- Open Access
Schlieren visualization of ultrasonic standing waves in mm-sized chambers for ultrasonic particle manipulation
© Möller et al.; licensee BioMed Central Ltd. 2013
Received: 17 June 2013
Accepted: 17 June 2013
Published: 28 June 2013
For the design and characterization of ultrasonic particle manipulation devices the pressure field in the fluid cavity is of great interest. The schlieren method provides an optical tool for the visualization of such pressure fields. Due to its purely optical nature this experimental method has got some unique advantages compared to methods like particle tracking.
A vertical schlieren setup and an investigation with the same of a mm-sized chamber used to agglomerate particles are presented here. The schlieren images show a two-dimensional representation of the whole pressure distribution recorded simultaneously with a good resolution in time. The gained description of the pressure field is shown to be in agreement with a numerical simulation. Thermal effects as well as streaming effects are shown.
The results show the great potential of schlieren visualization to investigate ultrasonic particle manipulation devices. Visualized are pressure fields, acoustic streaming, temperature effects and effects caused by fluid volumes of different density.
The schlieren method is a proven tool for the experimental characterization of optically transparent liquids and for the visualization of ultrasonic fields . This method makes visible spatial variations in the refractive index of the liquid. Thus any quantity related to the refractive index can be visualized, these are e.g. temperature variations or density variations caused by fluid flow or by acoustic waves. The method is similar to shadowgraphy and first known reports of a schlieren setup are from R. Hooke in the year 1665 for the visualization of thermal disturbances in air. The term schlieren goes back to A. Töpler who reported the first advanced schlieren setup in the year 1864. A more modern account on the schlieren technique in air is given by G.S. Settles . Reports suggest a large variety of different setups based on mirrors and transparent lenses including bidirectional setups with a mirror as background . Specialized or similar methods are e.g. the background oriented schlieren method , the colour schlieren method  or phase contrast and interferometry methods such as the differential interference contrast method.
Acoustic radiation forces  are exerted on particles in an ultrasonic standing pressure field. Particles which are denser and stiffer than the surrounding medium are driven towards the pressure nodes. These forces can be used for the transport and agglomeration of cells or micrometer-sized particles in suspension with a major area of application in life-sciences and biochemistry, such as drug screening or purification of nucleic acid solutions for sensitive molecular analysis [6, 7]. For ultrasonic particle manipulation devices, particle tracking  is also a very strong tool to characterize the pressure field or to visualize fluid flow. While ultimately both methods can be combined, the schlieren method offers some unique advantages. In contrast to particle tracking, its timescales are not limited by drag forces so it is much faster and, with the proper equipment, even able to measure traveling waves. In addition it is a non intrusive method, that is no seeding particles are required which might influence the fluid properties or the acoustic field. Other advantages of an optical method are that the complete area of interest can be imaged simultaneously and constantly even for fast frequency changes in real time while resolving complete pressure waves of a standing wave field. The resolution and frequencies of the ultrasonic field which can be imaged are mainly limited by the optical wavelength and the resolution of the optical components used. Reports show schlieren images of ultrasound in water of over 100 MHz . The structure containing the fluid, like any obstacle in the optical light path, causes interference fringes which can be a limiting factor for small devices or for imaging features close to a wall. The same problem can arise when using the schlieren method in combination with particles.
Method and setup
With a device that uses continuous frequency sweeping in combination with standing pressure fields, particles can be moved over larger distances using acoustic radiation forces . In macro scale chambers even a small change in frequency can change the pressure field significantly and thermal convection as well as acoustic streaming can be of significance, thus schlieren imaging is particularly suited to investigate such a device experimentally. The devices  investigated have a square fluid volume with a base of 21 x 21 mm and a height of 3 mm which is the same as the assumed acoustic beam diameter L. They are operated in the lower MHz frequency range. Assuming the optical axis to be parallel to the ultrasonic wave front and an optical wavelength λ of at least 700 nm the Klein-Cook parameter Q = 2 π λ L/Λ2 ≪ 1 and thus Raman-Nath diffraction  is expected. In other words, the acoustic beam diameter L or length that the optical waves cross the acoustic waves, is sufficiently small for the effect of multiple diffraction to be neglected. The devices are excited with a 1 mm thick piezo electric element, Pz26, which is either directly glued to one of the chamber walls or first glued to an aluminium waveguide and then clamped to the device. The excitation signal is a frequency sweep of 1.5 MHz to 2.5 MHz modulated with a saw-tooth frequency of 0.05 Hz. This type of excitation allows to move particles or cells from one side of the chamber to another or back by reversing the frequency range. The devices are built up with three layers, a PMMA frame with 0.5 mm thick walls and a planar top and bottom cover which are either 250 μ m PMMA foils or 0.5 mm glass slides. The glass is introduced to minimize disturbances in the optical path. A comparison of both cover materials did not show significant differences in the qualitative image of the acoustics.
