Information diffusion in online social networks: a simulation experiment

The advent of online social networks has completely transformed the way we communicate, with news, opinions, and ideas now spreading faster than ever before (Guille et al., 2013; Lee et al., 2022). That online social networks have a profound impact on the spread of information suggests further investigation of the relationship between network structure and information diffusion (Light & Moody, 2020). This honors thesis investigates degree assortativity – a measure of large-scale network structure that has often only been a footnote in relevant literature on infor- mation diffusion in online social networks – and its effect on the speed of informa- tion diffusion in online social networks. Two rewiring algorithms (Xulvi-Brunet & Sokolov, 2005) were applied to rewire a Facebook friend circle (n = 44) with varying degree assortativity, ranging from approximately −0.7 to 0.4. For each of the 160 rewired graphs, a random node was selected to infect (i.e., spread information to) its neighbors with probabilities ranging from 10 to 50 percent, and the number of infected nodes after each round of diffusion was recorded. Results suggest that degree assortativity and the speed of information dif- fusion have a strong inverse relationship – disassortative networks spread the same information faster. Moreover, degree assortativity appears to drive the speed of in- formation diffusion more than its correlates, clustering coefficient and average path length (Xulvi-Brunet & Sokolov, 2005)

Comets and Planetesimal Formation

In this chapter, we review the processes involved in the formation of planetesimals and comets. We will start with a description of the physics of dust grain growth and how this is mediated by gas-dust interactions in planet-forming disks. We will then delve into the various models of planetesimal formation, describing how these planetesimals form as well as their resulting structure. In doing so, we focus on and compare two paradigms for planetesimal formation: the gravitational collapse of particle over-densities (which can be produced by a variety of mechanisms) and the growth of particles into planetesimals via collisional and gravitational coagulation. Finally, we compare the predictions from these models with data collected by the Rosetta and New Horizons missions and that obtained via observations of distant Kuiper Belt Objects.Comment: Planetesimal Formation Review accepted for publication in Comets II

Use of a Robust Dehydrogenase from an Archael Hyperthermophile in Asymmetric Catalysis–Dynamic Reductive Kinetic Resolution Entry into (S)-Profens

Hyperthermophilic archaea are of great interest in evolutionary microbiology, owing to their ability to withstand high temperatures, and often extremes of pressure, pH and salinity. Enzymes from these organisms1 may offer particular opportunities for asymmetric synthesis, complementary to approaches with mesophilic enzymes,2 or those involving enzyme3 and pathway4 reengineering. However, perhaps due to a bias that hyperthermophilic enzymes have “narrow substrate specificities,”5 archaeal extremophiles remain a largely untapped resource in asymmetric synthesis.6 Herein, we disclose a remarkably general Dynamic Reductive Kinetic Resolution (DYRKR) entry into (S)-profens, including several important NSAIDs. The enzyme employed is alcohol dehydrogenase (ADH)-10, one of 13 annotated ADHs in the hyperthermophile Sulfolobus solfataricus. Protein phylogenetic analysis of this paralogous family indicates SsADH-10 is most closely related to homologues in distant taxa (Fig. 1). The highest identity between SsADH-10 and any other SsADHs is only 34%, suggesting that the SsADH family was established prior to the emergence of other archaeal lineages. Though not described as such, the SsADH-10 appears to be the only SsADH isozyme for which structural information is available in the pdb.

Use of a Robust Dehydrogenase from an Archael Hyperthermophile in Asymmetric Catalysis–Dynamic Reductive Kinetic Resolution Entry into (S)-Profens

Hyperthermophilic archaea are of great interest in evolutionary microbiology, owing to their ability to withstand high temperatures, and often extremes of pressure, pH and salinity. Enzymes from these organisms1 may offer particular opportunities for asymmetric synthesis, complementary to approaches with mesophilic enzymes,2 or those involving enzyme3 and pathway4 reengineering. However, perhaps due to a bias that hyperthermophilic enzymes have “narrow substrate specificities,”5 archaeal extremophiles remain a largely untapped resource in asymmetric synthesis.6 Herein, we disclose a remarkably general Dynamic Reductive Kinetic Resolution (DYRKR) entry into (S)-profens, including several important NSAIDs. The enzyme employed is alcohol dehydrogenase (ADH)-10, one of 13 annotated ADHs in the hyperthermophile Sulfolobus solfataricus. Protein phylogenetic analysis of this paralogous family indicates SsADH-10 is most closely related to homologues in distant taxa (Fig. 1). The highest identity between SsADH-10 and any other SsADHs is only 34%, suggesting that the SsADH family was established prior to the emergence of other archaeal lineages. Though not described as such, the SsADH-10 appears to be the only SsADH isozyme for which structural information is available in the pdb.

