Ultrasound microstreaming

The observation that sonochemistry and luminescent activities are not necessarily coincident had never been reported and suggests that the conditions at the root of sonoluminescence and sonochemistry could be different in nature (or in intensity). Chemical species as tracers should serve to describe the liquid flows the knowledge of which is of prime importance in medical ultrasound (microstreaming, acoustic streaming and/or rectified acoustic streaming zy Even if synthetic exploitations are difficult to imagine, some fundamental aspects of sonochemistry and cavitation still to be explored are very probably involved, and in this respect deserve further development. 

On the other hand stable cavitation (bubbles that oscillate in a regular fashion for many acoustic cycles) induce microstreaming in the surrounding liquid which can also induce stress in any microbiological species present [5]. This type of cavitation may well be important in a range of applications of ultrasound to biotechnology [6]. An important consequence of the fluid micro-convection induced by bubble collapse is a sharp increase in the mass transfer at liquid-solid interfaces. In microbiology there are two zones where this ultrasonic enhancement of mass transfer will be important. The first is at the membrane and/or cellular wall and the second is in the cytosol i. e. the liquid present inside the cell. 

At an appropriate intensity level of ultrasound, intracellular microstreaming has been observed inside animal and plant cells with rotation of organelles and eddying motions in vacuoles of plant cells [9]. These effects can produce an increase in the metabolic functions of the cell that could be of use in both biotechnology and microbiology, especially in the areas of biodegradation and fermentation. 

The mass transport can also be enhanced by ultrasound [62]. An ultrasonic source immersed in the solution produces a radial flux and the formation of bubbles [69]. When a bubble collapses at an electrode surface, a microjet of solution is formed [70]. This form of transport is called microstreaming. It can be utilized in ultrasound-enhanced electroanalysis [71-73]. 

Initial efforts focused upon the ability of ultrasound to aid in the translation of nanodroplets in an aqueous medium. We have determined that 2% perfluorocarbon nanodroplets were easily translated through an aqueous medium out of the blood flow by radiation force/microstreaming methodology. As expected, the translation of the nanodroplets varied inversely as a function of perfluorocarbon density (i.e. translation perfluoropentane > perfluorohexane > perfluoroheptane > perfluorooctane > PFOB, density perfluorooctane > perfluoroheptane > perfluorohexane... 

The latest studies analyzing the mechanisms of contact ultrasound-assisted mass transfer have revealed that ultrasound-related cell disruption is limited to the thin layer of tissue that is in direct contact with the vibrating surface. In any tissue layers deeper than 1 mm, the structural changes that occur are attributed to water removal moreover, ultrasound can enhance such water removal as a result of mechanical effects that can loosen cell-boundary layers, and remove moisture from solid-liquid interfaces. The transport of moisture through the pore network can also be increased, and low-pressure cycles vhll enable an improved evaporation in the tissues. 

Figures 17.11-17.13 present the elimination process of these floccules on LiCo02 surface with ultrasonic-assisted hydrothermal method. According to analysis of products [53, 78,79], cavitation caused by ultrasound in hydrothermal environment cannot only lead to the production of free radicals but also the occurrence of microstreaming. The former can promote organic degradation, and the latter is able to accelerate the heat and mass transfer process accompanying the renovation process. Shock waves from cavitation in liquid-solid slurries produce high-velocity inter-particle collisions. Therefore, the agglomerates on the surface of the LiCo02 can easily be broken.

---