through the horizontal channel, and the to-be-dispersed phase is pressurized through the vertical channel where it meets the continuous phase, is distorted, and eventually forms droplets at the T-junction. Typical images of this process in an experimental setup through Lattice Boltzmann simulations are shown in Fig. 4. 

Emulsion Preparation with Microstructured Systems, Table 2 Estimation of the device volume Vdevice and required area Adevice needed to handle oil at 1 m x h in various emulsification devices. The droplet size is between 5 and 10 pm ... 

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

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. 

Yamazaki N, Naganuma K, Nagai M, Ma GH, Omi S. 2003. Preparation of W/O (water-in-oil) emulsions using a PTFE (polytetrafluoroethylene) membrane A new emulsification device. / Dispersion Sci Technol 24 249-257. 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

Colloid mills which are employed for dispersion or for emulsification fall into four main groups the hammer or turbine, the smooth-surface disk, the rough-surface type, and valve or orifice devices. 

As noted above this type of mechanical transducer is predominantly used for homo-genisation/emulsification. These devices differ markedly from the more usual bath and probe types in that they derive their power from the medium (by mechanical flow across the blade) rather than by the transfer of energy from an external source to the medium. The majority of the chemical effects observed on using whistle type transducers for the sonication of homogeneous reactions can be attributed mainly to the generation of very fine emulsions rather than the ultrasonic irradiation itself. 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

Shear A strain resulting from applied forces that cause or tend to cause contiguous parts of a body to slide relative to one another in direction parallel to their plane of contact. In emulsification and suspensions, the strain produced upon passing a system through a homogenizer or other milling device. 

If we analyze different mechanical emulsification techniques and relate the pressure gradient to the work required W (energy input) we find that for most devices the mean radius of drops R scales with... 

More than 100 micro structured devices are listed on the homepage of the pChemTec consortium [24]. The devices cover physical applications such as flow distribution, mixing, heat transfer, phase transfer, emulsification and suspension, as well as chemical applications such as chemical and biochemical processing. Some separation units such as membrane separation and capillary electrophoresis are also offered. Control devices such as valves, micro pumps for product analysis and mass flow controllers supplement the catalog. 

Figure 21.1 Schematic representation (a) of membrane emulsification, where the membrane works as a high-throughput device to form droplets with regular dimensions (b) photo of an o/w emulsion...

The various membrane emulsification procedure can be practised by using appropriate membranes and devices configuration. 

Each type of device has specific advantages and disadvantages. The batch emulsification is suitable for laboratory-scale investigations. The construction of the device is simple and handling during emulsification as well as for cleaning. Crossflow membrane emulsification is used when it is important that a proper adjustment of all process parameters and larger amounts of emulsion have to be produced. 

In high force dispersion devices, ultrasonication is used today especially for the homogenization of small quantities, whereas rotor-stator dispersers with special rotor geometries, microfluidizers, or high-pressure homogenizers are best for the emulsification of larger quantities. 

Because of the availability of these new methods, devices, and purer materials, it has become more feasible to carry on effective research with adequate surface-chemical control of gas and liquid adsorption, wetting, adhesion, emulsification, foaming, boundary friction, corrosion inhibition, heterogeneous catalysis, electrophoresis, electrode surface potentials, and a variety of other subjects of interest in the surface-chemical and allied fields of research. In view of the present situation, serious investigators should now be able to report results in the scientific literature which will have much more value than ever before. There is no excuse for any investigator s taking such inadequate care in controlling surface composition or surface-active contaminants as was common in over 50% of the research publications in surface and colloid science in the past. 

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