Time reversal sub-wavelength focusing in bubbly media

TL;DR

This paper demonstrates a broadband time reversal technique to achieve deep subwavelength acoustic focusing inside bubbly media, with potential applications in microfluidics.

Contribution

It introduces a method combining multiple scattering theory and time reversal to focus energy at subwavelength scales in bubbly materials, validated by analytical and numerical results.

Findings

Subwavelength focus of λ/100 in ideal bubbles

Focus of λ/14 with realistic viscous and thermal losses

Effective wavenumber matches analytical predictions

Abstract

Thanks to a Multiple Scattering Theory algorithm, we present a way to focus energy at the deep subwavelength scale, from the far-field, inside a cubic disordered bubble cloud by using broadband Time Reversal (TR). We show that the analytical calculation of an effective wavenumber performing the Independant Scattering Approximation (ISA) matches the numerical results for the focal extension. Subwavelength focusings of lambda/100 are reported for simulations with perfect bubbles (no loss). A more realistic case, with viscous and thermal losses, allows us to obtain a $\lambda/14$ focal spot, with a low volume fraction of scatterers (phi = 0.01). Bubbly materials could open new perspective for acoustic actuation in the microfluidic context.