Field Theory Questions for String Theory Answers

We discuss the field theory of 3-brane probes in F-theory compactifications in two configurations, generalizing the work of Sen and of Banks, Douglas and Seiberg. One configuration involves several parallel 3-brane probes in F-theory compactified on $T^4/Z_2$, while the other involves a compactification of F-theory on $T^6/Z_2 x Z_2$ (which includes intersecting $D_4$ singularities). In both cases string theory provides simple pictures of the spacetime theory, whose implications for the three-brane world-volume theories are discussed. In the second case the field theory on the probe is an unusual N=1 superconformal theory, with exact electric-magnetic duality. Several open questions remain concerning the description of this theory.Comment: havmac, 24 pages, no figures; revised version; we have corrected the discussion concerning the relationship between the orientifolds and the F-theory compactification. Version to be published in Nucl. Phys.

Upscaling calcite dissolution rates in a tight reservoir sandstone

Calcite is a highly abundant mineral in the Earth’s crust and occurs as a cement phase in numerous siliciclastic sediments, where it often represents the most reactive component when a fluid percolates through the rock. Hence, the objective of this study is to derive calcite dissolution rates on different scales in a reservoir sandstone using mineral surface experiments combined with vertical scanning interferometry (VSI) and two types of core plug experiments. The 3D geometry of the calcite cement phase inside the rock cores was characterized by X-ray micro-computed tomography (µXCT) and was used to attempt dissolution rate upscaling from the mineral surface to the core scale. Initially (without upscaling), our comparison of the far-from-equilibrium dissolution rates at the mineral surface (µm-mm-scale, low fluid residence time) and the surface normalized dissolution rates obtained from the core experiments (cm-scale, high fluid residence time) revealed differences of 0.5–2 orders of magnitude. The µXCT geometric surface area connected to the open pore space (GSA$_{Cc,open}$) considers the fluid accessibility of the heterogeneously distributed calcite cement that can largely vary between individual samples, but greatly affects the effective dissolution rates. Using this parameter to upscale the rates from the µm- to the cm-scale, the deviation of the upscaled total dissolution rates from the measured total dissolution rates was less than one order of magnitude for all investigated rock cores. Thus, GSA$_{Cc,open}$ showed to be reasonably suitable for upscaling the mineral surface rates to the core scale

Simulating permeability reduction by clay mineral nanopores in a tight sandstone by combining computer X-ray microtomography and focussed ion beam scanning electron microscopy imaging

Computer X-ray microtomography (µXCT) represents a powerful tool for investigating the physical properties of porous rocks. While calculated porosities determined by this method typically match experimental measurements, computed permeabilities are often overestimated by more than 1 order of magnitude. This effect increases towards smaller pore sizes, as shown in this study, in which nanostructural features related to clay minerals reduce the permeability of tight reservoir sandstone samples. Focussed ion beam scanning electron microscopy (FIB-SEM) tomography was applied to determine the permeability effects of illites at the nanometre scale, and Navier–Stokes equations were applied to calculate the permeability of these domains. With these data, microporous domains (porous voxels) were defined using microtomography images of a tight reservoir sample. The distribution of these domains could be extrapolated by calibration against size distributions measured in FIB-SEM images. For this, we assumed a mean permeability for the dominant clay mineral (illite) in the rock and assigned it to the microporous domains within the structure. The results prove the applicability of our novel approach by combining FIB-SEM with X-ray tomographic rock core scans to achieve a good correspondence between measured and simulated permeabilities. This methodology results in a more accurate representation of reservoir rock permeability in comparison to that estimated purely based on µXCT images