Peter A. Kottke

Pressure-Viscosity Relationships for Elastohydrodynamics

The numerical simulation of elastohydrodynamic lubrication has evolved in terms of solution detail, speed and robustness. However, if EHL is to begin to solve practical quantitative problems involving differences among lubricants, practitioners must begin to utilize pressure-viscosity relationships in their numerical schemes that can describe real liquid response. The authors begin with the simple empiricisms that can be used to describe the pressure-viscosity response piecewise over the entire range from ambient to glass transition. Then various free volume formulations are introduced and compared. Finally, empirical expressions that can describe some essential features over the entire pressure range are presented. An example of the utility of one empirical equation is offered. Scheduled for Presentation at the 58th Annual Meeting in New York City April 28–May 1, 2003

Scanning mass spectrometry probe: a scanning probe electrospray ion source for imaging mass spectrometry of submerged interfaces and transient events in solution.

The scanning mass spectrometry (SMS) probe is a new electrospray ion source. Motivated by the need for untargeted chemical imaging of dynamic events in solution, we have exploited an approach to electrospray ionization (ESI) that allows continuous sampling from a highly localized volume (approximately picoliters) in a liquid environment, softly ionizes molecules in the sample to render them amenable for mass spectrometric analysis, and sends the ions to the mass spectrometer. The key underlying concepts for our approach are (1) treating the electrospray capillary inlet as a chemical scanning probe and (2) locating the electrospray point as close as possible to the sampling point, thus providing the shortest response time possible. This approach enables chemical monitoring or imaging of submerged interfaces, providing access to details of spatial heterogeneity and temporal changes within liquid samples. It also permits direct access to liquid/ liquid interfaces for ESI-MS analysis. In this letter we report the first demonstrations of these capabilities of the SMS probe and describe some of the probes basic characteristics.

Rapid Electron Beam Writing of Topologically Complex 3D Nanostructures Using Liquid Phase Precursor.

Advancement of focused electron beam-induced deposition (FEBID) as a versatile direct-write additive nanoscale fabrication technique has been inhibited by poor throughput, limited choice of precursors, and restrictions on possible 3D topologies. Here, we demonstrate FEBID using nanoelectrospray liquid precursor injection to grow carbon and pure metal nanostructures via direct decomposition and electrochemical reduction of the relevant precursors, achieving growth rates 10(5) times greater than those observed in standard gas-phase FEBID. Initiating growth at the free surface of a liquid pool enables fabrication of complex 3D carbon nanostructures with strong adhesion to the substrate. Deposition of silver microstructures at similar growth rates is also demonstrated as a promising avenue for future development of the technique.

THE MEASUREMENT OF VISCOSITY OF LIQUIDS UNDER TENSION

The rheological significance of a state of hydrostatic tension was investigated. A method for measuring the limiting low shear viscosity of liquids under tension was developed. The ability of nine liquids to withstand tension was verified, and the magnitudes of tension achievable through different methods were compared. The use of viscosity data from liquids under tension to more accurately determine the initial pressure viscosity coefficient was investigated. The continuity of the pressure viscosity coefficient across absolute zero pressure was verified.

A Review of Two-Phase Forced Cooling in Three-Dimensional Stacked Electronics: Technology Integration

Three-dimensional (3D) stacked electronics present significant advantages from an electrical design perspective, ranging from shorter interconnect lengths to enabling heterogeneous integration. However, multitier stacking exacerbates an already difficult thermal problem. Localized hotspots within individual tiers can provide an additional challenge when the high heat flux region is buried within the stack. Numerous investigations have been launched in the previous decade seeking to develop cooling solutions that can be integrated within the 3D stack, allowing the cooling to scale with the number of tiers in the system. Two-phase cooling is of particular interest, because the associated reduced flow rates may allow reduction in pumping power, and the saturated temperature condition of the coolant may offer enhanced device temperature uniformity. This paper presents a review of the advances in two-phase forced cooling in the past decade, with a focus on the challenges of integrating the technology in high heat flux 3D systems. A holistic approach is applied, considering not only the thermal performance of standalone cooling strategies but also coolant selection, fluidic routing, packaging, and system reliability. Finally, a cohesive approach to thermal design of an evaporative cooling based heat sink developed by the authors is presented, taking into account all of the integration considerations discussed previously. The thermal design seeks to achieve the dissipation of very large (in excess of 500 W/cm2) background heat fluxes over a large 1 cm × 1 cm chip area, as well as extreme (in excess of 2 kW/cm2) hotspot heat fluxes over small 200 μm × 200 μm areas, employing a hybrid design strategy that combines a micropin–fin heat sink for background cooling as well as localized, ultrathin microgaps for hotspot cooling.

Ambient nanoelectrospray ionization with in-line microdialysis for spatially resolved transient biochemical monitoring within cell culture environments.

