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Structure–Thermodynamic Relationship of a Polysaccharide Gel (Alginate) as a Function of Water Content and Counterion Type (Na vs Ca)
No full textBiofilms are the predominant mode of microbial life on Earth, and so a deep understanding of microbial communities and their impacts on environmental processes requires a firm understanding of biofilm properties. Because of the importance of biofilms to their microbial inhabitants, microbes have evolved
different ways of engineering and reconfiguring the matrix of extracellular polymeric substances (EPS) that constitute the main non-living component of biofilms. This ability makes it difficult to distinguish between the biotic and abiotic origins of biofilm properties. An important route toward establishing this distinction
has been the study of simplified models of the EPS matrix. This study builds on such efforts by using atomistic simulations to predict the nanoscale (≤10 nm scale) structure of a model EPS matrix and the sensitivity of this structure to interpolymer interactions and water content. To accomplish this, we use replica exchange molecular dynamics (REMD) simulations to generate all-atom configurations of ten 3.4 kDa alginate polymers at a range of water contents and Ca−Na ratios. Simulated systems are solvated with explicitly modeled water molecules, which allows us to capture the discrete structure of the hydrating water and to examine the thermodynamic stability of water in the gels as they are progressively dehydrated. Our primary findings are that (i) the structure of the hydrogels is highly sensitive to the identity of the charge-compensating cations, (ii) the thermodynamics of water within the gels (specific enthalpy and free energy) are, surprisingly, only weakly sensitive to cation identity, and (iii) predictions of the differential enthalpy and free energy of hydration include a shortranged enthalpic term that promotes hydration and a longer-ranged (presumably entropic) term that promotes dehydration, where short and long ranges refer to distances shorter or longer than ∼0.6 nm between alginate strands
ZeroWaste — Towards Computing Cooperative Robotic Sequences for the Disassembly and Reuse of Timber Frame Structures.
No full textZeroWaste is a project about repositioning existing timber building stock within a circular economy framework. Rather than disposing of these buildings at the end of their life, the goal is to view them as stores of valuable resources that can be readily reused. By doing this, material life cycle becomes an integral design consideration alongside planning for the efficient disas- sembly and reuse of these structures. In this paper, the computational workflow is presented for the first phase of the project: planning a cooperative robotic disassembly sequence for the scaf- fold-free removal of members from existing timber structures. A pavilion-scale prototype is first constructed, in the Embodied Computation Lab at Princeton University, to represent an existing timber structure built according to conventional North American stick frame construction prac- tices. A multi-directed graph data structure, representing structural member connectivity and support hierarchy, is then coupled with a breadth-first search algorithm to plan potential scaf- fold-free robotic disassembly sequences given a member removal target. In parallel, computer vision is integrated and implemented through the robotic setup to create an accurate as-built point cloud scan of the whole structure. This as-built information is then used to inform the evaluation of potential robotic sequences from the point of view of robotic reachability and structural performance. This work-in-progress paper first presents a high-level overview of the various components in this workflow, followed by its demonstration in planning the removal of a specific member in the prototype structure. Upcoming project developments will include the planning, and physical demonstration, of more complex disassembly sequences, coupled with reassembly and reuse of the removed members for various regions of the prototype structure
Neutral Silicon-Vacancy Centers in Diamond via Photoactivated Itinerant Carriers
No full textNeutral silicon-vacancy (Si-V0) centers in diamond are promising candidates for quantum network
applications because of their exceptional optical properties and spin coherence. However, the stabilization
of Si-V0 centers requires careful Fermi-level engineering of the diamond host material, making further
technological development challenging. Here, we show that Si-V0 centers can be efficiently stabilized by
photoactivated itinerant carriers. Even in this nonequilibrium configuration, the resulting Si-V0 centers
are stable enough to allow for resonant optical excitation and optically detected magnetic resonance. Our
