7: Ultrasound processing

Question 1

Describe the effects of ultrasound processing in liquid systems

Answer 1

1. C&R: When liquid is sonicated, sound waves propagate into liquid media and result in alternating compression (high P) and rarefaction (low P) cycles
2. Cavitation bubbles: During these cycles, cavitation bubbles start to form
3. Bubbles grow and collapse: Size of the bubbles grow through rectified diffusion and collapses to generate hotspots of high T and P.
4. Chemical effects: Components introduced during bubbles formation are subjected to high T and P on collapse, leading to chemical effects, e.g. molecular fragmentation
5. Mechanical effects: In the bulk liquid, collapse of bubbles generate shear forces leading to mechanical effects, such as size reduction.

Question 2

Describe the effects of ultrasound processing in solid-liquid systems.

Answer 2

1. High pressure microjets are generated from the collapse of cavitation bubbles.
2. Interfacial boundary layers are disturbed and surface becomes irregular.
3. Irregular surfaces cause the formation of new cavitation bubbles.
4. Collapse of bubbles also creates shock waves and cause particles to move rapidly, accelerating interparticle collisions.
5. Collisions cause erosion, surface cleaning, wetting effect and size reduction.

Question 3

What are the uses of ultrasound processing?

Answer 3

• MO inactivation (Cavitation bubbles leads to cell destruction)
• Inactivation of enzymes (Thermal effect, generation of free radicals and molecular changes)
• Processing aid (e.g., cleaning, mixing and homogenization)
• Foam destruction
• Temporarily reduce viscosity
• Airborne sonication
• Filtration
• Drying
• Improve efficiency of extraction at lower temperature.

Question 4

How does ultrasonic processing aid in foam destruction?

Answer 4

• High acoustic pressure creates partial negative pressure above the foam and cause release of gas
• By promoting bubble coalescence.
• Foam destruction from cavitation and acoustic streaming

Question 5

How does ultrasonic processing aid in reducing viscosity

Answer 5

• By disrupting molecular forces and causing molecular fragmentation
• Reducing viscosity can increase heat transfer, reduce fouling and contamination and reduce the frequency of cleaning.