Publications

Many researchers have argued that the Internet architecture would be more robust and more accommodating of heterogeneity if routers allocated bandwidth fairly. However, most of the mechanisms proposed to accomplish this, such as Fair Queueing [16, 6] and its many variants [2, 23, 15], involve complicated packet scheduling algorithms. These algorithms, while increasingly common in router designs, may not be inexpensively implementable at extremely high speeds; thus, finding more easily implementable variants of such algorithms may be of significant practical value. This paper proposes an algorithm called Approximate Fair Dropping (AFD), which bases its dropping decisions on the recent history of packet arrivals. AFD retains a simple forwarding path and requires an amount of additional state that is small compared to current packet buffers. Simulation results, which we describe here, suggest that the design provides a reasonable degree of fairness in a wide variety of operating conditions. The performance of our approach is aided by the fact that the vast majority of Internet flows are slow but the fast flows send the bulk of the bits. This allows a small sample of recent history to provide accurate rate estimates of the fast flows.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Nitinol's superelasticity, biocompatibilty and magnetic resonance compatibility has led to its increasing popularity for the manufacture of endovascular stents. Despite its growing importance in the biomedical industry, especially under conditions involving pulsatile loading, the cyclic fatigue resistance of Nitinol is not well understood. This paper provides a summary of existing data on the fatigue-crack propagation properties of the superelastic alloy and presents preliminary results on crack-growth behavior in samples cut from Nitinol tube used as the starting material for the manufacture of endovascular stents. In addition, a new test sample for measuring the total fatigue life properties is described involving the uniaxial loading of slotted-tubular samples, again machined from Nitinol tube.

Engineering materials exhibit an undesirable tradeoff between strength and resistance to crack propagation (fracture toughness). Here we demonstrate how this tradeoff can be circumvented by thermo-mechanical processing that produces a partially recrystallized, heterogeneous microstructure. An equimolar CrCoNi alloy was forged at room temperature (298 K) to produce high densities of three-dimensional crystallographic defect networks. Post-deformation heat treatments caused localized recrystallization that resulted in a bimodal microstructure with hard, non-recrystallized grains and soft, recrystallized grains. In this condition, the yield strength at 298 K is 2.75x the values previously obtained for the same alloy in the fully recrystallized state while the fracture toughness remains the same. The yield strength is further enhanced at 77 K without compromising the fracture toughness. This outstanding strength-toughness combination at 77 K exceeds those reported for other metallic materials and appears to result from the composite nature of the microstructure with non-recrystallized grains providing strength and recrystallized grains enabling plasticity that dissipates stresses during crack propagation. Our findings indicate that by tuning the degree of recrystallization through thermomechanical processing techniques, it will be possible to further expand the envelope bounding the strength and toughness of a range of structural metals at engineering component scales. Research into engineering alloys is often driven by the need to simultaneously improve strength and toughness. Here, an equimolar CrCoNi medium-entropy alloy achieves an almost three times increase in yield strength without sacrificing toughness, attributed to a partially recrystallized microstructure.

Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS). While the reliability of polysilicon MEMS has received extensive attention, the mechanisms responsible for this failure mode at the microscale have yet to be conclusively determined. We have used on-chip polycrystalline silicon side-wall friction MEMS specimens to study active mechanisms during sliding wear in ambient air. Worn parts were examined by analytical scanning and transmission electron microscopy, while local temperature changes were monitored using advanced infrared microscopy. Observations show that small amorphous debris particles ({approx}50-100 nm) are removed by fracture through the silicon grains ({approx}500 nm) and are oxidized during this process. Agglomeration of such debris particles into larger clusters also occurs. Some of these debris particles/clusters create plowing tracks on the beam surface. A nano-crystalline surface layer ({approx}20-200 nm), with higher oxygen content, forms during wear at and below regions of the worn surface; its formation is likely aided by high local stresses. No evidence of dislocation plasticity or of extreme local temperature increases was found, ruling out the possibility of high temperature-assisted wear mechanisms.

We report a mitochondrial-targeted activity-based sensor for formaldehyde, MitoRFAP-2, and apply this reagent to probe sources and sinks of the mitochondrial one-carbon pool.

Modern system design often employs virtualization to decouple the system service model from its physical realization. Two common examples are the virtualization of computing resources through the use of virtual machines and the virtualization of disks by presenting logical volumes as the storage interface. The insertion of these abstraction layers allows operators great flexibility to achieve operational goals divorced from the underlying physical infrastructure. Today, workloads can be instantiated dynamically, expanded at runtime, migrated between physical servers (or geographic locations), and suspended if needed. Both computation and data can be replicated in real time across multiple physical hosts for purposes of high-availability within a single site, or disaster recovery across multiple sites.