results pave the way for on-demand generation of Si-V0 centers as well as other emerging quantum defects
in diamond
Rallying Around the Flag: War Challenges and Civic Mobilization in Ukraine
No full textUkraine’s resilience in the first months of Russian aggression came as a great surprise to both its Moscow adversaries and Western partners. Very few experts expected Ukraine to withstand the all-out military assault of the alleged second-best army in the world, and virtually no one believed that it would be able to fight back. An overblown image of Russian strength and military prowess may be one reason for this, but probably more significant was a protracted neglect and depreciation of Ukraine in both Western media and political circles. All of a sudden, it appeared that neither the state—broadly described as weak, corrupt, and dysfunctional—collapsed under the tremendous military assault, nor the society—broadly stereotyped as divided, conflicted, and arguably balancing at the verge of a civil war—broke down for the proverbial two parts. One may presume either that some negative features and tendencies of Ukraine’s development were exaggerated or that some positive tendencies were neglected, undermined, or both. To elucidate the issue, I proceed in three steps. First, I outline briefly the real curses that plagued the Ukrainian state and society after the fall of communism and provide some reasons for international skepticism in regard to the newborn country. Second, I argue that very important and mostly positive (although incoherent and sluggish) changes had occurred in Ukraine in the past 30 years, so that the Russian aggression neither established any new patterns for Ukraine’s development, nor shifted the country into a new direction, but rather accelerated the prior processes and solidified the existing tendencies. Third, I examine the ongoing civic mobilization in Ukraine as a way of accumulating social capital that may play a crucial role in Ukraine’s postwar reconstruction and modernization
Charge-state stability of color centers in wide band gap semiconductors
No full textThe NV− color center in diamond has been extensively investigated for applications in quantum sensing, computation, and communication. Nonetheless, charge-state decay from the NV − to its neutral counterpart the NV 0 detrimentally affects the robustness of the NV − center and remains to be fully overcome. In this work, we provide an ab initio formalism for accurately estimating the rate of charge-state decay of color centers in wide band gap semiconductors. Our formalism employs density functional theory calculations in the context of thermal equilibrium. We illustrate the method using the transition of NV − to NV 0 in the presence of substitutional N [see Z. Yuan et al., Phys. Rev. Res. 2, 033263 (2020)]
Why MagNet: Quantifying the Complexity of Modeling Power Magnetic Material Characteristics
No full textThis paper motivates the development of sophisticated data-driven models for power magnetic material characteristics. Core losses and hysteresis loops are critical information in the design process of power magnetics, yet the physics behind them is not fully understood. Both losses and hysteresis loops
change for each magnetic material, are highly nonlinear, and depend heavily on the electrical operating conditions (e.g., waveform, frequency, amplitude, dc bias), the mechanical properties (e.g., pressure, vibration), as well as temperature and geometry of the magnetic components. Understanding the complexity of these factors is important for the development of accurate models and their applicability and limitations. Existing studies on power magnetics are usually developed based on a small amount of data and do not reveal the full magnetic behavior across a wide range of operating conditions. In this paper, based on a recently developed large-scale open-source database – MagNet – the core losses and hysteresis loops of Mn-Zn ferrites are analyzed over a wide range of amplitudes, frequencies, waveform shapes, dc bias levels, and temperatures, to quantify the complexity of modeling magnetic core losses, amplitude permeability, and hysteresis loops and provide guidelines for modeling power magnetics with data-driven methods
Microphysics of liquid water in sub-10 nm ultrafine aerosol particles
