David Newport

Transient natural convection in a conducting enclosure heated from above

Graphical Abstract

Development of interferometric temperature measurement procedures for microfluid flow

In order to understand heat transfer processes at the microscale, detailed temperature measurements are required. This article 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 article 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 microscale flow. Benchmark optical experiments quantifying the effect of noise on phase evaluation are described and the article concludes with an outline of the achievable resolution for a given channel length and fluid.

An optical counting technique with vertical hydrodynamic focusing for biological cells

A BARRIER IN SCALING LABORATORY PROCESSES INTO AUTOMATED MICROFLUIDIC DEVICES HAS BEEN THE TRANSFER OF LABORATORY BASED ASSAYS: Where engineering meets biological protocol. One basic requirement is to reliably and accurately know the distribution and number of biological cells being dispensed. In this study, a novel optical counting technique to efficiently quantify the number of cells flowing into a microtube is presented. REH, B-lymphoid precursor leukemia, are stained with a fluorescent dye and frames of moving cells are recorded using a charge coupled device (CCD) camera. The basic principle is to calculate the total fluorescence intensity of the image and to divide it by the average intensity of a single cell. This method allows counting the number of cells with an uncertainty +/-5%, which compares favorably to the standard biological methodology, based on the manual Trypan Blue assay, which is destructive to the cells and presents an uncertainty in the order of 20%. The use of a microdevice for vertical hydrodynamic focusing, which can reduce the background noise of out of focus cells by concentrating the cells in a thin layer, has further improved the technique. Computational fluid dynamics (CFD) simulation and confocal laser scanning microscopy images have shown an 82% reduction in the vertical displacement of the cells. For the flow rates imposed during this study, a throughput of 100-200 cellss is achieved.

Liquid Diffusion Measurement in Micro/Mini Channels From Full-Field Digital Phase Measurement Interferometry (PMI)

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.Copyright

Full-field low-frequency heterodyne interferometry using CMOS and CCD cameras with online phase processing

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 full field methods.

A heterodyne Mach-Zehnder Interferometer employing static and dynamic phase demodulation techniques for live-cell imaging

This paper describes a temporal carrier based Heterodyne Interferometer and associated phase demodulation techniques which are suitable for phase imaging of live cells. A Mach-Zehnder Interferometer is integrated to the microscope and two acousto-optic modulators are employed, to generate a temporal carrier that allows heterodyne approach to phase demodulation. Two demodulation schemes are presented: (a) Digital heterodyne phase extraction technique to extract the static phase information of the carrier signal, and (b) dynamic phase extraction technique for extracting phase variation in the carrier signal. The Heterodyne interferometer enables fast phase imaging and coupled with digital heterodyne phase extraction process, the system provides excellent temporal phase stability (standard deviation < 2 nm for 16 second measurement). This technique is employed for quantitative phase imaging of 3T3 fibroblast cells immersed in cell media. When there is phase variation, the temporal carrier signal is modulated and its instantaneous frequency is directly related to the variation. The dynamic phase extraction technique first determines the instantaneous frequency, which is then integrated with respect to time to obtain timevarying phase. The algorithm is able to extract a time varying phase, caused by a stimulated vibration at 30 Hz and 40 nm amplitude.

Free Convection Thermal Interaction Between 2D Components Mounted on a Vertically Oriented PCB

In this paper, measurements are presented of the temperature and velocity fields about two PCBs, with an array of five equally spaced two dimensional ribs. The ribs are two dimensional approximations of the Super Ball Grid Array (SuperBGA) package from Amkor electronics. The temperature and Nusselt number distributions are measured using Digital Moire Subtraction Interferometry and PIV is used to measure the velocity field. The effect of substrate conductivity is examined, and the level of thermal interaction is quantified. It is found that substrate conductivity significantly alters the induced boundary layer flow and also the recirculating vortex structure external to it. It is also found that there is a trade-off between a downstream component being heated by the thermal energy of the plume from a lower component, and cooled by the kinetic energy of that plume. The spacing to length ratio, above which the cooling effect is greater, is three for components mounted on a board with a high effective conductivity (15 W/m K). The ratio is greater than three for PCBs with lower effective conductivities. Previous work in the literature indicates a ratio greater than four for components mounted flush with an adiabatic substrate.Copyright

