This paper presents micro-interferometry as a measurement technique to extract temperature profiles and/ or mass transfer gradients rapidly and locally in microdevices. Interferometry quantifies the phase change between two or more coherent light beams induced by temperature and/or mass concentration. Previous work has shown that temporal noise is a limiting factor in microscale applications. This paper examines phase stepping and heterodyne phase retrieval techniques with both CCD and CMOS cameras. CMOS cameras are examined owing to the high speed at which images can be acquired which is particularly relevant to heterodyne methods. It is found that heterodyne retrieval is five times better than phase stepping being limited to 0.01 rad or k/628. This is twice the theoretical limit of k/1,000. The technique is demonstrated for mixing in a T-junction with a 500 lm square channel and compared favourably to a theoretical prediction from the literature. Further issues regarding application to temperature measurements are discussed.

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In order to understand heat transfer processes at the microscale, detailed temperature measurements are required. This paper begins with a review of the current state-of-the art in fluid temperature measurement at the microscale. At present, fluid temperature profiles are not measured, with verification of predicted heat transfer performance being based on global measurements. The paper describes a potential full-field technique based on micro-interferometry. The accuracy of extracting temperature data from small phase difference intensity maps is discussed, with particular reference to the high levels of signal to noise as would be found in a micro-scale flow. Benchmark optical experiments quantifying the effect of noise on phase evaluation are described and the paper concludes with an outline of the achievable resolution for a given channel length and fluid.

This paper considers division of amplitude interferometry as a means to extract fluid information from micro-systems. Initially the phase measurement technique is analysed and the measurement limitations of mixing measurement are assessed. Accurate phase measurements are then made of the concentration in a 3 dimensional channel flow. A mini sized channel with tow fluid flows at Reynolds numbers of 0.848 and 0.0848 is numerically analysed. The same channel is experimentally tested and the results for the mixing concentration gradients in channel flow are compared with those obtained numerically. The requirement for experimental measurement for accurate measurement of binary liquid diffusion is observed by the variation between experimental and numerical results. The diffusion coefficient measurement verifies PMI as a means of mixture measurement, or more broadly as a phase measurement technique for small-scale, or micro scale, fluidic analysis. PMI’s potential is finally discussed as a measurement technique for concentration, and hence fluidic analysis of micro channel mixing.

Most full-field heterodyne interferometry systems are based on complex electro-mechanical scanning devices. In this study, however, we present an alternative non-scanning approach based on a low frequency heterodyne interferometer employing standard CCD and CMOS cameras. Two frequency locked acousto-optical devices were used to obtain two laser beams with an optical frequency difference as low as 3 Hz. The interference of those beams generated a suitably low frequency carrier signal that allowed the use of a common 25 frame/second CCD camera. Using a digital CMOS camera and acquiring a limited number of randomly accessible pixels, measurements with much higher carrier frequencies were also possible. The advantages of the heterodyne technique with respect to common phase-stepping methods are the shorter response time and lower sensitivity to sources of uncertainty such as drift, vibrations and random electronic noises. In order to directly compare the heterodyne and phase-stepping techniques experimentally, the same interferometer was used for both methods. The switching between operation modes was achieved by simply altering the electronic driving signals of the acousto-optical devices where for the phase-stepping mode, the frequency difference of the driving signals was set to zero. The phase steps were obtained by a piezo-driven mirror. Comparing the phase difference between two pixels in an image, approximately 0.01 radian of standard deviation, corresponding to a resolution of λ /628, was achieved by heterodyne technique, as compared to 0.06 radian by the phase-stepping method. The interferometer with the CMOS camera was applied to monitor the refractive index variation across a micro-channel where two liquid flows were mixed. Also, the capability for fast, time-resolved full-field optical refractive index measurements was demonstrated. The examples presented show how the high sensitivity of the heterodyne technique allows the study of a number of sources of uncertainty that were not otherwise easily quantifiable using standard fullfield methods.The development of the acousto-optic modulators allows simple realisation of two laser beams with low optical frequency difference that can be recombined for heterodyne interferometric measurements. In the case of laser sources with long coherence length, the intensity signal is equal to that from classical interferometers with a constant velocity moving mirror in the reference arm with obviously not mechanical limitations. The equivalent moving mirror has very smooth displacement without vibrations and limitations on the velocity, vibrations and total displacement. Usual phase stepping methods are sensitive to the temperature drifts during the total stepping time and to the vibrations. The heterodyne systems are not sensitive to the vibrations and drifts provided the resulting Doppler shift remains smaller than the carrier frequency. Moreover, the high number of data acquired in heterodyne technique, provide more efficient averaging of the random noises. Hete...

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This paper describes the construction and performance of an Electronic Speckle Pattern Interferometer (ESPI) developed for measurements of concentration gradients in a binary mixture. The system uses a Mach-Zehnder interferometer with a commercially available CCD camera for image acquisition. A phase-shifting algorithm is employed to give full field measurements. The theoretical background to the optical process involved in these methods is also presented, with emphasis on using the system for analysing concentration gradients. Problems pertaining to the unknown Gladstone-Dale constants of a binary mixture are also discussed.