Techno-economic feasibility analysis of an extreme heat flux micro-cooler.

An estimated 70% of the electricity in the United States currently passes through power conversion electronics, and this percentage is projected to increase eventually to up to 100%. At a global scale, wide adoption of highly efficient power electronics technologies is thus anticipated to have a major impact on worldwide energy consumption. As described in this perspective, for power conversion, outstanding thermal management for semiconductor devices is one key to unlocking this potentially massive energy savings. Integrated microscale cooling has been positively identified for such thermal management of future high-heat-flux, i.e., 1 kW/cm2, wide-bandgap (WBG) semiconductor devices. In this work, we connect this advanced cooling approach to the energy impact of using WBG devices and further present a techno-economic analysis to clarify the projected status of performance, manufacturing approaches, fabrication costs, and remaining barriers to the adoption of such cooling technology.

Modes and break periods of electrowetting liquid bridge.

In this paper, we propose a microscale liquid oscillator using electrowetting-on-dielectric (EWOD). Specifically, a mesoscale liquid bridge (LB) between two horizontal surfaces with EWOD is considered. When EWOD is applied, the solid surface becomes more hydrophilic, and hence the contact angle (CA) is reduced. Following the activation of EWOD, the LB can remain connected or it can break into either symmetric or asymmetric shapes depending on the initial liquid volume and wettability of the two surfaces. The LB dynamics activated by EWOD is studied using the multibody dissipative particle dynamics (MDPD) method. Our numerical results show that the behavior of an LB under EWOD can be interpreted via three modes. In the first mode, the LB does not break after applying EWOD. In the second mode, the LB breaks and does not reform. The third mode happens when, depending on the interplay of the volume of the liquid and CA manipulation, the LB continuously breaks, recoils, and reforms. For asymmetric cases, it was observed that the LB may completely detach from one surface and may not reform. It was also observed that decreasing the wettability of a surface, for cases with a continuous breaking and reformation behavior, increases the connecting time interval and decreases the breaking time interval in one break-reform cycle. The results provided in this investigation facilitate fundamental understanding of LB dynamics and their application for the design of microscale liquid oscillators using EWOD.

Tailoring Permeability of Microporous Copper Structures through Template Sintering.

Microporous metals are used extensively for applications that combine convective and conductive transport and benefit from low resistance to both modes of transport. Conventional fabrication methods, such as direct sintering of metallic particles, however, often produce structures with limited fluid transport properties due to the lack of control over pore morphologies such as the pore size and porosity. Here, we demonstrate control and improvement of hydraulic permeability of microporous copper structures fabricated using template-assisted electrodeposition. Template sintering is shown to modify the fluid transport network in a manner that increases permeability by nearly an order of magnitude with a less significant decrease (∼38%) in thermal conductivity. The measured permeabilities range from 4.8 × 10-14 to 1.3 × 10-12 m2 with 5 µm pores, with the peak value being roughly 5 times larger than the published values for sintered copper particles of comparable feature sizes. Analysis indicates that the enhancement of permeability is limited by constrictions, i.e., bottlenecks between connecting pores, whose dimensions are highly sensitive to the sintering conditions. We further show contrasting trends in permeability versus conductivity of the electrodeposited microporous copper and conventional sintered copper particles and suggest these differing trends to be the result of their inverse structural relationship.

A method for quantifying in plane permeability of porous thin films.

The in-plane permeability of porous thin films is an important fluid mechanical property that determines wicking and pressure-driven flow behavior in such materials. This property has so far been challenging to measure directly due to the small sidewall cross-sectional area of thin films available for flow. In this work, we propose and experimentally demonstrate a novel technique for directly measuring in-plane permeability of porous thin films of arbitrary thicknesses, in situ, using a manifold pressed to the top surface of the film. We both measure and simulate the influence of the two dimensional flow field produced in a film by the manifold and extract the permeability from measurements of pressure drop at fixed flow rates. Permeability values measured using the technique for a periodic array of channels are comparable to theoretical predictions. We also determine in-plane permeability of arrays of pillars and electrodeposited porous copper films. This technique is a robust tool to characterize permeability of thin films of arbitrary thicknesses on a variety of substrates. In Supplementary material, we provide a solid model, which is useful in three-dimensional printer reproductions of our device.

Porous micropillar structures for retaining low surface tension liquids.

