16.1 Principles of acoustic levitation

Acoustic levitation uses sound waves to defy gravity, suspending small objects in mid-air. This phenomenon relies on high-frequency waves creating pressure nodes where objects can float, opening doors for innovative applications in various fields.

From material processing to biotechnology, acoustic levitation enables unique experiments and manufacturing techniques. By manipulating sound waves, scientists can control tiny particles, revolutionizing research and industrial processes in ways previously thought impossible.

Fundamentals of Acoustic Levitation

Principles of acoustic levitation

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• Acoustic radiation force generated by non-linear effect of sound waves counteracts gravity on small objects (dust particles, water droplets)
• High-frequency sound waves typically ultrasonic create intense sound field necessary for levitation
• Acoustic pressure variations in air pressure due to sound waves exert force on objects
• Sound waves reflect off surfaces and interfere to create stable levitation points (acoustic nodes)

Standing waves and pressure nodes

• Standing waves formed by interference of incident and reflected waves create stationary pattern of nodes and antinodes
• Pressure nodes points of minimum pressure variation occur at regular intervals in standing waves
• Acoustic potential wells regions of stability where objects can be trapped created at pressure nodes
• Distance between nodes is half the wavelength of sound $d = λ/2$

Parameters of levitation stability

• Frequency of sound waves higher frequencies allow smaller objects to be levitated (20-40 kHz range)
• Sound intensity stronger acoustic force with higher intensity affects lifting capacity
• Object properties:
  • Size must be smaller than the wavelength of sound (millimeter scale)
  • Density affects the required acoustic force (lighter objects easier to levitate)
• Environmental factors air temperature and humidity affect sound propagation and stability
• Transducer configuration:
  • Single-axis vs multi-axis setups determine levitation geometry
  • Phased array systems enable dynamic control and object manipulation

Applications in science and industry

• Material processing enables containerless processing of materials for studying properties without contamination (molten metals, alloys)
• Biotechnology facilitates manipulation of small biological samples and cell culturing in suspension (blood droplets, stem cells)
• Chemistry allows mixing small quantities of reactive substances and crystallization studies (protein crystals, pharmaceuticals)
• Pharmaceuticals aids in drug development, testing, and precise dosage control (microdroplets of medications)
• Space research enables microgravity simulations on Earth for studying material behavior (fluid dynamics experiments)
• Display technology supports acoustic holography and mid-air displays for interactive visualizations
• Manufacturing enables non-contact handling of delicate components in electronics assembly
• Fluid dynamics facilitates study of droplet behavior in isolation for improved understanding of surface tension and evaporation