T. J. John

Fabrication and testing of a CoNiCu/Cu CPP-GMR nanowire-based microfluidic biosensor

Giant magneto resistance (GMR)-based microfluidic biosensors are used in applications involving the detection, analysis, enumeration and characterization of magnetic nano-particles attached to biological mediums such as antibodies and DNA. Here we introduce a novel multilayered CoNiCu/Cu nanowire GMR-based microfluidic biosensor. The current perpendicular to the plane of multilayers (CPP)-nanowires GMR was used as the core sensing material in the biosensor which responds to magnetic fields depending on the concentration and the flow velocity of bio-nano-magnetic fluids. The device was tested with different control solutions such as DI-water, mineral oil, phosphate buffered saline (PBS), ferrofluid, polystyrene superparamagnetic beads (PSB) and Dynabeads sheep anti-rabbit IgG. The nanowire array resistance decreased with an increase in the ferrofluid concentration, and a maximum 15.8% relative GMR was observed for the undiluted ferrofluid. The sensor was also responding differently to various ferrofluid flow rates. The GMR device showed variation in the output signal when the PSB and Dynabeads of different dilutions were pumped through it. When the tests were performed with pulsing potentials (150 mV and 200 mV), an increased GMR response was identified at higher voltages for PSB and Dynabeads sheep anti-rabbit IgG.

Characteristic Study on the Optimization of Pin-Fin Micro Heat Sink

The need for dissipating heat from microsystems has increased drastically in the last decade. Several methods of heat dissipation using air and liquids have been proposed by many studies, and pin-fin micro heat sinks are one among them. Researchers have developed several effective pin-fin structures for use in heat sinks, but not much effort has been taken towards the optimization of profile and dimensions of the pin-fin. In this paper the authors studied the effect of different pin-fin shapes on the thermal resistance and pressure drop in a specific micro heat-sink. Optimization subjected to two different constraints is studied in this paper. The first optimization is subjected to constant flow rate and the second one is subjected to constant pressure drop. Both optimization processes are carried out using computer simulations generated using COVENTORWARE™. Two of the best structures from each of these optimization studies are selected and further analysis is performed for optimizing their structure dimensions such as width, height and length. A section of the total micro heat-sink is modeled for the initial optimization of the pin-fin shape. The model consists of two sections, the substrate and the fluid. Six different shapes: square, circle, rectangle, triangle, oval and rhombus were analyzed in the initial optimization study. Preliminary tests were conducted using the first model described above for a flow rate of 0.6ml/min. The non dimensional overall thermal resistance of the heat sink, and the nondimensional pumping power was calculated from the results. A figure of merit (FOM) was developed using the nondimensional thermal resistance and nondimensional pumping power for each structure with different pin-fin shapes. Smaller the value of FOM better the performance of the heat sink. The study revealed that the circle and ellipse structures have the best performance and the rectangle structure had the worst performance at low flow rates. At high flow rates rectangular and square structures have the best performance.© 2009 ASME

Characteristic Study on the Optimization of Micro PinFin Heat Sink with Staggered Arrangement

The effect of the Pinfin shapes on the overall per formance of the micro Pinfin heat sink with staggered arrangement is studied in this paper. Six different shapes of micro Pinfins, square, rectangle, circle, rhombus, triangle and ellipse are subjected to study in this paper. The optimization processes are carried out using computer simulations performed using COVE*TORWARE™. The study is carried out over a range of Reynolds number ranging from 50 to 500 and a figure of merit term (FOM) consisting of both the thermal resistance and the pumping power is developed for the overall performance evaluation of different models. A weighted average scheme is used for developing the FOM term. The results of the study revealed that at low values of Reynolds numbers (Re 200) rectangle showed the best performance. *omenclature C = specific heat capacity (J/kg K) h = total height of the heat sink (m) k = thermal conductivity (W/K m) L = length of the heat sink (m) n = constant P = pressure (N/m 2 )

Microfabrication of nanowires-based GMR biosensor

This study focuses on the development of current-perpendicular-to plane (CPP) Giant Magnetoresistance (GMR) of CoNiCu/Cu multilayered nanowire based microfluidic sensors for the detection of magnetic nanoparticles and fluids. The visible measurable variations in electrical voltage due to changes in external magnetic field are later to be monitored in microfluidic biosensor for the detection of toxicants in cells. An early prototype device was fabricated and tested using both an aqueous nonmagnetic medium (water) and a commercially available ferrofluid solution. A magnetic field of 0.01T caused a resistance change of 1.37% for ferrofluid, while a 1.1% GMR was recorded for the water baseline.

