This project aims to use optical coherence tomography (OCT) for velocity measurement in sub-millimetre flow channels. A major application for flow-measurement in this regime is in the design and assessment of microfluidic systems, which exemplify an increasing trend for miniaturization pervading many aspects of technology. In fluid-flow engineering, miniaturised instruments offer a reduced laboratory footprint, reduced requirements for energy and expensive or hazardous reagents and the ability to acquire data simultaneously from many systems within a single instrument. Microfluidics is a rapidly-growing field with benefits in healthcare (rapid, lab-on-a-chip, techniques for, e.g. immunoassays and biological phase separation), energy technology (membrane fuel cells) and high-throughput, portable systems for chemical analysis.
Currently, the main technique for investigation of micro-fluidic flows is micro particle image velocimetry, which uses high-resolution photography to determine positions of 'seed' particles within a plane illuminated by a thin sheet of laser light. Two images, acquired in rapid succession, allow the distance moved by particles between frames to be calculated, yielding velocity components in the image plane.
In very small ducts, a sufficiently thin light sheet cannot be generated. Illumination of a small volume, through a microscope arrangement, is usual, and the measurement plane thickness is defined using optics that exclude light from regions outside the focal plane. For the smallest channels, seed particles must be smaller than the illumination wavelength (approx. 1 micron). This causes difficulties, because light scattering decreases rapidly as particle diameter drops, and can fall so low that the signal-to-noise (SNR) is inadequate for acceptable images. Fluorescent particles and intensified cameras are then required, with filtering to separate the fluorescent signal from the background.
OCT is used mainly in medical environments, for detailed biological tissue imaging. However, its high spatial resolution, combined with particle-tracking techniques carried over from PIV, offer the possibility of 2- or 3-component velocity measurement in 2-D or 3-D regions, for flow velocities experienced in micro-fluidic systems. Micro-fluidic flow is not well described by classical flow theory, and experimental techniques are needed to validate models in designing micro-fluidic devices such as mixers, heat exchangers and fuel cells. It is important, for example, to eliminate 'dead zones' in the flow, and to understand the fluid motion in curved or bifurcated micro-channels. With appropriate processing, OCT can acquire images in three perpendicular planes with access from only one direction; a big advantage in micro-fluidics, when access is necessarily limited. High-resolution structural imaging of the channels is possible, alongside velocity measurement, which will help in detecting small variations or defects in wall structure that can have a large effect on flow.
OCT offers excellent optical sectioning capability, the image plane thickness being a few micrometres. Strong rejection of scattered light from outside the measurement region eliminates the need for fluorescent particles and eases near-wall measurements. The SNR of OCT is such that signals can be obtained from depths of hundreds of micrometres within turbid biological tissue, which suggests that flow measurements will be possible at higher seeding densities, or greater depths, than for comparable implementations of PIV. Typically, update rates for 2D OCT images are a few tens of Hz. Although micro-fluidic velocities are generally low, the update interval limits measurable velocities to a few mm/s. A shorter interval would be very advantageous in raising this limit. Multiplexing of images acquired from multiple illumination beams is proposed here, to reduce the inter-image interval and allow multiple image planes to be defined simultaneously
|