| Description |
Measurement of pressure and velocity field is vital for characterization of any flow field. While particle image velocimetry (PIV) is the gold standard for flow field velocity measurement, there is not an equally comprehensive technique to measure dynamic pressure. In this dissertation, we introduce a new pressure mapping methodology termed "particle imaging manometry". This method utilizes engineered microscopic pressure-sensitive particles seeded into the flows. These microscopic sensors can be optically interrogated nonintrusively to determine local pressure readings at many points within the flow. This dissertation describes several types of sensor microparticles of varying geometries and fabrication complexity. These include planar, pyramidal, and spherical microparticles batch fabricated by our research group using microfabrication technology, but these particles exhibit orientation-dependent readout. This dissertation is focused on spherical pressure-sensing microparticles using microfabrication techniques representing my contributions to this research. Spherical sensing particles offer orientation-independent pressure readouts. In this work, we introduce an innovative method for the construction of spherical microparticles using isotropic etching and micromolding. These microparticles consist of hollow spherical parylene-C shells that behave as compressible microballoons. The flexible shell radius thus expands or contracts in response to ambient pressure variations, and the radial change is detected optically. We also introduce two optical methods for the measurement of radius change in iv microballoons: 1) the spectral method where every sensor is treated as a Fabry-Perot optical resonator and the change in the particle spectral reflectance minima is representative of the radius change, and 2) the scattering ring method where a monochromatic beam of light incident on the sensing particle projects a forward scattered light pattern with interference rings. The spacing between the intensity lobes on the scattering ring can be used to determine the particle size. The leakage problem through entirely polymeric shells is also resolved by the incorporation of a thin metal oxide diffusion barrier layer that increases total equilibration time by 50 times. Our measurement methods are suitable for particles that are immobilized in the flow as the inmotion measurement requires the utilization of ultrafast hyperspectral imaging deemed beyond the scope of this dissertation. Fundamentally, after seeding, these particles remain at stationary locations. The immobilization of these particles in the domain of interest is itself an interesting research problem. In Chapter 6, we developed magnetic versions of these spherical particles that permit the immobilization at externally controlled locations. This was accomplished by the introduction of magnetic an Au-Ni-Au metal sandwich layer attached to the parylene support stems. By the application of an external magnetic field, the particle sensors can be immobilized at any location of interest to measure the pressure at that point. This novel pressure mapping technique offers a method that is independent of illumination wavelength, time and ambient condition, composition of medium, and chip design. Hence, this is a stronger and more comprehensive method. The key points of innovation are perfect sphere fabrication by standard MEMS technique, instrumentation and process developed for in-flow particle-radius detection, and pressure mapping capability in dynamic fluidic flow. |
