David E. Huber

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.

Descreening of field effect in electrically gated nanopores

This modeling work investigates the electrical modulation characteristics of field-effect gated nanopores. Highly nonlinear current modulations are observed in nanopores with nonoverlapping electric double layers, including those with pore diameters 100 times the Debye screening length. We attribute this extended field-effect gating to a descreening effect, i.e., the counter-ions do not fully relax to screen the gating potential due to the presence of strong ionic transport.

Highly efficient electrocaloric cooling with electrostatic actuation

A flexible and lightweight device uses an electrocaloric polymer film to provide exceptional cooling power. A solid way to keep cool Refrigeration relies on vapor compression that is noisy, takes up space, and is mechanically complex. Solid-state cooling requires changing an external field to drive cooling, but devices produced so far have not been efficient enough for practical applications. Ma et al. constructed a lightweight and flexible device using a thin electrocaloric polymer film, where toggling it in an electric field between a heat source and sink drives the cooling process (see the Perspective by Zhang and Zhang). The device rapidly cools down an overheated smartphone battery and has potential application for developing compact, low-profile electronics. Science, this issue p. 1130; see also p. 1094 Solid-state refrigeration offers potential advantages over traditional cooling systems, but few devices offer high specific cooling power with a high coefficient of performance (COP) and the ability to be applied directly to surfaces. We developed a cooling device with a high intrinsic thermodynamic efficiency using a flexible electrocaloric (EC) polymer film and an electrostatic actuation mechanism. Reversible electrostatic forces reduce parasitic power consumption and allow efficient heat transfer through good thermal contacts with the heat source or heat sink. The EC device produced a specific cooling power of 2.8 watts per gram and a COP of 13. The new cooling device is more efficient and compact than existing surface-conformable solid-state cooling technologies, opening a path to using the technology for a variety of practical applications.

Limiting and overlimiting conductance in field-effect gated nanopores

Numerical modeling of the coupled ionic and fluidic transport in field-effect gated nanopores reveals highly nonlinear current-voltage characteristics, including cross-over, rectification, and particularly limiting and overlimiting conductance. The limiting and overlimiting characteristics are shown to be greatly enhanced by the inherently coupled fluid flow and correlate with electrokinetic phenomena such as concentration polarization and vortex formation. The underlying reason for the observed nonlinear characteristics is explained by considering the symmetry properties of the electrical biasing.

A Microfluidic Platform for Characterization of Protein–Protein Interactions

Traditionally, expensive and time consuming techniques such as mass spectrometry and Western Blotting have been used for characterization of protein-protein interactions. In this paper, we describe the design, fabrication, and testing of a rapid and inexpensive sensor, involving the use of microelectrodes in a microchannel, which can be used for real-time electrical detection of specific interactions between proteins. We have successfully demonstrated detection of target glycoprotein-glycoprotein interactions, antigen-antibody interactions, and glycoprotein-antigen interactions. We have also demonstrated the ability of this technique to distinguish between strong and weak interactions. Using this approach, it may be possible to multiplex an array of these sensors onto a chip and probe a complex mixture for various types of interactions involving protein molecules.

Ballistic dispersion in temperature gradient focusing

Molecular dispersion is caused by both molecular diffusion and non-uniform bulk fluid motion. While the Taylor–Aris dispersion regime is the most familiar regime in microfluidic systems, an oft-overlooked regime is that of purely kinematic (or ballistic) dispersion. In most microfluidic systems, this dispersion regime is transient and quickly gives way to Taylor–Aris dispersion. In electrophoretic focusing methods such as temperature gradient focusing (TGF), however, the characteristic time scales for dispersion are fixed, and focused peaks may never reach the Taylor limit. In this situation, generalized Taylor dispersion analysis is not applicable. A heuristic model is developed here which accounts for both molecular diffusion and advective dispersion across all dispersion regimes, from pure diffusion to Taylor dispersion to pure advection. This model is compared to results from TGF experiments and accurately captures both the initial decrease and subsequent increase in peak widths as electric field strength increases. The results of this combined analytical and experimental study provide a useful tool for estimation of dispersion and optimization of TGF systems.

Temperature Gradient Focusing in a Microfluidic Device

Thermal gradient focusing leverages a temperature gradient imposed along the axial direction of a microchannel to effect a gradient in electrophoretic mass flux. When a bulk flow is imposed in the opposite direction, charged analytes separate and focus at points where their net bulk velocities (advective plus electrophoretic) sum to zero (a). The experimental setup (b) consists of epifluorescence optics (collecting at two wavelengths), thermoelectric-regulated temperature blocks at each end of the microchannel, a high voltage power supply, and a custom pressure controller. Embedded RTDs provide reference temperatures for system calibration.

Magnetic Bar Array With Linker Technology for Detection and Investigation of Nonmagnetic Molecules

We have developed a simple method using a magnetic bar array to anchor nonmagnetic biomolecules for time-lapse analysis and identification. Biomolecules of interest are attached, using linker molecules, to the superparamagnetic beads which are in turn captured at gaps of the magnetic bar array. This micro-array system can be used for various studies and applications such as yeast growth studies, as illustrated in this work, and pyrosequencing. The technology is applicable to a wide variety of biomolecules using the suitable linker molecules, with the merits of low lost and reusability.

Experimental demonstration and analysis of DNA passage in nanopore-based nanofluidic transistors

We investigate DNA passage through a nanofluidic transistor (NFT) composed of large diameter nanopores (~200nm) with an embedded metal gate. The nanopores are large enough to be made by currently available photolithography process for manufacturability. We observe that the NFT is capable of electrostatic control of DNA capture rate, the equivalent of current in the electron transistor, with similar operating principle as their electron device counterpart.

The Measurement of Diffusion Coefficient Using Nanofluidic Channels

The behavior of molecules confined within nanochannels may provide new insight into important chemical and biological mechanisms. As such, the ability to measure the diffusion coefficient of molecules accurately and rapidly is critical. Toward this end, we demonstrate a new method for measuring the diffusion coefficient of analytes in nanoscale environments. Based on nanoscale electrokinetic transport theory, a numerical simulation was developed to determine the spatial concentration of analyte within a nanofluidic T-shaped channel for a given Peclet number. The resulting concentration profiles were compared to experimental data to determine the Peclet number that gives the minimum least-square error. Using this approach, the diffusion coefficient of sodium fluorescein was measured and found to be within the error of previously published values. This method is envisioned to be a novel analytical tool to rapidly and accurately measure diffusion coefficients of small analytes, and to measure the effective diffusion coefficients of more complex species such as DNA and peptides when confined within a nanochannel.© 2007 ASME