Microfluidics is a field of research that experienced a strong increase in popularity over the past two decades. In addition to applications in biotechnology and the chemical industry, a main focus of microfluidic developments is the miniaturization of medical devices, with the aim to carry out examinations and diagnoses faster, more cost-effectively, and directly on the patient in order to make quick decisions for further treatment. The miniaturization of laboratory diagnostics to the size of microchips has been summarized in recent years with the term “lab-on-a-chip”. However, the development of miniaturized laboratory components also involves numerous new challenges. For instance, the systematic manipulation of particles and cells on microscopic scales and the efficient mixing of small amounts of liquid on small scales are difficult to implement. The desired goals can be achieved by using special actuators in microchannels. In the present work, this is done by resonant ultrasonic actuation of bubbles. This induces a secondary streaming flow which is to be used for the applications mentioned above. The bubbles are introduced into a transparent microchannel and the generated flow is investigated three-dimensionally by Particle Tracking Velocimetry (PTV). In order to ensure precise measurements, it is crucial that the density of the tracer particles is adapted to the density of the liquid in which they are dissolved. A chapter of this work is, therefore, concerned with the preparation of suitable particle solutions. Another challenge in measurement technology is the large velocity dynamics of the generated flow. In order to achieve meaningful measurements, existing PTV measurement methods have been improved so that they can also be used at frame rates of over 100,000 frames per second. It is particularly important to ensure the reproducibility of the bubble flow for use in lab-on-a-chip applications. Here, the time-dependent change of the bubble size, which has been observed by many research groups and not yet explained, is particularly problematic. This question is answered in this work. On this basis, a system is presented that enables automated stabilization of bubbles in microchannels. With the aid of improved measurement methods and bubble stabilization, the generated bubble streaming flow is investigated three-dimensionally. The behavior of particles of different sizes is then analyzed. The aim of these investigations is to explain the frequent observation that large particles are trapped and retained by the bubble. With such “acoustic tweezers”, particles can not only be trapped but also exposed to a shear flow. Several physical mechanisms can be responsible for these effects, ranging from acoustic radiation to particle inertia. In this work, it is shown that particles in the bubble streaming flow are already trapped by taking into account only the steric interaction with the bubble and the channel walls. In the last chapter of this thesis, a concept is presented which allows an efficient mixing of liquids on microscopic scales. Due to the typical laminar flows in microfluidics, this is a special challenge. For this purpose, several bubbles of the same size are introduced into a microchannel in order to evoke the folding and stretching of two liquids by the bubble streaming flow. This enlarges their mutual interface in order to speed up the natural diffusion process. With the help of three-dimensional PTV measurement methods, an insight into three-dimensional mixing processes of such an arrangement can be obtained. The present work thus contributes to making acoustically excited bubble flows reproducible. They can be used as actuators for microfluidic applications in order to enable and further develop applications in biotechnology and medical technology.    «

Microfluidics is a field of research that experienced a strong increase in popularity over the past two decades. In addition to applications in biotechnology and the chemical industry, a main focus of microfluidic developments is the miniaturization of medical devices, with the aim to carry out examinations and diagnoses faster, more cost-effectively, and directly on the patient in order to make quick decisions for further treatment. The miniaturization of laboratory diagnostics to the size of m...    »