Acoustic radiation- and streaming-induced microparticle velocities determined by microparticle image velocimetry in an ultrasound symmetry plane

Abstract

We present microparticle image velocimetry measurements of suspended microparticles of diameters from 0.6 to 10 μm undergoing acoustophoresis in an ultrasound symmetry plane in a microchannel. The motion of the smallest particles is dominated by the Stokes drag from the induced acoustic streaming flow, while the motion of the largest particles is dominated by the acoustic radiation force. For all particle sizes we predict theoretically how much of the particle velocity is due to radiation and streaming, respectively. These predictions include corrections for particle-wall interactions and ultrasonic thermoviscous effects and match our measurements within the experimental uncertainty. Finally, we predict theoretically and confirm experimentally that the ratio between the acoustic radiation- and streaming-induced particle velocities is proportional to the actuation frequency, the acoustic contrast factor, and the square of the particle size, while it is inversely proportional to the kinematic viscosity.

Theoretical study of time-dependent, ultrasound-induced acoustic streaming in microchannels

“…However, for smaller particles, the focusing by the acoustic radiation force is hindered by the drag force from the suspending liquid, which is set in motion by the generation of an acoustic streaming flow [2,3]. This limits the use of acoustophoresis to manipulate submicrometer particles, relevant for application within medical, environmental, and food sciences, and it underlines a need for better understanding of acoustic streaming and ways to circumvent this limitation.…”

“…We only consider acoustic perturbation in fluids, and treat the surrounding solid material as ideal rigid walls, a good approximation for water channels in glass-silicon systems. Moreover, extensive theoretical and experimental work [2,3,10,11,19,21] has shown that the adiabatic model describes the observed phenomena qualitatively correctly, while thermoviscous effects may lead to relative quantitative changes up to 30%. Thus by employing an adiabatic model, we can provide an analysis of experimental relevance while at the same time restricting the complexity and vast parameter space of the full problem.…”

Three-dimensional matrixlike focusing of microparticles in flow through minichannel using acoustic standing waves: An experimental and modeling study

“…The visualization and tracking of particles are generally performed using particle image velocimetry (PIV) as in Manneberg et al [8,16] and Barnkob et al [17]. The tracking of particle motion under ASWs using the astigmatism particle tracking velocity (APTV) technique was also reported by Muller et al [18].…”

Performance Study of Acoustophoretic Microfluidic Silicon-Glass Devices by Characterization of Material- and Geometry-Dependent Frequency Spectra

“…Despite the very different displacement fields in the surrounding chip material, which is represented as the magnitude of the real part of the displacement jReðuÞj by the color plot (from blue zero, via green, to red maximum), both systems exhibit a nearly perfect horizontal standing pressure wave ReðpÞ (the color plot from blue minimum, through white, to red maximum or line plot in the inset) in the fluid. This occurrence has been observed experimentally [25][26][27], and it is ideal for forcing the particles to the vertical pressure nodal plane in the center of the channel. By definition of f acp , the orientation θ is close to unity in both systems.…”

“…The first numerical optimization studies of acoustophoretic devices have also been performed recently, illustrating a procedure to obtain optimal acoustophoretic forces by changing the geometrical parameters of the device [21]. Other studies involve numerical characterization of the acoustic pressure wave in the microchannel and subsequent computation of particle trajectories by means of numerical integration [22][23][24][25][26][27].…”