The ability to manipulate fluid interfaces, e.g., to retain liquid behind or within porous structures, can be beneficial in multiple applications, including microfluidics, biochemical analysis, and the thermal management of electronic systems. While there are a variety of strategies for controlling the disposition of liquid water via capillarity, such as the use of chemically modified porous adhesive structures and capillary stop valves or surface geometric features, methods that work well for low surface tension liquids are far more difficult to implement. This study demonstrates the microfabrication of a silicon membrane that can retain exceptionally low surface tension fluorinated liquids against a significant pressure difference across the membrane via an array of porous micropillar structures. The membrane uses capillary forces along the triple phase contact line to maintain stable liquid menisci that yield positive working Laplace pressures. The micropillars have inner diameters and thicknesses of 1.5-3 µm and ∼1 µm, respectively, sustaining Laplace pressures up to 39 kPa for water and 9 kPa for Fluorinert™ (FC-40). A theoretical model for predicting the change in pressure as the liquid advances along the porous micropillar structure is derived based on a free energy analysis of the liquid meniscus with capped spherical geometry. The theoretical prediction was found to overestimate the burst pressure compared with the experimental measurements. To elucidate this deviation, transient numerical simulations based on the Volume of Fluid (VOF) were performed to explore the liquid pressure and evolution of meniscus shape under different flow rates (i.e., Capillary numbers). The results from VOF simulations reveal strong dynamic effects where the anisotropic expansion of liquid along the outer micropillar edge leads to an irregular meniscus shape before the liquid spills along the micropillar edge. These findings suggest that the analytical prediction of burst Laplace pressure obtained under quasi-static condition (i.e., equilibrium thermodynamic analysis under low capillary number) is not applicable to highly dynamic flow conditions, where the liquid meniscus shape deformation by flow perturbation cannot be restored by surface tension force instantaneously. Therefore, the critical burst pressure is dependent on the liquid velocity and viscosity under dynamic flow conditions. A numerical simulation using Surface Evolver also predicts that surface defects along the outer micropillar edge can yield up to 50% lower Laplace pressures than those predicted with ideal feature geometries. The liquid retention strategy developed here can facilitate the routing and phase management of dielectric working fluids for application in heat exchangers. Further improvements in the retention performance can be realized by optimizing the fabrication process to reduce surface defects.

Ion selectivity in capacitive deionization with functionalized electrode: Theory and experimental validation.

Capacitive deionization (CDI) is a promising technique for salt removal and may have potential for highly selective removal of ion species. In this work, we take advantage of functional groups usually used with ionic exchange resins and apply these to CDI. To this end, we functionalize activated carbon with a quaternary amines surfactant and use this surface to selectively and passively (no applied field) trap nitrate ions. We then set the cell voltage to a constant value to regenerate these electrodes, resulting in an inverted capacitive deionization (i-CDI) operation. Unlike resins, we avoid use of concentrated chemicals for regeneration. We measure the selectivity of nitrate versus chloride ions as a function of regeneration voltage and initial chloride concentration. We experimentally demonstrate up to about 6.5-fold (observable) selectivity in a cycle with a regeneration voltage of 0.4 V. We also demonstrate a novel multi-pass, air-flush i-CDI operation to selectively enrich nitrate with high water recovery. We further present a dynamic, multi-species electrosorption and equilibrium solution-to-surface chemical reaction model and validate the model with detailed measurements. Our i-CDI system exhibits higher nitrate selectivity at lower voltages; making it possible to reduce NaNO3 concentrations from ∼170 ppm to below the limit of maximum allowed values for nitrate in drinking water of about 50 ppm NaNO3.

Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes.

Ion adsorption and equilibrium between electrolyte and microstructure of porous electrodes are at the heart of capacitive deionization (CDI) research. Surface functional groups are among the factors which fundamentally affect adsorption characteristics of the material and hence CDI system performance in general. Current CDI-based models for surface charge are mainly based on a fixed (constant) charge density, and do not treat acid-base equilibria of electrode microstructure including so-called micropores. We here expand current models by coupling the modified Donnan (mD) model with weak electrolyte acid-base equilibria theory. In our model, surface charge density can vary based on equilibrium constants (pK's) of individual surface groups as well as micropore and electrolyte pH environments. In this initial paper, we consider this equilibrium in the absence of Faradaic reactions. The model shows the preferential adsorption of cations versus anions to surfaces with respectively acidic or basic surface functional groups. We introduce a new parameter we term "chemical charge efficiency" to quantify efficiency of salt removal due to surface functional groups. We validate our model using well controlled titration experiments for an activated carbon cloth (ACC), and quantify initial and final pH of solution after adding the ACC sample. We also leverage inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC) to quantify the final background concentrations of individual ionic species. Our results show a very good agreement between experiments and model. The model is extendable to a wide variety of porous electrode systems and CDI systems with applied potential.

