A note on compactly generated co-t-structures

The idea of a co-t-structure is almost "dual" to that of a t-structure, but with some important differences. This note establishes co-t-structure analogues of Beligiannis and Reiten's corresponding results on compactly generated t-structures.Comment: 10 pages; details added to proofs, small correction in the main resul

Thermal Conductivity of Ultra High Molecular Weight Polyethylene: From Fibers to Fabrics

Unique combinations of properties such as mechanical compliance and chemical stability make polymers attractive for many applications. However, the intrinsic low thermal conductivity of bulk polymers has generally limited their potential for heat dissipation applications, and in fact they are widely used as thermal insulators. But in recent years, gel-spun, ultraoriented fibers made of ultrahigh molecular weight polyethylene (UHMW-PE) have sparked interest in the thermal management community due to their exceptionally high thermal conductivity. These fibers are typically used in commercially produced protective gear such as motorcycle jackets and ballistic vests due to their high mechanical strength, but they have not been widely utilized for heat spreading and thermal management applications. While recent studies have characterized individual fibers and ultradrawn films, the thermal properties of fabrics constructed from these materials remain virtually unexplored. Here, we synthesize plain-weave fabrics from yarns of commercially available gel-spun UHMW-PE and measure the thermal properties of the individual microfibers, yarns, and woven fabrics using an in-house thermal characterization technique based on infrared microscopy. For the woven fabric, we report an effective in-plane thermal conductivity of ∼10 W m−1 K−1 in the direction aligned with the weft yarns, which is 2−3 orders of magnitude higher than conventional textile materials. This work reveals the high thermal conductivity of UHMW-PE fabrics that can be realized by using a scalable textile manufacturing platform and lays the foundation for exploiting their unique thermomechanical properties for heat spreading functions in flexible/wearable devices

Identification of Nucleate Boiling as the Dominant Heat Transfer Mechanism during Confined Two-Phase Jet Impingement

Thermal management of high-power electronics requires cooling strategies capable of dissipating high heat fluxes while maintaining the device at low operating temperatures. Two-phase jet impingement offers a compact cooling technology capable of meeting these requirements at a low pressure drop. Generally, confined impingement geometries are used in electronics cooling applications, where the flow is constrained between the hot surface and orifice plate. Understanding the primary heat transfer mechanisms occurring as boiling takes place on the surface during jet impingement is important, specifically under such confined conditions. In this study, heat transfer from a copper surface is experimentally characterized in both confined jet impingement and pool boiling configurations. The dielectric liquid HFE- 7100 is used as the working fluid. For the jet impingement configuration, the jet issues through a single 2 mm-diameter orifice, at jet exit velocities of 1, 3, 6, and 9 m/s, into a confinement gap with a spacing of 3 jet diameters between the orifice and heat source. Additional orifice-to-target spacings of 0.5, 1, and 10 jet diameters are tested at the lowest (Vj = 1 m/s) and highest (Vj = 9 m/s) jet velocities. By incrementing the heat flux applied to the surface and observing the steady-state response at each flux, the single-phase and two-phase heat transfer performance is characterized; all experiments were carried through to critical heat flux conditions. The jet impingement data in the fully boiling regime either directly overlap the pool boiling data, or coincide with an extension of the trend in pool boiling data beyond the pool boiling critical heat flux limit. This result confirms that nucleate boiling is the dominant heat transfer mechanism in the fully boiling regime in confined jet impingement; the convective effects of the jet play a negligible role over the wide range of parameters considered here. While the presence of the jet does not enhance the boiling heat transfer coefficient, the jet does greatly increase single-phase heat transfer performance and extends the critical heat flux limit. Critical heat flux displays a linear dependence on jet velocity while remaining insensitive to changes in the orifice-to-target spacing

Grindr Users Take More Risks, but Are More Open to Human Immunodeficiency Virus (HIV) Pre-exposure Prophylaxis: Could This Dating App Provide a Platform for HIV Prevention Outreach?

