Yitshak Zohar

Forced convection boiling in a microchannel heat sink

Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 /spl mu/m at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves.

Surface-chemistry technology for microfluidics

A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.

Phase change in microchannel heat sinks with integrated temperature sensors

A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel sizes and liquid flow-rates. The results suggest that the bubble dynamic mechanism in the microchannel might be different compared with conventional channels.

Subsonic gas flow in a straight and uniform microchannel

A nonlinear equation based on the hydrodynamic equations is solved analytically using perturbation expansions to calculate the flow field of a steady isothermal, compressible and laminar gas flow in either a circular or a planar microchannel. The solution takes into account slip-flow effects explicitly by utilizing the classical velocity-slip boundary condition, assuming the gas properties are known. Consistent expansions provide not only the cross-stream but also the streamwise evolution of the various flow parameters of interest, such as pressure, density and Mach number. The slip-flow effect enters the solution explicitly as a zero-order correction comparable to, though smaller than, the compressible effect. The theoretical calculations are verified in an experimental study of pressure-driven gas flow in a long microchannel of sub-micron height. Standard micromachining techniques were utilized to fabricate the microchannel, with integral pressure microsensors based on the piezoresistivity principle of operation. The integrated microsystem allows accurate measurements of mass flow rates and pressure distributions along the microchannel. Nitrogen, helium and argon were used as the working fluids forced through the microchannel. The experimental results support the theoretical calculations in finding that acceleration and non-parabolic velocity profile effects were found to be negligible. A detailed error analysis is also carried out in an attempt to expose the challenges in conducting accurate measurements in microsystems.

Measurements of residual stresses in thin films using micro-rotating-structures

Abstract In the present study, micro-rotating-structures for local measurements of residual stresses in a thin film were simulated by the finite element method (FEM). A sensitivity factor – the ratio of the deflection of the micro-structure to the normalized residual stress is introduced and tabulated from the FEM results. Thereafter, a formula to calculate the residual stress is given so that the residual stress can be easily evaluated from the deflection of the rotating beam. A variety of optimized micro-rotating-structures were then designed and fabricated to verify the FEM results. Residual stresses in both silicon nitride and polysilicon thin films were determined by this technique and compared with measurements by the wafer-curvature method. The two methods lead to comparable results. In addition, the micro-rotating-structures have the ability to measure spatially and locally a large range of residual tensile or compressive stresses.

Rapid thermal annealing of polysilicon thin films

In comparison with conventional heat treatment, high-temperature rapid thermal annealing (RTA) in a radio frequency (RF) induction-heated system can reduce or eliminate residual stresses in thin films in a few seconds. In this work, changes in the stress level due to the RTA of polycrystalline silicon thin films were studied as a function of annealing time and temperature. The corresponding variations in the microstructure and surface layer of the thin films were experimentally investigated by a variety of analytical tools. The results suggest that the residual stress evolution during annealing is dominated by two mechanisms: 1) microstructure variations of the polysilicon thin film and 2) effects of a surface layer formed during the heat treatment. The fact that the microstructure changes are more pronounced in samples after conventional heat treatment implies that the effects of the formed surface layer may dominate the final state of the residual stress in the thin film.

Size and shape effects on two-phase flow patterns in microchannel forced convection boiling

An integrated microchannel heat sink consisting of shallow, nearly rectangular microchannels has been fabricated using standard micromachining techniques to highlight the effects of the micrometer sized channel shape on the evolving flow patterns and, consequently, on the thermal performance of the microsystem. An integrated heater serves as a local heat source, while an array of micro thermistors is used for temperature distribution measurements. The working fluid, DI water, is pressurized through the microchannels for forced convection heat transfer studies. Boiling curves for different flow rates have been recorded and analyzed based on the visualized flow patterns. Local nucleation, including bubble formation and bubble dynamics, is documented and found to be negligible. Although detected, in contrast with triangular microchannels, annular flow is observed to be unstable. Instead, the dominant flow pattern is an unsteady transition region connecting an upstream vapor zone to a downstream liquid zone with an average location depending on the input power. A physical mechanism based on the force balance across the vapor–liquid interface, and the development of a restoring force, is proposed to explain the flow visualization results.