Energy breakdown in capacitive deionization.

We explored the energy loss mechanisms in capacitive deionization (CDI). We hypothesize that resistive and parasitic losses are two main sources of energy losses. We measured contribution from each loss mechanism in water desalination with constant current (CC) charge/discharge cycling. Resistive energy loss is expected to dominate in high current charging cases, as it increases approximately linearly with current for fixed charge transfer (resistive power loss scales as square of current and charging time scales as inverse of current). On the other hand, parasitic loss is dominant in low current cases, as the electrodes spend more time at higher voltages. We built a CDI cell with five electrode pairs and standard flow between architecture. We performed a series of experiments with various cycling currents and cut-off voltages (voltage at which current is reversed) and studied these energy losses. To this end, we measured series resistance of the cell (contact resistances, resistance of wires, and resistance of solution in spacers) during charging and discharging from voltage response of a small amplitude AC current signal added to the underlying cycling current. We performed a separate set of experiments to quantify parasitic (or leakage) current of the cell versus cell voltage. We then used these data to estimate parasitic losses under the assumption that leakage current is primarily voltage (and not current) dependent. Our results confirmed that resistive and parasitic losses respectively dominate in the limit of high and low currents. We also measured salt adsorption and report energy-normalized adsorbed salt (ENAS, energy loss per ion removed) and average salt adsorption rate (ASAR). We show a clear tradeoff between ASAR and ENAS and show that balancing these losses leads to optimal energy efficiency.

Burst behavior at a capillary tip: Effect of low and high surface tension.

Liquid retention in micron and millimeter scale devices is important for maintaining stable interfaces in various processes including bimolecular separation, phase change heat transfer, and water desalination. There have been several studies of re-entrant geometries, and very few studies on retaining low surface tension liquids such as fluorocarbon-based dielectric liquids. Here, we study retention of a liquid with very low contact angles using borosilicate glass capillary tips. We analyzed capillary tips with outer diameters ranging from 250 to 840 µm and measured Laplace pressures up to 2.9 kPa. Experimental results agree well with a numerical model that predicts burst pressure (the maximum Laplace pressure for liquid retention), which is a function of the outer diameter (D) and capillary exit edge radius of curvature (r).

Rapid Slow Off-Rate Modified Aptamer (SOMAmer)-Based Detection of C-Reactive Protein Using Isotachophoresis and an Ionic Spacer.

We present an on-chip electrophoretic assay for rapid protein detection with a SOMAmer (Slow Off-Rate Modified Aptamer) reagent. We used isotachophoresis (ITP) coupled with an ionic spacer to both react and separate SOMAmer-protein complex from free SOMAmer reagent. ITP accelerates the reaction kinetics as the ionic spacer concurrently separates the reaction products. We developed a numerical and analytical model to describe ITP spacer assays, which involve low-mobility, nonfocusing targets that are recruited into the ITP zone by higher-mobility, ITP-focused probes. We demonstrated a proof-of-concept of this assay using C-reactive protein (CRP) in buffer, and achieved a 2 nM limit of detection (LOD) with a combined 20 min assay time (10 min off-chip preparation of reagents and 10 min on-chip run). Our findings suggest that this approach has potential as a simple and rapid alternative to other homogeneous immunoassays. We also explore the extension of this assay to a diluted serum sample spiked with CRP, where we observe decreased sensitivity (an LOD of 25 nM in 20-fold diluted serum). We describe the challenges in extending this assay to complex samples and achieving higher sensitivity and specificity for clinical applications.

Increasing hybridization rate and sensitivity of bead-based assays using isotachophoresis.

We present an electrokinetic technique to increase the reaction rate and sensitivity of bead-based assays. We use isotachophoresis (ITP) to preconcentrate and co-focus target molecules and beads into a single ITP zone. The process achieves rapid mixing, stirring, and strongly increases the binding reaction rate. We demonstrate our assay with quantitative detection of 24 nt single-stranded DNA over a dynamic range of three orders of magnitude and multiplexed detection of ten target species per sample. We show that ITP can achieve approximately the same sensitivity as a well-stirred standard reaction in 60-fold reduced reaction time (20 min versus 20 h). Alternately, compared to standard reaction times of 30 min, we show that 20 min ITP hybridization can achieve 5.3-fold higher sensitivity.