Characterization of vertical confinement of Brownian-diffusive particles within quarter-wavelength ultrasonic...

The confinement of Brownian-diffusive particles in ultrasonic standing wave fields is crucial for enabling acoustic manipulation of submicrometer and nanometer particulate materials in chemical and biochemical analytical applications. In this study, we fabricated a cylindrical micro-resonator with multi-layered structures capable of stably generating quarter-wavelength ultrasonic standing wave fields in the vertical direction, and investigated the confinement of Brownian-diffusive particles by acoustic radiation force. To validate the design of the micro-resonator, numerical simulations were conducted, confirming its ideal resonance frequency of 2 MHz and demonstrating its capability to generate quarter-wavelength ultrasonic standing wave fields both numerically and experimentally. The concentration profiles of polystyrene fluorescent spheres in a quasi-steady state, vertically confined toward the pressure nodal plane at the bottom surface of the micro-cavity by acoustic radiation force, were experimentally measured and compared with theoretical predictions. An experimental analysis of Brownian-diffusive particle confinement in quarter-wavelength ultrasonic standing wave fields demonstrated that the acoustic boundary layer on a single particle can reduce the acoustic radiation force, leading to an underestimation of the average energy density of standing acoustic waves. By characterizing the confinement of Brownian-diffusive particles within a quarter-wavelength standing wave field, this study proposes a semi-empirical correlation for the average energy density of the induced acoustic field, based on experimental data across a wide range of voltage amplitudes applied to the micro-resonator. The correlation also showed excellent agreement with experimental values, regardless of the Brownian-diffusive particle radius.