No full textUltrafine aerosol particles with sizes smaller than 50 nm have been shown in recent studies to serve as a large source of cloud condensation nuclei that can promote additional cloud droplet formation under supersaturation conditions. Knowledge of the microphysics of liquid water in these droplets remains limited, particularly in the sub-10 nm particle size range, due to experimental and theoretical challenges associated with the complexity of aerosol components and the small length scales of interest (e.g., difficulty of precisely sampling the liquid–air interface, questionable validity of mean-field theoretical representations). Here, we carried out molecular dynamics (MD) simulations of aerosol particles with diameters between 1 and 10 nm and characterized atomistic-level structure and water dynamics in well-mixed and phase-separated systems with different particle sizes, NaCl salinities, and pimelic acid (PML) organic surface loadings as a function of distance from the time-averaged Gibbs dividing interface or instantaneous water–air interface. We define a sphericity factor (ϕ) that can shed light on the phase-mixing state of nanodroplets, and we reveal an unexpected dependence of mixing state on droplet size. Our results also evidence an ion concentration enhancement in ultrafine aerosols, which should modulate salt nucleation kinetics in sub-10 nm droplets, and provide detailed characterization of the influence of droplet size on surface tension and on water self-diffusivity near the interface. Analysis of water evaporation free energy and water activity demonstrates the validity of the Kelvin equation and Köhler theory at droplet sizes larger than 4 nm under moderate salinities and organic loadings and the need for further extension to account for ion concentration enhancement in sub-10 nm aerosols, droplet-size-dependent phase separation effects, and a sharp decrease in the cohesiveness of liquid water in sub-4 nm droplets. Finally, we show that an idealized fractional surface coating factor (fs) can be used to categorize and reconcile water accommodation coefficients (α*) observed in MD simulations and experimental results in the presence of organic coatings, and we resolve the droplet size dependence of α*
From Ludendorff to Lenin? World War I and the Origins of Soviet Economic Planning
No full textThe planned economy was a defining element of the Bolshevik dictatorship. In contrast to scholars who have located its intellectual roots in the classic texts of nineteenth-century Marxism, this essay situates the origins of economic planning in World War I. The text analyses the link between war and planning in Russian and German thought. In doing so, we argue that the Bolsheviks’ positive assessment of the techniques of wartime mobilisation, influenced by the work of German economist Rudolf Hilferding, was foundational to their vision of organising the economy through the state
Trions in twisted bilayer graphene
No full textThe strong coupling phase diagram of magic angle twisted bilayer graphene (TBG) predicts a
series of exact one particle charge ±1 gapped excitations on top of the integer-filled ferromagnetic
ground-states. Finite-size exact diagonalization studies showed that these are the lowest charge ±1
excitations in the system (for 10nm screening length), with the exception of charge +1 at filling −1
in the chiral limit. In the current paper we show that this “trion bound state”, a 3-particle, charge 1
excitation of the insulating ferromagnetic ground-state of the projected Hamiltonian of TBG is the
lowest charge +1 overall excitation at ν = −1, and also for some large (≈ 20nm) screening lengths at
ν = −2 in the chiral limit and with very small binding energy. At other fillings, we show that trion
bound states do exist, but only for momentum ranges that do not cover the entire moir´e Brillouin
zone. The trion bound states (at different momenta) exist for finite parameter range w0/w1 but they
all disappear in the continuum far below the realistic values of w0/w1 = 0.8. We find the conditions
for the existence of the trion bound state, a good variational wavefunction for it, and investigate its
behavior for different screening lengths, at all integer fillings, on both the electron and hole sides
Dynamical response of a pinned two-dimensional Wigner crystal
No full textWe re-examine a long-standing problem of a finite-frequency conductivity of a weakly pinned two-dimensional classical Wigner crystal. In this system an inhomogeneously broadened absorption line (pinning mode) centered at disorder and magnetic field dependent frequency is known to appear. We show that the relative linewidth of the pinning mode is of the order of one in weak magnetic fields, exhibits a power-law decrease in intermediate fields, and eventually saturates at a small value in strong magnetic fields. The linewidth narrowing is due to a peculiar mechanism of mixing between the stiffer longitudinal and the softer transverse components of the collective excitations. The width of the high-field resonance proves to be related to the density of states in the low-frequency tail of the zero-field phonon spectrum. We find a qualitative agreement with recent experiments and point out differences from the previous theoretical work on the subject