Measuring the local pressure amplitude in microchannel acoustophoresis

Abstract

A new method is reported on how to measure the local pressure amplitude and the Q factor of ultrasound resonances in microfluidic chips designed for acoustophoresis of particle suspensions. The method relies on tracking individual polystyrene tracer microbeads in straight water-filled silicon/glass microchannels. The system is actuated by a PZT piezo transducer attached beneath the chip and driven by an applied ac voltage near its eigenfrequency of 2 MHz. For a given frequency a number of particle tracks are recorded by a CCD camera and fitted to a theoretical expression for the acoustophoretic motion of the microbeads. From the curve fits we obtain the acoustic energy density, and hence the pressure amplitude as well as the acoustophoretic force. By plotting the obtained energy densities as a function of applied frequency, we obtain Lorentzian line shapes, from which the resonance frequency and the Q factor for each resonance peak are derived. Typical measurements yield acoustic energy densities of the order of 10 J/m 3 , pressure amplitudes of 0.2 MPa, and Q factors around 500. The observed half wavelength of the transverse acoustic pressure wave is equal within 2% to the measured width w ¼ 377 mm of the channel.

Acoustic streaming in the transducer plane in ultrasonic particle manipulation devices

“…Typically, in a low MHz field the motion of particles of diameters larger than about 2 μm is dominated by the ARF. Recent experimental work [3][4][5] has shown excellent agreement with theoretical predictions based on well-established theory 6,7 . However, during the process of particle manipulation, acoustic streaming can disrupt the manipulation of particles with diameter smaller than 2 μm.…”

A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics

“…52,53 It has been found that at diameters of around 1 μm there is a transition to drag-dominated behaviour for operating frequencies of ~2 MHz. 52,54 For these reasons, few studies have demonstrated acoustofluidic concentration of flowing particles with diameter <2 μm 49,55 (Table I). Antfolk et al recently demonstrated focusing of bacteria using a combination of streaming and a two-dimensional resonance; their paper is not included in Table I, as comparable concentration data was not included; however, impressive performance with polystyrene beads as small as 0.5 µm was shown.…”

Observation of Metal Nanoparticles for Acoustic Manipulation

“…The results indicate that the size parameter is not important for acoustic manipulation when the particle size is much smaller than the acoustic wavelength. According to the previous theory,23, 24, 45, 46, 47, 48 the primary acoustic radiation force F on small particles can be calculated as Equation (1) $F_{\max }=\frac{\pi P^2{V}_{\mathrm{normalP}}}{2\lambda }\frac{1}{{\rho }_{\mathrm{normalM}}{C}_{\mathrm{normalM}}^2}\left(\frac{5{\rho }_{\mathrm{P}}-2{\rho }_{\mathrm{M}}}{2{\rho }_{\mathrm{P}}+{\rho }_{\mathrm{M}}}-\frac{{\rho }_{\mathrm{M}}{C}_{\mathrm{M}}^2}{{\rho }_{\mathrm{P}}{C}_{\mathrm{P}}^2}\right)$where P is acoustic pressure, V P is the volume of metal particle, ρ P and ρ M are the density of metal particle and medium, C P and C M are the speed of sound in particle and medium, respectively. Details are listed in Table S1 (Supporting Information).…”