Multi Layer Micro Pin-Fins Heat Sinks for Better Performance

This study numerically investigates the feasibility and advantages of using a multilayer pin-fin heat sink to increase the overall performance of the heat sink. For the purpose of determining overall performance of the pin-fin heat sink a figure of merit (FOM) term is introduced in this paper, which constituted of both the thermal resistance and the pumping power of the heat sink. Higher the FOM of a heat sink better is its overall performance. A computational fluid dynamics software CoventorWARE™ is used for the analysis of micro heat sink performance. A small portion of the entire heat sink is modeled in this study assuming repeatability towards both sides for the ease of analysis. The developed models consist of two sections, the substrate (silicon) and the fluid (water at 278K). A uniform heat flux is applied to the base of the heat sink. A single layer micro pin-fin heat sinks with same dimensions as of the multi layer heat sink was also modeled for the comparison purpose. Temperature distribution at five different locations from the inlet to the outlet section is also analyzed to study the temperature distribution over the heat sink. Circular pin-fins were used in both the multilayer and single layer micro heat sinks. Feasibility of using micro channels as the second layer was also investigated in this paper and it proved to have advantages over using pin-fin structures on both layers. A geometric optimization based on the substrate thickness of the second layer of the double layer heat sink showed that the substrate thickness of the second layer doesn’t have any effect on the overall thermal resistance of the heat sink.Copyright

Diffusion Analysis of Chloride in Concrete Following Electrokinetic Nanoparticle Treatment

Concrete is a highly porous material which is susceptible to the migration of highly deleterious species such as chlorides and sulfates. Various external sources including sea salt spray, direct sea water wetting, deicing salts and brine tanks harbor chlorides that can enter reinforced concrete. Chlorides diffuse into the capillary pores of concrete and come into contact with the rebar. When chloride concentration at the rebar exceeds a threshold level it breaks down the passive layer of oxide, leading to chloride induced corrosion. Application of electrokinetics using positively charged nanoparticles for corrosion protection in reinforced concrete structures is an emerging technology. This technique involves the principle of electrophoretic migration of nanoparticles to hinder chloride diffusion in the concrete. The re-entry of the chlorides is inhibited by the electrodeposited assembly of the nanoparticles at the rebar interface. In this work electrochemical impedance spectroscopy (EIS) combined with equivalent circuit analysis was used to predict chloride diffusion coefficients as influenced by nanoparticle treatments. Untreated controls exhibited a diffusion coefficient of 3.59 × 10−12 m2 /s which is slightly higher than the corrosion initiation benchmark value of 1.63 × 10−12 m2 /s that is noted in the literature for mature concrete with a 0.5 water/cement mass ratio. The electrokinetic nanoparticle (EN) treated specimens exhibited a diffusion coefficient of 1.41 × 10−13 m2 /s which was 25 times lower than the untreated controls. Following an exposure period of three years the mature EN treated specimens exhibited lower chloride content by a factor of 27. These findings indicate that the EN treatment can significantly lower diffusion coefficients thereby delaying the initiation of corrosion.Copyright

Performance of Cross Flow Microchannel Heat Exchangers Subjected to Viscous Dissipation

This paper analyzes the effect of viscous dissipation on the thermal performance of balanced flow cross flow microchannel heat exchangers. The cross flow microchannel heat exchanger analyzed in this paper is one that is subjected to axial heat conduction. Governing equations are developed for each of the fluids and the wall separating the fluids. The equations are solved simultaneously using the numerical technique of finite difference method to obtain the temperature profile. The effectiveness of each fluid is determined using the temperature profile thus obtained. The effectiveness and the temperature of the fluids are found to depend on NTU, axial heat conduction parameters and the viscous dissipation parameter. In the presence of axial heat conduction the effectiveness of the fluid decreases for a specific NTU. In addition, the effectiveness of the fluids decreases with increase in axial heat conduction parameters at a particular NTU. The effectiveness of the hot fluid in the presence of viscous heat dissipation alone decreased at a particular NTU. On the other hand the effectiveness of the cold fluid for the same amount of viscous heating improved at a specific NTU. The combined effect of axial heat conduction and viscous dissipation on the hot fluid is to decrease its effectiveness. With regard to the cold fluid effectiveness it can either increase or decrease due to the combined effect of axial heat conduction parameter and viscous dissipation.Copyright

