Gerhard Lindner

Multi Reflection of Lamb Wave Emission in an Acoustic Waveguide Sensor

Recently, an acoustic waveguide sensor based on multiple mode conversion of surface acoustic waves at the solid—liquid interfaces has been introduced for the concentration measurement of binary and ternary mixtures, liquid level sensing, investigation of spatial inhomogenities or bubble detection. In this contribution the sound wave propagation within this acoustic waveguide sensor is visualized by Schlieren imaging for continuous and burst operation the first time. In the acoustic waveguide the antisymmetrical zero order Lamb wave mode is excited by a single phase transducer of 1 MHz on thin glass plates of 1 mm thickness. By contact to the investigated liquid Lamb waves propagating on the first plate emit pressure waves into the adjacent liquid, which excites Lamb waves on the second plate, what again causes pressure waves traveling inside the liquid back to the first plate and so on. The Schlieren images prove this multi reflection within the acoustic waveguide, which confirms former considerations and calculations based on the receiver signal. With this knowledge the sensor concepts with the acoustic waveguide sensor can be interpreted in a better manner.

Measurement of the emission of Lamb waves by a PVDF membrane hydrophone

The emission of sound waves resulting from mode conversion of Lamb waves propagating on thin glass plates submerged into water was investigated by a PVDF membrane hydrophone. The antisymmetrical zero order Lamb wave mode was excited by a piezoelectric single phase transducer operated at a frequency of 1 MHz. With the hydrophone the emission pressure distribution of the emitted sound waves in the liquid was recorded and the phase velocities of the Lamb waves were calculated from the direction of the emitted sound waves, i. e. the Lamb angles. Due to the dispersion of Lamb waves the velocity changes with the plate thickness. The measured phase velocities coincide well with the calculated values using the global matrix method for plate thicknesses between 0.5 and 4 mm.

Droplet propulsion on non-piezoelectric substrates induced by Lamb waves

Microliter droplet can be propelled on a non-piezoelectric substrate, on which antisymmetrical zero order Lamb waves with 1 MHz center frequency are propagating. Different droplet volume has different ratio of absorbed power, which is described by droplet propulsion distance/electrical input power. Moreover, with increasing droplet volume (10μl, 20μl, 50μl), the excitation frequency (1.03 MHz, 1.04 MHz, 1.06 MHz) must be slightly increased, for the optimization of the propulsion process.

On-line surveillance of lubricants in bearings by means of surface acoustic waves

The acoustic wave propagation in bearings filled with lubricants and driven by pulsed excitation of surface acoustic waves has been investigated with respect to the presence and the distribution of different lubricants. Experimental setups, which are based on the mode conversion between surface acoustic waves and compression waves at the interface between a solid substrate of the bearing and a lubricant are described. The results of preliminary measurements at linear friction bearings, rotation ball bearings and axial cylinder roller bearings are presented.

High-speed camera observation of multi-component droplet coagulation in an ultrasonic standing wave field

With an acoustic levitator small particles can be aggregated near the nodes of a standing pressure field. Furthermore it is possible to atomize liquids on a vibrating surface. We used a combination of both mechanisms and atomized several liquids simultaneously, consecutively and emulsified in the ultrasonic field. Using a high-speed camera we observed the coagulation of the spray droplets into single large levitated droplets resolved in space and time. In case of subsequent atomization of two components the spray droplets of the second component were deposited on the surface of the previously coagulated droplet of the first component without mixing.

Influence of surface acoustic waves induced acoustic streaming on the kinetics of electrochemical reactions

The kinetics of electrochemical reactions is controlled by diffusion processes of charge carriers across a boundary layer between the electrode and the electrolyte, which result in a shielding of the electric field inside the electrolyte and a concentration gradient across this boundary layer. In accumulators the diffusion rate determines the rather long time needed for charging, which is a major drawback for electric mobility. This diffusion boundary can be removed by acoustic streaming in the electrolyte induced by surface acoustic waves propagating of the electrode, which results in an increase of the charging current and thus in a reduction of the time needed for charging. For a quantitative study of the influence of acoustic streaming on the charge transport an electropolishing cell with vertically oriented copper electrodes and diluted H3PO4-Propanol electrolytes were used. Lamb waves with various excitation frequencies were exited on the anode with different piezoelectric transducers, which induced acoustic streaming in the overlaying electrolytic liquid. An increase of the polishing current of up to approximately 100 % has been obtained with such a set-up.

Monitoring of Soft Deposition Layers in Liquid-Filled Tubes with Guided Acoustic Waves Excited by Clamp-on Transducers

The monitoring of liquid-filled tubes with respect to the formation of soft deposition layers such as biofilms on the inner walls calls for non-invasive and long-term stable sensors, which can be attached to existing pipe structures. For this task a method is developed, which uses an ultrasonic clamp-on device. This method is based on the impact of such deposition layers on the propagation of circumferential guided waves on the pipe wall. Such waves are partly converted into longitudinal compressional waves in the liquid, which are back-converted to guided waves in a circular cross section of the pipe. Validating this approach, laboratory experiments with gelatin deposition layers on steel tubes exhibited a distinguishable sensitivity of both wave branches with respect to the thickness of such layers. This allows the monitoring of the layer growth.