5632 research outputs found
Sort by
Effect of Salts on the Formation and Hypervelocity-Induced Fragmentation of Icy Clusters with Embedded Amino Acids
No full textThe search for biomolecules via flyby or orbiting missions is prime for hypervelocity sampling where there is a water-rich plume or exosphere that can be sampled without landing (e.g., Europa, Enceladus, and possibly Triton). Under hypervelocity conditions, meaning relative speeds of km/s, these molecules may fragment upon impact with spacecraft surfaces or instrument inlets in ways that are not fully understood, potentially leading to incorrect identification and/or quantitation. Experiments on earth have attempted to reproduce the fragmentation process; however, accelerating single neutral molecules above several km/s over short distances (in a lab) is extremely challenging, and even if successful, such experiments are hard-pressed to yield insights into molecular reaction pathways. In this work, we use first-principles-based simulations to describe the effect of salts in the hypervelocity fragmentation processes of the amino acids arginine (Arg), alanine (Ala), and histidine (His) when encased in ice grains at different concentrations of sodium chloride (NaCl), between 0.25 and 2.0 M, under normal impacts at velocities between 3 and 10 km/s. We find that salt ions affect the fragmentation pathways and velocity thresholds of encased amino acids. Although most fragmentation starts by 3 km/s, the salinity effect can be considered second order, compared to differences resulting from velocity. These changes are attributed to weak interactions between Na⁺ and Cl⁻ with particular amino acid groups, during the flash-freezing process of ejected particles from Enceladus (and possibly Europa). Such interactions may weaken amino acid bonds (e.g., N–H), electrostatically shield them from surrounding waters undergoing high-strain rates, change the amino acid placement and conformation within the ice clusters (due to salting-in and salting-out effects), or disrupt the mechanical response of the ice clusters (interfere with hydrogen-bond network). These effects become more pronounced at higher velocities and provide valuable information for the interpretation of data from the Cassini spacecraft, and motivate future missions to characterize ocean worlds via hypervelocity sampling of atmospheres and plumes
A review of current state-of-the-art control methods for lower-limb powered prostheses
No full textLower-limb prostheses aim to restore ambulatory function for individuals with lower-limb amputations. While the design of lower-limb prostheses is important, this paper focuses on the complementary challenge—the control of lower-limb prostheses. Specifically, we focus on powered prostheses, a subset of lower-limb prostheses, which utilize actuators to inject mechanical power into the walking gait of a human user.
In this paper, we present a review of existing control strategies for lower-limb powered prostheses, including the control objectives, sensing capabilities, and control methodologies. We separate the various control methods into three main tiers of prosthesis control: High-level control for task and gait phase estimation, mid-level control for desired torque computation (both with and without the use of reference trajectories), and low-level control for enforcing the computed torque commands on the prosthesis. In particular, we focus on the high- and mid-level control approaches in this review. Additionally, we outline existing methods for customizing the prosthetic behavior for individual human users. Finally, we conclude with a discussion on future research directions for powered lower-limb prostheses based on the potential of current control methods and open problems in the field
Experimental cheat-sensitive quantum weak coin flipping
No full textAs in modern communication networks, the security of quantum networks will rely on complex cryptographic tasks that are based on a handful of fundamental primitives. Weak coin flipping (WCF) is a significant such primitive which allows two mistrustful parties to agree on a random bit while they favor opposite outcomes. Remarkably, perfect information-theoretic security can be achieved in principle for quantum WCF. Here, we overcome conceptual and practical issues that have prevented the experimental demonstration of this primitive to date, and demonstrate how quantum resources can provide cheat sensitivity, whereby each party can detect a cheating opponent, and an honest party is never sanctioned. Such a property is not known to be classically achievable with information-theoretic security. Our experiment implements a refined, loss-tolerant version of a recently proposed theoretical protocol and exploits heralded single photons generated by spontaneous parametric down conversion, a carefully optimized linear optical interferometer including beam splitters with variable reflectivities and a fast optical switch for the verification step. High values of our protocol benchmarks are maintained for attenuation corresponding to several kilometers of telecom optical fiber