Dynamics of Fluid Flow in a Heated Zig-Zag Square Microchannel

This paper deals with the dynamics of flow in a zigzag square microchannel subjected to uniform external heat flux (H2 thermal boundary condition). The heat transfer and fluid flow characteristics is studied using CoventorWare™. *umerical studies are performed using microchannels with hydraulic diameter ranging from 100 -m to 300 -m for Reynolds number between 25 and 500. The angle which each arm of the zigazg microchannel makes with the axial direction is varied from between 10 o and 30 o with increments of 10 o . The effect of number of turns in a repeating unit for a specific length of the microchannel is also investigated in this paper. The *usselt number and Poiseuille number in a zigzag microchannel is found to be totally different from that in a straight square microchannel of the same geometric dimensions. These two parameters are found to increase with increase in Re for zigzag microchannels with fixed geometry. T he individual effect of each of these parameters is to increase both *usselt number and Poiseuille number for a specific Reynolds number. The enhancement is highest with increase in number of turns while it is the lowest with increase in hydraulic diameter. The enhancement in *usselt number and Poiseuille number is explained using the flow profile in the microchannels.

Analysis of Effectiveness of Parallel Flow Microchannel Heat Exchangers With Heat Transfer From Surroundings

The performance of a parallel flow micro channel heat exchanger (MCHX) is examined in this paper with the help of a mathematical model developed for this study. The thermal performance of a balanced parallel flow MCHX is numerically evaluated with the effect of heat transfer from the surroundings taken into consideration. Two non-dimensional governing equations are developed for modeling this MCHXPF . This model also considers a case in which the temperature of surrounding that is adjacent to the hot fluid channel is assumed to be different from that in contact with the cold fluid channel. The equations are numerically solved using the Runge-Kutta-Fehlberg method. MATLAB software is used to develop the program for solving the equations that constitute this model. The axial temperatures along both channels are obtained upon numerically solving the governing equations. The effectiveness of each fluid is then calculated using its inlet and outlet temperatures. The heat transfer between the fluids and that between the surrounding and each fluid are also numerically calculated using the axial temperatures of the fluids. The effectiveness of the fluids depends on the NTU, temperatures of the surroundings, and the thermal resistance between the individual fluids and their corresponding surrounding. When the heat transfer from the surrounding and the respective fluid is increased the effectiveness of the hot fluid decreased and that of the cold fluid increased. The temperature of the surroundings will determine the direction of heat transfer between the individual fluid and its surrounding. When the temperature of the surroundings is higher than the inlet temperature of the hot fluid then there would be degradation of the effectiveness of the hot fluid and an improvement in the effectiveness of the cold fluid. And the opposite trend happens when the temperature of the surroundings are lower than the inlet temperature of the cold fluid.Copyright

Experimental Analysis of Poiseuille Number in Square Microchannels

The applications involving fluid flow through microchannels in industry and research have increased significantly with the evolution of microfluidic devices such as lab-on-chip systems. Most of the previous studies concerning fluid flow were done using circular microchannels. However, there is an increased usage of noncircular microchannels, especially square microchannels, in microfluidic devices. Thus there is need for experimental studies on the behavior of fluid flow in square microchannels, and the comparison of the results with the results obtained from the conventional fluid flow equations is relevant. In this study the authors are focusing on the analysis of the friction factor associated with square microchannels of rounded edges under laminar flow conditions. Microchannels with hydraulic diameters of 200, 300, 400 and 500 micrometers and length of 10 cm and 5 cm are used in the analysis. DI-water and ethylene glycol at room temperature is used as the liquid for experiments. A constant liquid flow rate is achieved in the channels using a syringe pump that can pump from 50 μl/hr to 7,500 ml/hr using a 60 ml syringe, and a high precision pressure gauge is used to measure the pressure drop across the channel. The Reynolds number of the liquid flow in all the channels is kept constant between 20 and 120 by varying the flow rate. The friction factor at each Reynolds number is calculated and the results are compared with the friction factor of conventional channels. Experiments are conducted to measure the pressure drop across the channels. The pressure drop obtained across the 5 cm channel is subtracted from the pressure drop obtained across the 10 cm channel so that the effect of entrance effect can be eliminated from the results. The fiction factor obtained from the experiments is used to calculate the Poiseuille number. The experimental values of Poiseuille number are showing a median deviation of around 9% from the conventional values for all the different channels. The uncertainty is observed to be ca.9% for all the channels at all values of Reynolds numbers. The major factor contributing towards the total uncertainty is the uncertainty in the measurement of liquid flow rate.© 2009 ASME