Manipulating Thermoacoustic Streaming : Measuring the Synergy of Laser-induced Temperature Gradients and Ultrasonic Pressure Fields in Acoustofluidic Microchannels

In Acoustofluidics, sound pressure waves can cause a force field on the liquid in a microfluidic channel. The forces depend on density and compressibility of the medium as well as the pressure and velocity amplitude of the sound. The forces lead to a steady flow in the bulk of the fluid, namely acoustic streaming. If a localized change in temperature introduces a gradient in compressibility and density, the force landscape changes accordingly. As a consequence, thermoacoustic streaming arises and replaces the acoustic streaming in velocity and shape. In the experiments for this thesis, a laser beam with a diameter of a third of the microchannel's width creates a localized temperature hot spot through absorption of the light, resulting in a temperature gradient.In the first study, the thermoacoustic streaming effects of such a gradient are classified in 3D. Firstly, the setup, that was built specifically for the experiments in this thesis, is introduced. The shape of the temperature gradient is varied through the amount of absorbed light across the channel height z and the position of the laser spot in x and y. A simulation is shown to support our understanding of the effect.In the second study, the time scales for the build-up and decay of the thermoacoustic streaming are investigated through imaging at a high frame rate and the temperature gradient is measured using a temperature sensitive fluorophore. A video is included, to demonstrate how toggling of the ultrasound with pre-established temperature gradient can deflect single particles from their original position in the channel, demonstrating the effects potential for sorting applications.For the third study, we introduce a substance called DMSO to the channel.Its compressibility changes with increasing temperatures such that a reversal of the thermoacoustic streaming takes place in the temperature range of 20-50°C. By controlling those quantities, compressibility and density, through the background temperature of the whole channel, the acoustic force landscape and the resulting thermoacoustic flow is manipulated. The temperature gradient originating from the laser remains the same and we show through varying concentrations of the substance and by changing the background temperature, that the acoustic body force landscape can be manipulated in a way that the direction of the streaming reverses. A video shows how the streaming slows down, changes direction and speeds up again, during a gradually increasing background temperature for unchanged laser- and ultrasound actuation. The aims of this thesis work are: to classify the newly found thermoacoustic streaming for a localized, laser-induced temperature gradient, to show how rapid the response of an acoustofluidic system is, to demonstrate how the phenomenon can be shaped and manipulated and to present an outlook on application possibilities for future studies.