The advection of microparticles, MCF-7 and MDA-MB-231 breast cancer cells in response to very low Reynolds numbers

The lymphatic system is an extensive vascular network that serves as the primary route for the metastatic spread of breast cancer cells (BCCs). The dynamics by which BCCs travel in the lymphatics to distant sites, and eventually establish metastatic tumors, remain poorly understood. Particle tracking techniques were employed to analyze the behavior of MCF-7 and MDA-MB-231 BCCs which were exposed to lymphatic flow conditions in a 100 μm square microchannel. The behavior of the BCCs was compared to rigid particles of various diameters (η = dp/H= 0.05-0.32) that have been used to simulate cell flow in lymph. Parabolic velocity profiles were recorded for all particle sizes. All particles were found to lag the fluid velocity, the larger the particle the slower its velocity relative to the local flow (5%-15% velocity lag recorded). A distinct difference between the behavior of BCCs and particles was recorded. The BCCs travelled approximately 40% slower than the undisturbed flow, indicating that morphology and size affects their response to lymphatic flow conditions (Re < 1). BCCs adhered together, forming aggregates whose behavior was irregular. At lymphatic flow rates, MCF-7s were distributed uniformly across the channel in comparison to the MDA-MB-231 cells which travelled in the central region (88% of cells found within 0.35 ≤ W ≤ 0.64), indicating that metastatic MDA-MB-231 cells are subjected to a lower range of shear stresses in vivo. This suggests that both size and deformability need to be considered when modelling BCC behavior in the lymphatics. This finding will inform the development of in vitro lymphatic flow and metastasis models.

Development of Compact Thermal–Fluid Models at the Electronic Equipment Level

The introduction of compact thermal models (CTM) into computational fluid dynamics (CFD) codes has significantly reduced computational requirements when representing complex, multilayered, and orthotropic heat generating electronic components in the design of electronic equipment. This study develops a novel procedure for generating compact thermal–fluid models (CTFM) of electronic equipment that are independent over a boundary condition set. This boundary condition set is estimated based on the information received at the preliminary design stages of a product. In this procedure, CFD has been used to generate a detailed model of the electronic equipment. Compact models have been constructed using a network approach, where thermal and pressure-flow characteristics of the system are represented by simplified thermal and fluid paths. Data from CFD solutions are reduced for the compact model and coupled with an optimization of an objective function to minimize discrepancies between detailed and compact solutions. In turn, an accurate prediction tool is created that is a fraction of the computational demand of detailed simulations. A method to successively integrate multiple scales of electronics into an accurate compact model that can predict junction temperatures within 10% of a detailed solution has been demonstrated. It was determined that CTFM nodal temperatures could predict the corresponding area averaged temperatures from the detailed CFD model with acceptable accuracy over the intended boundary condition range. The approach presented has the potential to reduce CFD requirements for multiscale electronic systems and also has the ability to integrate experimental data in the latter product design stages. [DOI: 10.1115/1.4006715]

Thermal Performance Characteristics of Integrated Cooling Solutions Consisting of Multiple Miniature Fans

Thermal performance characteristics are assessed for multiple miniature axial fans of 24.6 mm diameter that provide impingement cooling on a finned surface. Combined experimental and numerical analyses indicate that fans positioned adjacently in an array can influence heat transfer performance both positively and negatively by up to 35% compared to an equivalent single fan – heat sink unit operating standalone. However the level of thermal performance reductions, coupled with greater geometrical flexibility, makes the design approach a viable alternative to current single fan – heat sink units. Experimental measurements also suggest that for a fixed spacing, fan operating point is a sensitive criterion for ensuring optimal thermal performance over an equivalent single fan unit. Numerical simulations, modelled using experimental inputs, have provided an insight into the flow fields produced by the interaction between adjacent fans and the finned geometry. Fluid recirculation occurs beneath the fan hub of the centrally located fan in the array, with the adjacent fans on the periphery experiencing cross flow in the hub region. A novel experimental approach utilising infrared thermography has been developed to assess the validity of the numerical model. Indeed, the previously stated flow features were confirmed using this assessment, while limitations in the modelling assumptions have been outlined. Overall, the results provide recommendations in the design of fan cooled heat sinks utilising multiple axial fans for jet impingement and an understanding of the flow physics which occur within this compact cooling solution design.