Stroboscopic Imaging Interferometry for visualizing ultrasonic waves

MSc. project Stroboscopic Imaging Interferometry for visualizing ultrasonic waves

Ultrasound (frequency of 20 kHz - 1 GHz) has many technological applications, from medical (making kidney-stones explode or looking at human embryos) to various lab-on-a-chip usages for particle manipulation. At Physics of Complex Fluids, we use ultrasonic standing wave patterns in micro-flowchannels to concentrate dangerous bacteria and spores from drinking water.

Video: Slow motion movie of polystyrene particles in water, concentrated by ultrasonic standing waves in a microchannel. First, the field at the fundamental resonance (n = 1) is switched on, causing microbeads to be concentrated in the node exactly in the middle of the channel. Later, the movement of the beads in the first harmonic (n = 2) mode can be observed. Both the wavy path and the trapping of the particles are caused by interference from higher order longitudinal modes along the direction of the flowchannel, as seen in the figure below.

Figure: An example of a higher order longitudinal mode (n = 16) of the ultrasonic pressure field on a substrate containing a flowchannel [1]. Longitudinal modes are basically vibration modes of the substrate and are unavoidable when working with non-infinitely big structures. The interference of all modes give rise to a complex pressure field inside the flowchannel and influences the motion of suspended particles.

State-of-the-art methods to measure the pressure field are based on observing the motion of suspended particles by using advanced Particle Tracking methods like μPIV. However, for small particles (<1 μm) or at low powers, other forces than the acoustic force dominate, making it impossible to resolve the complex pressure field.

A direct measurement of the field is possible by looking at the change of index of refraction of the water, as function of pressure. This change dn/dP is very small, approximately 1.25 · 10-10 Pa-1, but since the instantaneous pressure of the ultrasonic field can be on the order of megaPascals, the effect can still be measured. Depending on the pressure, light going through the anti-node of a standing pressure wave acquires a relative phase of 1,5 to 60 degrees (or an optical path difference of only 3 to 100 nm!) compared light going through the node.

By combining an imaging Michelson interferometer with an stroboscopic coherent light source (<10 ns pulses) and using a smart timing scheme, it should be possible to map the phase differences over a region of the flowchannel. This way the pressure field can be directly visualized but also quantitatively measured as function of space and time. This has not been done for these kind of systems and if experimental proof of principle is obtained the project will result in a paper.

The ideal candidate for this project likes designing and building advanced optical setups and has affinity with sensitive detection methods. Knowledge of the Fourier transform and interferometry in general is a plus. In addition to working on a fun project, you also get to join the most fun optical group at UT!

Contact Jorick van ‘t Oever in CR 4.542 or by email:

[1] R. Barnkob et al., Lab Chip, vol. 10, no. 5, pp. 563–570, Mar. 2010