Total synthesis of (−)-scabrolide A and (−)-yonarolide
No full textThe complete account of the total syntheses of scabrolide A and yonarolide is disclosed. This article describes an initial approach involving a bio-inspired macrocyclization/transannular Diels–Alder cascade, which ultimately failed due to undesired reactivity during macrocycle construction. Next, the evolution of a second and third strategy, which both involve an initial intramolecular Diels–Alder reaction followed by a late-stage closure of the seven-membered ring of scabrolide A are detailed. The third strategy was first validated on a simplified system, but problems were encountered during a key [2 + 2] photocycloaddition on the fully elaborated system. An olefin protection strategy was employed to circumvent this problem, ultimately leading to the completion of the first total synthesis of scabrolide A and the closely related natural product yonarolide
A convergent fragment coupling strategy to access quaternary stereogenic centers
No full textThe formation of quaternary stereogenic centers via convergent fragment coupling is a longstanding challenge in organic synthesis. Here, we report a strategy for the formation of quaternary stereogenic centers in polycyclic systems based upon the semi-pinacol reaction. In the key transformation, two fragments of a similar size and complexity are joined by a 1,2-addition of an alkenyl lithium to an epoxy ketone, and the resulting epoxy silyl ether undergoes a semi-pinacol rearrangement catalyzed by N-(trimethylsilyl)bis(trifluoromethanesulfonyl)imide (TMSNTf2) or trimethylsilyl trifluoromethanesulfonate (TMSOTf). Polycyclic scaffolds were generated in high yields and the reaction conditions tolerated a variety of functional groups including esters, silyl ethers, enol ethers, and aryl triflates. This method provides a useful strategy for the synthesis of complex polycyclic natural product-like scaffolds with quaternary stereogenic centers from simplified fragments
Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes
No full textBimolecular excited-state proton-coupled electron transfer (PCET*) was observed for reaction of the triplet MLCT state of [(dpab)₂Ru(4,4′-dhbpy)]²⁺ (dpab = 4,4′-di(n-propyl)amido-2,2′-bipyridine, 4,4′-dhbpy = 4,4′-dihydroxy-2,2′-bipyridine) with N-methyl-4,4′-bipyridinium (MQ⁺) and N-benzyl-4,4′-bipyridinium (BMQ⁺) in dry acetonitrile solutions. The PCET* reaction products, the oxidized and deprotonated Ru complex, and the reduced protonated MQ+ can be distinguished from the excited state electron transfer (ET*) and the excited state proton transfer (PT*) products by the difference in the visible absorption spectrum of the species emerging from the encounter complex. The observed behavior differs from that of reaction of the MLCT state of [(bpy)₂Ru(4,4′-dhbpy)]²⁺ (bpy = 2,2′-bipyridine) with MQ⁺, where initial ET* is followed by diffusion-limited proton transfer from the coordinated 4,4′-dhbpy to MQ⁰. The difference in observed behavior can be rationalized based on changes in the free energies of ET* and PT*. Substitution of bpy with dpab results in the ET* process becoming significantly more endergonic and the PT* reaction becoming somewhat less endergonic
A telescopic paradox: the artisans of the Accademia del Cimento, their instruments and their (in)visibility
No full textThe brief life of the Accademia del Cimento (1657–1667), the first known society with a purely experimental programme, is entangled with the most surprising advancements in the history of scientific instruments of that century, from the telescope to the microscope, the thermometer to the barometer, the hygrometer to the pendulum as a time-regulator, and more. The making of instruments at the Florentine court shows the interaction of princely, scholarly and artisanal actors. This paper explores this collaboration and shows how the supposed “invisibility’ of artisans depended on their proximity to the academicians and princes, who mainly communicated verbally with them, directly or through middlemen. The visibility of artisans increases proportionally to their physical distance from the Court. In this essay I unveil the identity of the artisans of the Cimento and, finally, attempt to attribute five instruments (some lost and others still extant) to specific makers, shedding light also on relations between the artisan and his patron
