A study of nanoparticle manipulation using ultrasonic standing waves

There has been considerable interest in the noninvasive manipulation of particles in dispersions during the last 15 years. The manipulation techniques based on acoustic radiation forces are particularly interesting as they allow for the manipulation of particles based on their density, compressibility and/or size. The majority of the acoustic research has been focused on particles above a few μm. In the current work we investigate the manipulation of particles in the range of 100 nm to 10 μm using ultrasonic standing waves. The setup consisted of a small rectangular cell. Within the cell, two sides of piezo material were meticulously aligned (square: 30 × 30 mm², center frequency 2 MHz). The distance between the piezo slabs was 9 mm. One piezo slab was excited using continuous wave signals with frequencies around 2 MHz; the frequency was optimized such that the power transfer was maximized. The liquid in the tank consisted of SiO₂ nanoparticle dispersions with varying mean sizes: 100 nm, 500 nm, 1000 nm and 10 μm. The concentration of the particles was 0.03 mass%. If the ultrasound was switched off, the nanoparticle dispersion appeared as a homogeneous milky white substance. If the ultrasound was switched on, the particles moved to the nodes of the standing wave field. The distance between the planes in which the particles were localized corresponded to half the wavelength. Good localization was achieved in roughly ≪1s, 5s and 10s for the 10 μm, the 1 μm and the 500 nm particles, respectively. No particle localization was observed for the 100 nm particles, which was likely caused by mixing due to acoustic streaming of the liquid and compounded by Brownian motion. The peak power dissipated into the piezo slabs was 6W. Due to the small size of the cell and the fairly high losses in the piezo material, the temperature increased considerably during the experiments (1.3°C/min). Nanopar- icles down to 500 nm were successfully manipulated using the radiation force induced by ultrasonic standing waves. Regarding 100 nm particles, the mixing effects due to acoustic streaming prevented successful manipulation.