Results and discussion
The schlieren images are shown as grain extract images. For images with a linear intensity range from 0 (black) to 1 (white) a grain extract image G is obtained with G = min(1, max(0,B−F+0.5)) where B is the background image and F the front image. The front image is the schlieren image of interest. The resulting grain extract images have a pressure node where the local intensity field is the darkest, for both knife edge filter and dark field filter. Areas within the fluid chamber without a standing pressure wave present, or with one of low amplitude show no local variation in the schlieren image. Figure 2 b) and c) show grain extract images where B is a schlieren image of the device without ultrasound and F a snap-shot image during sweeping at 2.105 MHz and 1.650 MHz obtained with the schlieren setup. For the image in b) a dark field filter has been used, revealing both the standing pressure field as well as changes in x- and y-direction. These changes appear to be in line with the numerical simulation, both in shape and response to small frequency changes. The schlieren image in c) is obtained with a knife edge filter in x-direction. The visualized nodal pressure planes show to be almost parallel with some disturbances around the four boundaries. The bright white stripe at the boundary along x = 0 mm close to the piezo element, as well as the other disturbances close to it are due to thermal gradients and convection caused by heat dissipation of the transducer. For prolonged experiments with high driving voltages and without cooling these thermal effects appear very clearly on the schlieren images.
The rather weakly developed schlieren image at the side boundaries (y = 0 mm and y = 21 mm) are in good agreement with the results from the numerical simulation as well as with the experiments with particles. They all indicate that close to these boundaries the pressure amplitudes are quite low. The schlieren images given here are from devices with 1 mm openings in two opposing corners (x,y) = (0,21) and (21,0) which will also have an effect on the pressure on the boundary. However experiments with closed cover slides as well as the numerical simulation which are both without these openings, indicate that the low pressure is only partly due to these openings. Some disturbances can also be caused by convection in the air, even though they should be small in a vertical setup where the airflow of rising air is parallel to the light path.
Conclusion and outlook
Schlieren visualization has proven to be a strong and simple tool to visualize standing pressure waves in ultrasonic micro manipulation devices. Both local and large scale pressure distributions can be imaged. Streaming effects as well as thermal effects can be studied at the same time as the pressure field. The method appears to be particularly suited to image changes over time while keeping the full resolution of the field. The quality of the imaging setup has the potential to be improved in order to record with higher sensitivity and homogeneity which can be achieved by introducing colour filters and light blocking filters of different shape  or with a more sophisticated lens setup and light source.
This work was supported by ETH Zurich.
- Ohno M, Tanaka N, Matsuzaki Y: Schlieren Imaging by the Interference of Two Beams in Raman-Nath Diffraction. Japanese J Appl Phys. 2003, 42 (Part 1, No. 5B): 3067-3071. 10.1143/JJAP.42.3067.View ArticleGoogle Scholar
- Settles G: Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transparent Media. Experimental Fluid Mechanics,. Berlin: Springer, 2001.View ArticleGoogle Scholar
- Speak G, Walters D: Optical considerations and limitations of the schlieren method. Br Aeronautical Res Counc. 1950,, (Reports and Memoranda 2859): 825–849Google Scholar
- Richard H, Raffel M: Principle and applications of the background oriented schlieren (BOS) method. Meas Sci Technol. 2001, 12 (9): 1576-1585. 10.1088/0957-0233/12/9/325.View ArticleGoogle Scholar
- Bruus H: Acoustofluidics 7: The acoustic radiation force on small particles. Lab On A chip. 2012, 12 (6): 1014-1021. 10.1039/c2lc21068a.View ArticleGoogle Scholar
- Evander M, Nilsson J: Acoustofluidics 20: applications in acoustic trapping. Lab On A chip. 2012, 12 (22): 4667-4676. 10.1039/c2lc40999b.View ArticleGoogle Scholar
- Yasuda K, Kiyama M, Umemura S, Takeda K: Deoxyribonucleic acid concentration using acoustic radiation force. J Acoust Soc Am. 1996, 99 (2): 1248-10.1121/1.414635.View ArticleGoogle Scholar
- Barnkob R, Augustsson P, Laurell T, Bruus H: Measuring the local pressure amplitude in microchannel acoustophoresis. Lab On A chip. 2010, 10 (5): 563-570. 10.1039/b920376a.View ArticleGoogle Scholar
- Zanelli CI, Howard SM: Schlieren metrology for high frequency medical ultrasound. Ultrasonics. 2006, 44 Suppl 1 (August): e105-e107.View ArticleGoogle Scholar
- Dual J, Hahn P, Leibacher I, Möller D, Schwarz T, Wang J: Acoustofluidics 19: Ultrasonic microrobotics in cavities: devices and numerical simulation. Lab On A chip. 2012, 12 (20): 4010-4021. 10.1039/c2lc40733g.View ArticleGoogle Scholar
- Möller D, Dual J: Flow-free transport of particles in a macro scale chamber. USWNet 2009 KTH-AlbaNova. 2009, Stockholm: USWNet, 42-42.Google Scholar
- Raman C, Nagendra Nathe N: The diffraction of light by high frequency sound waves: Part I. Proc Indian Acad Sci - Sect A. 1935, 2: 406-412.Google Scholar
- Settles G: Colour-coding schlieren techniques for the optical study of heat and fluid flow. Int J Heat Fluid Flow. 1985, 6: 3-15. 10.1016/0142-727X(85)90024-4.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.