Research on transient heat transfer, ultra rapid bubble formation and collapse on a small surface is important for removal of high heat flux from microelectronic chips, microelectrical mechanical systems (MEMS) and MicroOptoElectroMechanical systems (MOEMS). Furthermore, total glass microdevices have attracted wide interest in the development of micro total analysis systems (μTAS), laser microsurgery, micro heat pipes and microfluidics researches because of the possibility for optical diagnostics and observations. However, when it comes to microscale, it also calls for different experimental methodology, which is far from mature. Due to lack of measurement techniques and experimental data, ultra rapid heat transfer and bubble formation processes are not well understood. Therefore it is...[ Read more ]

Research on transient heat transfer, ultra rapid bubble formation and collapse on a small surface is important for removal of high heat flux from microelectronic chips, microelectrical mechanical systems (MEMS) and MicroOptoElectroMechanical systems (MOEMS). Furthermore, total glass microdevices have attracted wide interest in the development of micro total analysis systems (μTAS), laser microsurgery, micro heat pipes and microfluidics researches because of the possibility for optical diagnostics and observations. However, when it comes to microscale, it also calls for different experimental methodology, which is far from mature. Due to lack of measurement techniques and experimental data, ultra rapid heat transfer and bubble formation processes are not well understood. Therefore it is crucial to find a better experimental approach (Including micro device fabrication, temperature and velocity field measurement) to obtain more precise measurements thus better understanding in this field. In this thesis, transient bubble nucleation and dynamics of liquid on a mini/micro polysilicon flat surface and in microchannels having different dimensions are investigated. To achieve ultrafast heating and boiling process on a mini/micro surface, an Nd YAG pulse laser with several nanoseconds pulse duration is utilized. Micro-machined temperature sensors and non-intrusive temperature sensing techniques have been developed to measure the transient interface temperature changes. The flow fields and the motion of liquid-gas interface during the bubble formation process are studied by a recently improved micro resolution particle image velocimetry (μPIV) system and a newly developed optical diagnostic method.

To study microscale ultrafast laser-liquid interaction and consequent flow/heat transfer problem, a transparent microchannel integrated with temperature sensors is necessary. The integration of a detection system, such as integrated sensors into a glass microchannel, however, was difficult due to technology constraints. A microfabrication process integrating a micromachined fast response temperature sensor array inside a glass microchannel was developed with a novel MEMS technology. This fabrication is made possible by combining newly developed anodic bonding of two different glass materials, channel etching and fabrication of silicon temperature detectors. The implantation process and the channel etching technology were ameliorated to adapt to the new material introduced. This is the very first time anodic bonding is realized between two different glass materials with micro sensors sandwiched in the channel. The rigid structure, total transparency and easiness for sensor integration make it overwhelm other microchannel design in microfluidics. The dynamic response of the integrated temperature array has also been calibrated. The response time of the integrated temperature sensor array was examined to be 1.5 μs. Success of this device fabrication made possible optical investigation of laser heating and upcoming PIV measurement of flow around bubble in channel.

Considering that the available temperature measurement method cannot meet the requirement of ultrafast process at time order smaller than μs, some new detecting methodologies were attempted. A 2D fringe probing transient temperature measurement technique based on photothermal deflection (PTD) theory was developed. It utilizes material’s refractive index dependence on temperature gradient to obtain temperature information from laser deflection. Instead of single beam, this method applies multiple laser beams to obtain 2D temperature information. The laser fringe was generated with a Mach-zehnder interferometer. A transient heating experiment was conducted using an electric wire to demonstrate this technique. Temperature field around a heating wire and variation with time was obtained utilizing the scattering fringe patterns. This technique provides a non-invasive 2D temperature measurement with spatial and temporal resolutions of 3.5 μm and 4 ms, respectively. It is possible to achieve temporal resolution to 500 μs utilizing the existing high speed camera.

Micro temperature sensor and LIF temperature imaging combined with high speed camera were used to study the heating process of DI water in microchannel with a high intensity pulsed laser. At fixed laser fluence, the complete physical process including bubble dynamics and heat transfer is detected with different methods at different time scales. Channel wall is found to have significant effect on bubble nucleation and rebounding of bubble at 10 μs time scale is observed from both bubble dynamics and channel wall temperature results.

Gas bubble residing on channel wall is a consequence of bubble nucleation and also common under microscale. In the last part of this study, flow around bubble sticking to channel wall was investigated with Shadowgraphy Micro PIV and unusual velocity distribution around bubble was dig out from PIV data. The fluctuation of velocity field and shear stress around bubble indicates that the slip coefficient along bubble boundary is non-consistent. This phenomenon is detected for the first time and it needs to be further addressed.