Ultrasonic homogenization applications

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 ... 

In contrast to the opaque, milky conventional emulsions and ml-nlemulslons, mlcroemulslons are isotropic, transparent or translucent, and thermodynamically stable. They form spontaneously when oil and water are mixed with surfactant and cosurfactant (usually 1-pen-tanol or 1-hexanol). Vigorous agitation, homogenization, or ultrason-Iflcatlon are not needed. Mlcroemulslons are postulated to conq>rlse dispersions of droplets of size smaller than 100 nm or blcontlnuous lamellar layers. Both structures are consistent with their transparency or translucency. Which structure is more applicable is the subject of some controversy, a discussion of which is beyond the scope of this paper. 

Emulsions are usually prepared by the application of mechanical energy produced by a wide range of agitation techniques. These disrupt droplets by the application of either shear forces in laminar flow or inertial forces in turbulent flow. Emulsifying devices ranging from simple hand mixers and stirrers to the use of propeller or turbine mixers, static mixers, colloid mills, homogenizers, and ultrasonic devices have been used. 

To create an emulsion either industrially or in the laboratory, it is necessary to use the appropriate homogenization procedure. If an application does not require that the droplets in an emulsion be particularly small, it is usually the easiest to use a high-speed blender. When the involving ingredients are limited in availability or are expensive, an ultrasonic piezoelectric transducer can be ideally used, whereas for monodispersed emulsions a membrane homogenizer is preferred. 

Ultrasonic irradiation not only has a remarkable effect on the rate of this homogeneous reaction, but is also essential to reach full conversion. Such results are unattainable imder silent conditions. Sp-Hydroxy-A -pregnenes and -androstenes give identical yields, thus ratifying the wide applicability of this one-pot oxidation. 

The amorphous polymers are proeessed only in the visco-elastie or flowing state. However, quite frequent are the situations in which, prior to their proeessing the polymers are subjected, at inferior temperatures, either to vibratory milling or ultrasonic irradiaton, in view of their homogenization from the viewpoint of the particle size. Also, vibratory milling under T is applied for the recovery of wastes that is why, knowledge of the chemical transformations suffered - under such conditions - by the amorphous polymers and, respectively, activation of certain chemical reactions under the action of mechanical stresses, may result in significant practical applications. 

The most popular ultrasonic nondestructive testing application has been associated with thickness measurement of a test object and defect location within the particular test object. Most of the applications to date have been associated with the testing of homogeneous isotropic materials. Recent work has extended the basic ultrasonic test philosophy to the field of composite materials and adhesive bonding inspection. Unfortunately, many difficulties occur because of the inhomogeneous and anisotropic characteristic of a composite material. This section includes a review of the physical principles associated with ultrasonic testing and the particular items that must receive special attention when inspecting composite materials or adhesively bonded sections of a structure. 

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