HOW THE WAVE IS DRIVEN

Fingers that alternate

An IDT looks like two interlocking combs of metal fingers. Driving alternating positive and negative potential across the fingers launches a traveling acoustic wave — with a period set by a quarter of the wavelength.

+ − + − + − + −

Manufacturing limits and existing chip designs put a practical floor on wavelength around 240 µm.

FREQUENCY

Setting the resonant frequency

f = v / λ

The resonant frequency depends on the Rayleigh wave velocity of the substrate (about 3980 m/s for lithium niobate) divided by the IDT's wavelength.

Shrink the wavelength, and the operating frequency goes up — the same relationship that governs any wave.

ACOUSTIC FORCE

What pushes the particle

Traveling wave force Depends on absorption coefficient, wave intensity, and speed of sound

Standing wave (SSAW) force Depends on pressure amplitude, particle volume, compressibility, density, and wave number

The standing wave version is what actually sorts particles inside the channel.

ACOUSTIC CONTRAST FACTOR

Which way does it move?

The acoustic contrast factor decides the direction a particle migrates.

Positive

Force points from antinodes toward pressure nodes (center)

Negative

Direction flips — force pushes toward antinodes (outside)

Whether a particle heads for the center or the edges of the channel comes down to the sign of a single factor.

THE OPPOSING FORCE

Viscosity pushes back

As a particle moves through the fluid, viscous drag resists it. That drag force depends on the particle's radius, its velocity, and the viscosity of the surrounding medium.

🌊

A particle's final position is a balance point — where acoustic force and viscous drag settle into equilibrium.

UNIT 26 STUDY COMPLETE

💧

Ready for the Fab Challenge?

You've covered The Physics of Acoustic Forces.