Juan G. Santiago

A review of micropumps

We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

A PIV Algorithm for Estimating Time-Averaged Velocity Fields

A PIV algorithm is presented for estimating time-averaged or phase-averaged velocity fields. The algorithm can be applied to situations where signal strength is not sufficient for standard cross correlation techniques, such as a low number of particle images in an interrogation spot, or poor image quality. The algorithm can also be used to increase the spatial resolution of measurements by allowing smaller interrogation spots than those required for standard cross correlation techniques. The quality of the velocity measurements can be dramatically increased by averaging a series of instantaneous corelation functions, before determining the location of the signal peak, as opposed to the commonly used technique of estimating instantaneous velocity fields first and then averaging the velocity fields. The algorithm is applied to a 30 μm×300 μm microchannel flow

Measurements and modeling of two-phase flow in microchannels with nearly constant heat flux boundary conditions

Two-phase forced convective flow in microchannels is promising for the cooling of integrated circuits. There has been limited research on boiling flow in channels with dimensions below 100 /spl mu/m, in which bubble formation and flow regimes can differ from those in larger channels. This work develops single and multi-channel experimental structures using plasma-etched silicon with pyrex glass cover, which allow uniform heating and spatially-resolved thermometry and provide optical access for visualization of boiling regimes. Boiling was observed with less than 5/spl deg/C of super-heating in rectangular channels with hydraulic diameters between 25 and 60 /spl mu/m. The channel wall widths are below 350 /spl mu/m, which minimizes solid conduction and reduces variations in the heat flux boundary condition. Pressure drop and wall temperature distribution data are consistent with predictions accounting for solid conduction and homogeneous two-phase convection.

A planar electroosmotic micropump

Electroosmotic (EO) micropumps use field-induced ion drag to drive liquids and achieve high pressures in a compact design with no moving parts. An analytical model applicable to planar, etched-structure micropumps has been developed. This model consists of pressure and flow relations in addition to an analytical expression that can be used to estimate the thermodynamic efficiency of planar EO pumps. The analytical model was applied to guide the design of a pump consisting of an etched EO flow chamber for near-optimal hydraulic power performance. To achieve high efficiency, the working fluid used was deionized (DI) water with a conductivity of 3.0 /spl times/ 10/sup -4/ S/m (pH = 5.7). The EO micropump was fabricated on a soda-lime glass substrate using standard microlithography and chemical wet etching techniques. The active pumping volume of the device consists of a wet-etched flow channel 1-mm long in the flow direction and 0.9 /spl mu/m by 38-mm in cross section. The pump performance agrees well with the theoretical model. The pump can produce a maximum pressure of 0.33 atm and a maximum flow rate of 15 /spl mu/L/min min at 1 kV.

Closed-loop electroosmotic microchannel cooling system for VLSI circuits

The increasing heat generation rates in VLSI circuits motivate research on compact cooling technologies with low thermal resistance. This paper develops a closed-loop two-phase microchannel cooling system using electroosmotic pumping for the working fluid. The design, fabrication, and open-loop performance of the heat exchanger and pump are summarized. The silicon heat exchanger, which attaches to the test chip (1 cm/sup 2/), achieves junction-fluid resistance near 0.1 K/W using 40 plasma-etched channels with hydraulic diameter of 100 /spl mu/m. The electroosmotic pump, made of an ultrafine porous glass frit with working volume of 1.4 cm/sup 3/, achieves maximum backpressure and flowrate of 160 kPa and 7 ml/min, respectively, using 1 mM buffered de-ionized water as working fluid. The closed-loop system removes 38 W with pump power of 2 W and junction-ambient thermal resistance near 2.5 K/W. Further research is expected to strongly reduce the thermal resistance for a given heating power by optimizing the saturation temperature, increasing the pump flowrate, eliminating the thermal grease, and optimizing the heat exchanger dimensions.

Porous glass electroosmotic pumps: theory

This paper presents an analytical study of electroosmotic (EO) pumps with porous pumping structures. We have developed an analytical model to solve for electroosmotic flow rate, pump current, and thermodynamic efficiency as a function of pump pressure load for porous-structure EO pumps. The model uses a symmetric electrolyte approximation valid for the high-zeta-potential regime and numerically solves the Poisson-Boltzmann equation for charge distribution in the idealized pore geometry. Generalized scaling of pumping performance is discussed in the context of a parameterization that includes porosity, tortuosity, pore size, bulk ionic density, and the nonuniform conductivity distribution over charge layers. The model also incorporates an approximate ionic-strength-dependent zeta potential formulation.

