There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Damascene copper electroplating for on-chip interconnections, a process that we conceived and developed in the early 1990s, makes it possible to fill submicron trenches and vias with copper without creating a void or a seam and has thus proven superior to other technologies of copper deposition. We discuss here the relationship of additives in the plating bath to superfilling, the phenomenon that results in superconformal coverage, and we present a numerical model which accounts for the experimentally observed profile evolution of the plated metal.

Damascene copper electroplating for chip interconnections

http://web.mit.edu/bazant/www/papers/pdf/Bazant_2009_ACIS_preprint.pdf

Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions

Networks of nanofilaments derived from natural amino acids can have metallic-like conductivity.

/pdf/tunable-metallic-like-conductivity-in-microbial-nanowire-2l3d9tb4kx.pdf

Tunable metallic-like conductivity in microbial nanowire networks

The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatic forces. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt, which are often exceeded, e.g. in microfluidic pumps and electrochemical sensors. Since the 1950s, several modifications of the PB equation have been proposed to account for the finite size of ions in equilibrium, but in this two-part series, we consider steric effects on diffuse charge dynamics (in the absence of electro-osmotic flow). In this first part, we review the literature and analyze two simple models for the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance typically varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. In PB theory, the capacitance blows up exponentially with voltage, but steric effects actually cause it to decrease above a threshold voltage where ions become crowded near the surface. Other nonlinear effects in PB theory are also strongly suppressed by steric effects: The net salt adsorption by the double layers in response to the applied voltage is greatly reduced, and so is the tangential "surface conduction" in the diffuse layer, to the point that it can often be neglected compared to bulk conduction (small Dukhin number).

Steric effects in the dynamics of electrolytes at large applied voltages: I. Double-layer charging

Microfluidic devices with integrated electrical detection will enable fast, low-cost or portable sensing and processing of biological and chemical samples. By employing microfabrication, microelectrical impedance spectroscopy detectors are designed to take advantage of AC electroosmosis to rapidly concentrate bioparticles, leading to enhanced sensitivity due to a reduction of the transport time to the detector. Electrodes were fabricated by integrated circuit technology. Experiments were performed to find the optimal voltage and frequency ranges such that a trapping converging flow exists on the electrodes and the assembled cells exhibit sensitive impedance spectrum signatures. Preliminary results show resolution at a concentration of 10 4 bacteria/mL, indicating that combining AC electroosmosis with impedance measurement can improve the sensitivity and speed of detection of bacteria in solutions with conductivity comparable to that in environmental applications. The bacterial impedance signatures are, however, different in tap water and PBS buffers, due perhaps to different ion transport properties across the cell membranes in the two solutions.

Long-Range AC Electroosmotic Trapping and Detection of Bioparticles

Recently, considerable interest and effort have been devoted to the on-site detection of low-concentration pathogenic bacteria in order to deter infectious diseases and bioterrorism. Conventionally, bacteria detection involves culturing, which is time-consuming and unfeasible under field conditions. Microfluidic devices with integrated electrical detection will enable fast, low-cost, and portable sensing and processing of biological and chemical samples. AC electroosmosis (EO) is well-suited for integration into microsystems due to its low-voltage operation and no-moving-part implementation and microelectrical impedance spectroscopy can be integrated with AC EO for particle manipulation, leading to enhanced sensitivity due to a reduction of the transport time to the detector. Experiments are performed to find optimal conditions for obtaining particle and bacterial assembly lines on electrodes by AC EO and preliminary results show good resolution at a concentration of 104 bacteria/ml, indicating that combining AC EO with impedance measurement can improve the sensitivity of particle electrical detection.

Particle detection by electrical impedance spectroscopy with asymmetric-polarization AC electroosmotic trapping

AC electrokinetics has shown great potential for microfluidic functions such as pumping, mixing and concentrating particles. So far, electrokinetics are typically applied on fluids that are not too conductive (<0.02 S/m), which excludes most biofluidic applications. To solve this problem, this paper seeks to apply AC electrothermal (ACET) effect to manipulate conductive fluids and particles within. ACET generates temperature gradients in the fluids, and consequently induces space charges that move in electric fields and produce microflows. This paper reports two new ACET devices, a parallel plate particle trap and an asymmetric electrode micropump. Preliminary experiments were performed on fluids with conductivity at 0.224 S/m. Particle trapping and micropumping were demonstrated at low voltages, reaching approximately 100 microm/s for no more than 8 Vrms at 200 kHz. The fluid velocity was found to depend on the applied voltage as V(4), and the maxima were observed to be approximately 20 microm above the electrodes.

AC electrothermal manipulation of conductive fluids and particles for lab-chip applications

Electrokinetics is a preferred technique for microfluidic systems, but it is typically applied on fluids that are not too conductive (lower than 0.02S∕m), which excludes most biological applications. To solve this problem, this letter investigates microfluidic actuation by ac electrothermal (ACET) effect that was largely overlooked by the community. ACET originates from temperature gradients in the fluids, and it becomes more pronounced in more conductive fluids. This letter discusses two ACET pump designs, and pumping was demonstrated with biobuffers (e.g., lysogeny broth at 0.754S∕m).

Jie Wu

Papers

Long-Range AC Electroosmotic Trapping and Detection of Bioparticles

Particle detection by electrical impedance spectroscopy with asymmetric-polarization AC electroosmotic trapping

AC electrothermal manipulation of conductive fluids and particles for lab-chip applications

Micropumping of biofluids by alternating current electrothermal effects

AC electrokinetics-enhanced capacitive immunosensor for point-of-care serodiagnosis of infectious diseases