In situ estimation of local acoustic pressure amplitude by force balancing with a ferrofluid droplet probe

Abstract

Acoustic tweezers enable non-contact manipulation of microscale objects, but quantitative in situ evaluation of the peak local pressure amplitude remains difficult in confined devices. Conventional hydrophone-based measurements are often limited at the microscale by probe size and installation constraints. Here, we present a force-balance method in which a trapped ferrofluid droplet serves as a local probe in a standing-wave acoustic field and an externally applied magnetic-field gradient is tuned so that the magnetic force balances the maximum primary acoustic radiation force on the droplet. From the magnetic force on the ferrofluid droplet, determined at the balance point, we estimate a peak local pressure amplitude of $2.6\times10^{5}$~Pa for 7.2~MHz operation at 10~V$_{\mathrm{pp}}$. This approach provides a practical route for quantitative in situ characterization of microscale acoustic fields and for setting operating conditions in compact acoustofluidic devices.