Detachment of captured cancer cells under flow acceleration in a bio-functionalized microchannel

Attachment, deformation and detachment of N-cadherin expressing prostate and breast cancer cell lines in a functionalized microchannel under hydrodynamic loading have been studied. N-cadherin antibodies are immobilized on the microchannel surface to capture the target cancer cells, PC3N and MDA-MB-231-N, from a homogeneous cell suspension. Although difficult, a significant fraction of moving cells can be captured under a low flow rate. More than 90% of the target cells are captured after a certain incubation time under no flow condition. The mechanical response of a captured cancer cell to hydrodynamic flow field is investigated and, in particular, the effect of flow acceleration is examined. The observed cell deformation is dramatic under low acceleration, but is negligible under high acceleration. Consequently, the detachment of captured cells depends on both flow rate and flow acceleration. The flow rate required for cell detachment is a random variable that can be described by a log-normal distribution. Two flow acceleration limits have been identified for proper scaling of the flow rate required to detach captured cells. A time constant for the mechanical response of a captured cell, on the order of 1 min, has been identified for scaling the flow acceleration. Based on these acceleration limits and time constant, an exponential-like empirical model is proposed to predict the flow rate required for cell detachment as a function of flow acceleration.

A high-performance microsystem for isolating circulating tumor cells

A unique flow field pattern in a bio-functional microchannel is utilized to significantly enhance the performance of a microsystem developed for selectively isolating circulating tumor cells from cell suspensions. For high performance of such systems, disposal of maximum non-target species is just as important as retention of maximum target species; unfortunately, most studies ignore or fail to report this aspect. Therefore, sensitivity and specificity are introduced as quantitative criteria to evaluate the system performance enabling a direct comparison among systems employing different techniques. The newly proposed fluidic scheme combines a slow flow field, for maximum target-cell attachment, followed by a faster flow field, for maximum detachment of non-target cells. Suspensions of homogeneous or binary mixtures of circulating breast tumor cells, with varying relative concentrations, were driven through antibody-functionalized microchannels. Either EpCAM or cadherin-11 transmembrane receptors were targeted to selectively capture target cells from the suspensions. Cadherin-11-expressing MDA-MB-231 cancer cells were used as target cells, while BT-20 cells were used as non-target cells as they do not express cadherin-11. The attachment and detachment of these two cell lines are characterized, and a two-step attachment/detachment flow field pattern is implemented to enhance the system performance in capturing target cells from binary mixtures. While the system sensitivity remains high, above 0.95, the specificity increases from about 0.85 to 0.95 solely due to the second detachment step even for a 1 : 1000 relative concentration of the target cells.

High-throughput design and fabrication of an integrated microsystem with high aspect-ratio sub-micron pillar arrays for free-solution micro capillary electrophoresis

A new technology approach for the design, fabrication and application of an integrated free-solution capillary electrophoresis microsystem is presented. Combining the advantages of projection, contact photolithography and deep-reactive-ion-etching, this approach allows fast and flexible formation of micron-sized channels integrated with extremely high aspect-ratio (>50:1) sub-micron pillar arrays on a silicon substrate. Utilizing fluorescence video microscopy, free-solution DNA separation has been demonstrated. Furthermore, the detailed DNA molecular interaction with the pillars inside the microsystem can be analysed. In comparison with the previously reported fabrication technologies, such as electron beam lithography, the newly presented technology approach offers a significant improvement in fabrication time and design flexibility; both are highly desirable not only for potential commercialization of the free-solution electrophoresis microsystem in applications such as lab-on-a-chip but also for systematic studies of micro-scale DNA kinetics.