14th International Symposium on Particle Image Velocimetry – ISPIV 2021
August 1–5, 2021

Measurement of the acoustic streaming pattern in a
standing surface acoustic wave field

S. Sachs1∗, C. Cierpka1, J. König1

1 Technische Universität Ilmenau, Institute of Thermodynamics and Fluid Mechanics, Ilmenau, Germany
∗ sebastian.sachs@tu-ilmenau.de

1 Introduction
The application of standing surface acoustic waves (sSAW) has enabled the development of many flexible
and easily scalable concepts for the fractionation of particle solutions in the field of microfluidic lab-on-
a-chip devices. In this context, the acoustic radiation force (ARF) is often employed for the targeted ma-
nipulation of particle trajectories, whereas acoustically induced flows complicate efficient fractionation in
many systems [Sehgal and Kirby (2017)]. Therefore, a characterization of the superimposed fluid motion is
essential for the design of such devices. The present work focuses on a structural analysis of the acoustically
excited flow, both in the center and in the outer regions of the standing wave field. For this, experimental
flow measurements were conducted using astigmatism particle tracking velocimetry (APTV) [Cierpka et al.
(2010)]. Through multiple approaches, we address the specific challenges for reliable velocity measure-
ments in sSAW due to limited optical access, the influence of the ARF on particle motion, and regions
of particle depletion caused by multiple pressure nodes along the channel width and height. Variations in
frequency, channel geometry, and electrical power allow for conclusions to be drawn on the formation of a
complex, three-dimensional vortex structure at the beginning and end of the sSAW.

2 Experimental microfluidic setup
The investigated microfluidic system (see Fig. 1) consists of a straight channel made of Polydimethylsiloxan
(PDMS) with a rectangular cross-section (500 µm in width, heights between 85 µm and 480 µm) centrally
located between two interdigital transducers (IDT) on a piezoelectric substrate consisting of LiNbO3. Two
coherent counter propagating surface acoustic waves (SAW) were excited that superimposed in the region
of the microchannel to form a standing wave field within the carried fluid. To study the influence of the
frequency, different pairs of IDTs with resonance frequencies ranging from 25.72 MHz to 194.8 MHz were
used. A syringe pump (neMESYS, cetoni GmbH) was employed to provide a mixture of 80 % v/v de-ionized
water and 20 % v/v glycerol at a constant flow rate. The glycerol was added to match the density between
the fluid and suspended 1.14 µm polystyrene particles. In this way, not only sedimentation was reduced but
also the influence of ARF due to a lower acoustic contrast factor. 3D3C flow measurements are performed
by APTV in three distinct regions of interest (ROI) through the birefringent LiNbO3, which requires the use
of an additional polarization filter [Kiebert et al. (2017)].

Figure 1: Schematic illustration of the microfluidic device with the observed regions at the beginning (ROI1),
center (ROI2) and the end (ROI3) of the sSAW. The directions of SAW propagation are indicated by arrows.



Figure 2: Experimentally determined particle motion transverse to the main flow in ROI2 for frequencies of
194.8 MHz (a) and 25.72 MHz (b). Flow field in sectional planes lengthwise (c, x′ = 250 µm) and crosswise
(d, y′ = 90.7 µm) to the microchannel at the beginning (dashed line) of the sSAW.

3 Results
The flow field transverse to the imposed main flow at the center of the sSAW (ROI2) is depicted for fre-
quencies of 194.8 MHz and 25.72 MHz in Fig. 2a and 2b, respectively. The irregularly distributed velocity
vectors were interpolated onto a regular grid with a voxel size of 10×800×10 µm3. White areas indicate
regions where no particles were detected. While two acoustically induced vortex pairs in Fig. 2a extend
across the entire channel cross-section, periodic structures are visible in the center of the channel as the fre-
quency decreases. With regard to the beginning of the sSAW (ROI1, Fig. 2c), a 3D vortex is visible, which
has significant velocity components in the main flow direction. Combined with Fig. 2d, it is evident that
recirculation occurs near the channel ceiling, while two separate maxima are formed at the lower corners.

4 Conclusion and outlook
In this study, the acoustically induced velocity field in the center and outer regions of the sSAW was revealed
by 3D3C APTV measurements. More advanced investigations of the complex 3D vortices and results on
the variation of frequency, channel geometry and electrical power will be presented at the conference.

Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support within the priority
program SPP2045 ”MehrDimPart” (CI 185/8-1). Furthermore, support by the Center of Micro- and Nan-
otechnologies (ZMN), a DFG-funded core facility of TU Ilmenau, is gratefully acknowledged.

References
Cierpka C, Segura R, Hain R, and Kähler CJ (2010) A simple single camera 3C3D velocity measurement

technique without errors due to depth of correlation and spatial averaging for microfluidics. Meas Sci
Technol 21:045401

Kiebert F, Wege S, Massing J, König J, Cierpka C, Weser R, and Schmidt H (2017) 3D measurement and
simulation of surface acoustic wace driven fluid motion: a comparison. Lab on a Chip 17:2104–2114

Sehgal P and Kirby BJ (2017) Separation of 300 and 100 nm particles in fabry-perot acoustofluidic res-
onators. Analytical Chemistry 89:12192–12200


	Introduction
	Experimental microfluidic setup
	Results
	Conclusion and outlook