BackgroundTechnology has changed the way that men who have sex with men (MSM) seek sex. More than 60% of MSM in the United States use the internet and/or smartphone-based geospatial networking apps to find sex partners. We correlated use of the most popular app (Grindr) with sexual risk and prevention behavior among MSM.MethodsA nested cohort study was conducted between September 2018 and June 2019 among MSM receiving community-based human immunodeficiency virus (HIV) and sexually transmitted infection (STI) screening in central San Diego. During the testing encounter, participants were surveyed for demographics, substance use, risk behavior (previous 3 months), HIV pre-exposure prophylaxis (PrEP) use, and Grindr usage. Participants who tested negative for HIV and who were not on PrEP were offered immediate PrEP.ResultsThe study included 1256 MSM, 1090 of whom (86.8%) were not taking PrEP. Overall, 580 of 1256 (46%) participants indicated that they used Grindr in the previous 7 days. Grindr users reported significantly higher risk behavior (greater number of male partners and condomless sex) and were more likely to test positive for chlamydia or gonorrhea (8.6% vs 4.7% of nonusers; P = .005). Grindr users were also more likely to be on PrEP (18.7% vs 8.7% of nonusers; P < .001) and had fewer newly diagnosed HIV infections (9 vs 26 among nonusers; P = .014). Grindr users were also nearly twice as likely as nonusers to initiate PrEP (24.6% vs 14%; P < .001).ConclusionsGiven the higher risk behavior and greater acceptance of PrEP among MSM who used Grindr, Grindr may provide a useful platform to promote HIV and STI testing and increase PrEP uptake

A super-analogue of Kontsevich's theorem on graph homology

In this paper we will prove a super-analogue of a well-known result by Kontsevich which states that the homology of a certain complex which is generated by isomorphism classes of oriented graphs can be calculated as the Lie algebra homology of an infinite-dimensional Lie algebra of symplectic vector fields.Comment: 15 page

The wetting state of water on a rose petal

The rose petal features surface structures that offer unique wetting properties. A water droplet placed on a rose petal forms a high contact angle but exhibits significant contact angle hysteresis, such that relatively large droplets remain stuck to the surface when it is tilted. Understanding this distinctive ‘parahydrophobic’ wetting behavior can provide insight into the design of highly nonwetting, yet highly adhesive, synthetic surfaces. Surface features of the rose petal are characterized via focused ion beam scanning electron microscopy, which reveals microscale papillae that are partially covered with nanoscale striae. The wetting state of water on a rose petal is directly visualized using confocal microscopy. This experimental evidence confirms the microscale wetting behavior on the papillae, but cannot resolve the wetting behavior on the nanoscale striae. To infer the wetting state on the striae, an energyminimization- based model is developed and the results from the model are compared to the experimental evidence. In combination, the experimental findings and the model results reveal the wetting state of water on the hierarchical surface structure and explain the macroscopic wetting behavior of the rose petal

Evaporation-Driven Micromixing in Sessile Droplets for Miniaturized Absorbance-Based Colorimetry

We demonstrate the use of an evaporating, sessile droplet on a nonwetting substrate as a miniature micromixing device to conduct sample−dye reactions for absorbance-based colorimetry. The nonwetting substrate supports buoyancy-induced mixing inside the droplet for rapid completion of the measurement. The Bradford assay is used as a proof of concept, where a protein-containing sample is reacted with a reagent dye to measure the protein concentration. Viability of absorbance measurement through the droplet is first established using droplets in which the reactants are mixed prior to their deposition onto the substrate. In a second set of experiments involving in situ mixing, the reagent is directly added to a sessile droplet of the protein-containing sample, allowing the reactants to mix while the absorbance is being measured. Interplay between buoyancyinduced mixing, protein−reagent reaction, and protein adsorption onto the substrate leads to a complex temporal absorbance measurement signal. Videos corresponding to the signal data show that each of these mechanisms dominates during different phases of droplet evolution, causing a signal pattern containing peaks and valleys having a strong monotonic trend with the protein concentration. Overall, the second absorbance peak at which the reaction nears completion is the most sensitive to sample concentration. Heating of the substrate is demonstrated to dramatically speed up the mixing process. These protein concentration measurements, obtained with a simpler system and low reactant volumes, demonstrate that this droplet micromixing concept is a viable alternative to microtiter plates for colorimetric applications

Effect of Superhydrophobic Surface Morphology on Evaporative Deposition Patterns

Prediction and active control of the spatial distribution of particulate deposits obtained from sessile droplet evaporation are vital in printing, nanostructure assembly, biotechnology, and other applications that require localized deposits. This Letter presents surface wettability-based localization of evaporation-driven particulate deposition and the effect of superhydrophobic surface morphology on the distribution of deposits. Sessile water droplets containing suspended latex particles are evaporated on non-wetting textured surfaces with varying microstructure geometry at ambient conditions. The droplets are visualized throughout the evaporation process to track the temporal evolution of contact radius and apparent contact angle. The resulting particle deposits on the substrates are quantitatively characterized. The experimental results show that superhydrophobic surfaces suppress contact-line deposition during droplet evaporation, thereby providing an effective means of localizing the deposition of suspended particles. A correlation between deposit size and surface morphology, explained in terms of the interface pressure balance at the transition between wetting states, reveals an optimum surface morphology for minimizing the deposit coverage area