Doppler-free Spectroscopy of Buffer-Gas-Cooled Calcium Monohydroxide
No full textIn this study, we report the Doppler-free spectra of buffer-gas-cooled CaOH. We observed five Doppler-free spectra containing low-J Q₁ and R₁₂ transitions, which were only partially resolved by previous Doppler-limited spectroscopies. The spectra frequencies were corrected using the Doppler-free spectra of iodine molecules; accordingly, the uncertainty was estimated to be below 10 MHz. We determined the spin–rotation constant in the ground state, which agrees with the values reported in the literature obtained based on millimeter-wave data within 1 MHz. This suggests that the relative uncertainty is much smaller. The present study demonstrates the Doppler-free spectroscopy of a polyatomic radical and the broad applicability of the buffer gas cooling method to molecular spectroscopy. CaOH is the only polyatomic molecule that can be directly laser-cooled and trapped in a magneto-optical trap. High-resolution spectroscopy of such molecules is useful for establishing efficient laser cooling schemes of polyatomic molecules
A Bayesian level set method for identifying subsurface geometries and rheological properties in Stokes flow
No full textWe aim to simultaneously infer the shape of subsurface structures and material properties such as density or viscosity from surface observations. Modelling mantle flow using incompressible instantaneous Stokes equations, the problem is formulated as an infinite-dimensional Bayesian inverse problem. Subsurface structures are described as level sets of a smooth auxiliary function, allowing for geometric flexibility. As inverting for subsurface structures from surface observations is inherently challenging, knowledge of plate geometries from seismic images is incorporated into the prior probability distributions. The posterior distribution is approximated using a dimension-robust Markov-chain Monte Carlo sampling method, allowing quantification of uncertainties in inferred parameters and shapes. The effectiveness of the method is demonstrated in two numerical examples with synthetic data. In a model with two higher-density sinkers, their shape and location are inferred with moderate uncertainty, but a trade-off between sinker size and density is found. The uncertainty in the inferred is significantly reduced by combining horizontal surface velocities and normal traction data. For a more realistic subduction problem, we construct tailored level-set priors, representing “seismic” knowledge and infer subducting plate geometry with their uncertainty. A trade-off between thickness and viscosity of the plate in the hinge zone is found, consistent with earlier work
Growth Mechanism and Kinetics of Diamond in Liquid Gallium from Quantum Mechanics Molecular Dynamics Simulations
No full textRuoff and co-workers recently demonstrated low-temperature (1193 K) homoepitaxial diamond growth from liquid gallium solvent. To develop an atomistic mechanism for diamond growth underlying this remarkable demonstration, we carried out density functional theory-based molecular dynamics (DFT-MD) simulations to examine the mechanism of single-crystal diamond growth on various low-index crystallographic diamond surfaces (100), (110), and (111) in liquid Ga with CH4. We find that carbon linear chains form in liquid Ga and then react with the growing diamond surface, leading first to the formation of carbon rings on the surface and then initiation of diamond growth. Our simulations find faster growth on the (110) surface than on the (100) or (111) surfaces, suggesting the (110) surface as a plausible growth surface in liquid Ga. For (110) surface growth, we predict the optimum growth temperature to be ∼1300 K, arising from a balance between the kinetics of forming carbon chains dissolved in Ga and the stability of carbon rings on the growing surface. We find that the rate-determining step for diamond growth is dehydrogenation of the growing hydrogenated (110) surface of diamond. Inspired by the recent experimental studies by Ruoff and co-workers demonstrating that Si accelerates diamond growth in Ga, we show that addition of Si into liquid Ga significantly increases the rate of dehydrogenating the growing surface. Extrapolating from the DFT-MD predicted rates at 2800 to 3500 K, we predict the growth rate at the experimental growth temperature of 1193 K, leading to rates in reasonable agreement with the experiment. These fundamental mechanisms should provide guidance in optimizing low-temperature diamond growth