Radiation Force Due to Beamed Energy
Maxwell’s electromagnetic stress tensor – Radiation pressure due to plane
wave on surface = k*E. ( E = Energy density) Absorbing surface: k =1. Reflective
surface: k=2. Gradient forces: Beam waist acts as particle trap for transmitting
particles, due to intensity gradient.
Optical Tweezers: Particles are forced to the focus / waist of a CW laser
beam interpreted using geometric optics and refractive index for particles
>> l. also works using Mie theory where particle size ~ l recently found
to work for Rayleigh regime – nanoparticles << l
Radiation pressure on objects due to coherent
beams is used in optics and acoustics.
Ultrasonic beams - “Fingers of Sound / Space Drums” Used to hold and manipulate
levitated / suspended particles.
Acoustic Radiation Force: Applications
Rayleigh – proposed expression for radiation pressure in acoustic fields,
analogous to Maxwell’s stress tensor.
King 1934: Theory for radiation force in acoustic fields – formation of
dust striations in water tanks. Forces considered to be insignificant except
with ultrasonic frequencies and neutrally-buoyant particles in water.
Levitation experiments: Ultrasonic levitators used to lift steel spheres
– to demonstrate utility in non-contact melting and positioning within
STS experiments: Holding molten drop of metal inside a container in micro-gravity.
Problem: Radiation force lost when phase change / cooling occurred. Attributed
to reversal of force due to formation of envelope of heated gas around
the sphere. [Wang 1998]
Liquid manipulation using ultrasonics: NASA Glenn research
NASA Hybrid electrostatic levitator / ultrasonic manipulator facility.