Ultrasound external pressure

It is now widely recognized that microbubbles can be destroyed by ultrasound irradiation [70]. This may be explained simply by the fact that when microbubbles are subjected to external pressure, they shrink and gas leaks out and dis-... 

The first observations concerning the role of the gas bubbles existent into a liquid supposed to the ultrasounds action belong to Boyle and Lemann (1923) and V.C. Sorensen (1936). The first ones underlined that the gaseous bubbles can penetrate into the spaces formed by cavitation and discovered the tendency of sound to remove the gases dissolved in the liquid medium. The cavitation phenomenon depends on the external pressure that can increase up to a certain limit from which gas removal starts [1145]. According to V.C. Sorensen the removing of 1 cm of gas from saturated water requires 51.2 kW at 197 kHz, 72.6 and 87.4 kW at 380 kW and 530 kHz, respectively. Therefore, the gas removing essentially depends by the ultrasound intensity [1146]. 

Sonochemistry is strongly affected by a variety of external variables, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, ambient gas, and solvent (47). These are the important parameters which need consideration in the effective appHcation of ultrasound to chemical reactions. The origin of these influences is easily understood in terms of the hot-spot mechanism of sonochemistry. 

The role of cavitation in ultrasound degradation has been confirmed repeatably in most experiments where cavitation was prevented, either by applying an external hydrostatic pressure, by degassing the solution, by reducing the sound intensity or the temperature, polymer chain scission was also largely suppressed [117]. 

Acoustic cavitation (AC), formation of pulsating cavities in a fluid, occurs when a powerful ultrasound is applied to a non-viscous fluid. The cavities are formed when the variable acoustic pressure in the rarefaction phase exceeds the cohesive strength of the fluid. Under acoustic treatment (AT), cavities grow to resonance dimensions conditioned by frequency, amplitude of oscillations, stiffness properties and external conditions, and start to pulsate synchronously (self-consistently) with acoustic pressure in the medium. The cavities undergo significant strains (compared to their dimensions) and their size decreases under compression up to collapsing. This nonlinear behavior determines the active, destructional character of the cavities near which significant shear velocities, local pressure and temperature bursts occur in the fluid. Cavitation determines the specific character of acoustic treatment of the fluid and effects upon objects resident in the fluid, as well as all consequences of these effects. 

Active micromixers use an external energy source to introduce perturbation in the fluid stream and are then categorized with respect to the type of energy source used. Pressure field, elecrokinetic, dielectrophoretic, electrowetting shaking, magneto-hydrodynamic, ultrasound, and thermal-assisted micromixers have been presented and discussed. 

In a different strategy to achieve ultrasound-guided motion, Wang and coworkers prepared microbullets with an inner Au layer which permits conjugation to a monolayer of thiolated cysteamine. The entire functionalisation also enables electrostatic attachment to perfluorocarbon (PFC) (either perfluoropentane or perfluorohexane) droplets to be carried out. Under ultrasonic irradiation, the PFC droplets are vaporised, leading to the net motion of microbullets towards lamb kidney tissue. Propulsion can be modulated by adjusting external parameters such as acoustic pressure, pulse duration or surfactant concentration [131]. 

The term active mixer or active microimxef refers to a microfluidic device in which species mixing is enhanced by the application of some form of external energy disturbance. Typically, this disturbance is generated either by moving components within the micromixer itself, e.g. magnetically-actuated stirrers, or by the application of an external force field, e. g. pressure, ultrasound, acoustic, electrohydrodynamic, electrokinetic, dielectrophoretic, magneto-hydrodynamic, thermal, and so forth [1]. 

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