Instability of electrokinetic microchannel flows with conductivity gradients

Electrokinetic flow is leveraged in a variety of applications, and is a key enabler of on-chip electrophoresis systems. An important sub-class of electrokinetic devices aim to pump and control electrolyte working liquids with spatial gradients in conductivity. These high-gradient flows can become unstable under the application of a sufficiently strong electric field. In this work the instability physics is explored using theoretical and numerical analyses, as well as experimental observations. The flow in a long, rectangular-cross-section channel is considered. A conductivity gradient is assumed to be orthogonal to the main flow direction, and an electric field is applied in the streamwise direction. It is found that such a system exhibits a critical electric field above which the flow is highly unstable, resulting in fluctuating velocities and rapid stirring. Modeling results compare well with experimental observations. The model indicates that the fluid forces associated with the thin dimension of the c...

Velocity Measurements of a Jet Injected into a Supersonic Crossflow

This paper presents a quantitative, experimental study of a single, sonic, underexpanded, transverse, round jet injected into a Mach 1.6 crosse ow. This investigation is applicable to studies of supersonic combustors, thrust vector control of rocket nozzles, the cooling of nozzle walls, and jet reaction force prediction. Schlieren/shadowgraph photography and two-component, frequency preshifted laser Doppler velocimetry are used to visualize the e ow and to measure three mean velocity components, e ve of the six kinematic Reynolds stresses, and turbulent kinetic energy at over 4000 locations throughout the e owe eld. The study focuses on the transverse, midline plane and on two crosse ow planes. These measurements are used to study the size and orientation of the recirculation regions upstream and downstream of the jet; the structure and strength of the bow shock, barrel shock, and Mach disk; the structure, strength, and development of the kidney-shaped, counter-rotating vortex pair; the growth of the annular shear layer between the jet plume and the crosse ow; and the growth of the boundary layer beneath the jet. In addition, the present study provides validation data for analytical and numerical predictions of the transverse jet e owe eld.

Porous glass electroosmotic pumps: design and experiments.

An analytical model for electroosmotic flow rate, total pump current, and thermodynamic efficiency reported in a previous paper has been applied as a design guideline to fabricate porous-structure EO pumps. We have fabricated sintered-glass EO pumps that provide maximum flow rates and pressure capacities of 33 ml/min and 1.3 atm, respectively, at applied potential 100 V. These pumps are designed to be integrated with two-phase microchannel heat exchangers with load capacities of order 100 W and greater. Experiments were conducted with pumps of various geometries and using a relevant, practical range of working electrolyte ionic concentration. Characterization of the pumping performance are discussed in the terms of porosity, tortuosity, pore size, and the dependence of zeta potential on bulk ion density of the working solution. The effects of pressure and flow rate on pump current and thermodynamic efficiency are analyzed and compared to the model prediction. In particular, we explore the important tradeoff between increasing flow rate capacity and obtaining adequate thermodynamic efficiency. This research aims to demonstrate the performance of EOF pump systems and to investigate optimal and practical pump designs. We also present a gas recombination device that makes possible the implementation of this pumping technology into a closed-flow loop where electrolytic gases are converted into water and reclaimed by the system.

Convective and absolute electrokinetic instability with conductivity gradients

Electrokinetic flow instabilities occur under high electric fields in the presence of electrical conductivity gradients. Such instabilities are a key factor limiting the robust performance of complex electrokinetic bio-analytical systems, but can also be exploited for rapid mixing and flow control for microscale devices. This paper reports a representative flow instability phenomenon studied using a microfluidic T-junction with a cross-section of 11 µm by 155 µm. In this system, aqueous electrolytes of 10:1 conductivity ratio were electrokinetically driven into a common mixing channel by a steady electric field. Convectively unstable waves were observed with a nominal threshold field of 0. 5k V cm −1 , and upstream propagating waves were observed at 1. 5k V cm −1 . A physical model has been developed for this instability which captures the coupling between electric and flow fields. A linear stability analysis was performed on the governing equations in the thin-layer limit, and Briggs–Bers criteria were applied to select physically unstable modes and determine the nature of instability. The model predicts both qualitative trends and quantitative features that agree very well with experimental data, and shows that conductivity gradients and their associated bulk charge accumulation are crucial for such instabilities. Comparison between theory and experiments suggests the convective role of electro-osmotic flow. Scaling analysis and numerical results show that the instability is governed by two key controlling parameters: the ratio of dynamic to dissipative forces which governs the onset of instability, and the ratio of electroviscous to electro-osmotic velocities which governs the convective versus absolute nature of instability.