We have developed a new mass spectrometry (MS) based approach for continuous, spatially resolved in vitro biochemical detection and demonstrated its utility in a 3-D cell culture system. Extracellular liquid is passively extracted at a low flow rate (~10 nL/s) through a small bore silica capillary (ID 50 μm); inline microdialysis (MD) removes ions that would interfere with mass spectrometric analysis, and the sample is ionized by nanoelectrospray ionization (nano-ESI) and mass analyzed in a time-of-flight mass spectrometer. The system successfully detects low-volume, low-concentration releases of a small protein (8 μL of 5 μM cytochrome-c, molecular mass ~12 kDa) and exhibits ~1 min temporal resolution. The system also displays sensitivity to probe proximity to the sample release point. Due to the sensitivity of ESI-MS and its ability to simultaneously detect and identify multiple unanticipated biochemicals, this approach shows considerable potential as a biomarker discovery tool.

Microfabricated ultrarapid desalting device for nanoelectrospray ionization mass spectrometry.

Salt removal is a prerequisite for electrospray ionization mass spectrometry (ESI-MS) analysis of biological samples. Rapid desalting and a low volume connection to an electrospray tip are required for time-resolved measurements. We have developed a microfabricated desalting device that meets both requirements, thus providing the foundational technology piece for transient ESI-MS measurements of complex biological liquid specimens. In the microfabricated device, the sample flows in a channel separated from a higher flow rate, salt-free counter solution by a monolithically integrated nanoporous alumina membrane, which can support pressure differences between the flow channels of over 600 kPa. Salt is removed by exploiting the large difference in diffusivities between salts and the typical ESI-MS target bioanalytes, e.g., peptides and proteins. We demonstrate the capability to remove 95% of salt from a sample solution in ∼1 s while retaining sufficiently high concentration of a relatively low molecular weight protein, cytochrome-c, for ESI-MS detection.

Scale Analysis of Combined Thermal Radiation and Convection Heat Transfer

Simple order-of-magnitude relationships predicting heat transfer in limiting cases of combined convection and radiation problems in boundary layer flows are found using the method of scale analysis. Key nondimensional groups are developed that help identify the fundamental interactions between thermal radiation and convection modes of heat transfer for two cases of laminar, two-dimensional convection heat transfer from a semi-infinite vertical wall. The first case is that of heat transfer from a gray wall with a specified heat flux to a surrounding non-participating medium and considers both forced and natural convection effects. The second case is that of heat transfer from a black wall with a specified temperature to an optically thick, gray, nonscattering medium and considers forced, natural, and mixed convection effects. The scale analysis results are presented in terms of the local Nusselt number. They include closed form expressions for order-of-magnitude estimates of the heat transfer rate, and dimensionless parameters that indicate the dominant mode of heat transfer and correlate experimental data well

Two-Phase Convective Cooling for Ultrahigh Power Dissipation in Microprocessors

We present results of modeling for the design of microgaps for the removal of high heat fluxes via a strategy of high mass flux, high quality, and two-phase forced convection. Modeling includes (1) thermodynamic analysis to obtain performance trends across a wide range of candidate coolants, (2) evaluation of worst-case pressure drop due to contraction and expansion in inlet/outlet manifolds, and (3) 1D reduced-order simulations to obtain realistic estimates of different contributions to the pressure drops. The main result is the identification of a general trend of improved heat transfer performance at higher system pressure.

Multiscale Mass Transport in Porous Silicon Gas Sensors

Porous silicon (PS) is a material that has garnered considerable research attention over the past 15 years. It is formed by the dissolution of single crystalline silicon. The resulting materials morphology depends upon the silicon doping and the dissolution process. The dissolution process can be varied by changing the applied current and illumination, solvent conditions, and etching time, producing a diverse range of pore diameters (1–12) which can be made to vary from the 1 to 10 nm2–6 range (nanoporous silicon) to sizes in the 1–3 μm range (9) (microporous silicon). Interestingly, different dissolution processes lead to very different pore sizes. One can fabricate a range of hybrid structures between two limiting well-defined PS morphologies: (1) PS fabricated from aqueous electrolytes which consists of highly nanoporous, structures, and (2) PS fabricated from nonaqueous electrolytes, which is comprised of open and accessible microporous structures with deep, wide, well-ordered channels that display a crystalline Si (100) influenced pyramidal termination. The ability to control the interplay of these two regimes of porosity provides a means to exploit both the bulk and surface properties of the resulting porous membrane. In fact, the hybrid microporous/nanoporous structure etched into a silicon framework as depicted in Fig. 1, representing an extrapolation of the Probst and Kohl study (10), provides a useful platform for the construction of a conductometric PS-based sensor. All dissolution processes seem to result in mono- or bidisperse pore size distributions (13), with the typical diameters for the two sizes of pores being of the order ∼ 1 μm and <20 nm. In this chapter, the larger (∼ 1 μm pores) will be called micropores, and the smaller (<20 nm) pores will be called nanopores. This terminology is not universal! For monodisperse pore diameter porous silicon, either micro or nanopores may be present. Because the synthesis conditions that lead to a given morphology have been much perfected, reproducible PS production is now possible, a feature that is necessary for practical utility.