Accessing the spatiotemporal heterogeneities of single nanocatalysts by optically imaging gas nanobubbles

Catalysts that catalyze the generation of products in the gas phase, especially those involved in the hydrogen evolution reaction (HER), hold great promise for ecofriendly and sustainable energy development. In general, gas chromatography is widely used to measure catalytic activity. Unfortunately, it gives an averaged output that washes out the heterogeneities among individuals. To assess the unique catalytic properties at the single nanoparticle level, various methods based on single particle catalysis have been proposed. Over the past fifteen years, tremendous breakthroughs have been achieved, which uncovered hidden spatial and temporal heterogeneities. Although powerful, effectively quantifying the activities of single HER nanocatalysts remains challenging because of the fast diffusion of hydrogen (H₂). In 2017, a novel approach based on a nanobubble indicator was proposed to correlate the kinetics of gas bubble evolution with the catalytic activities of individual nanoentities during the HER process. Since then, a plethora of optical microscopy techniques have been utilized to monitor dynamically evolved nanobubbles and to measure the catalytic activities of single HER catalysts. In this minireview, we summarized state-of-the-art optical microscopy for in operando imaging of dynamic nanobubbles involved in gas-generating reactions while highlighting some important discoveries, including the blinking photocatalytic activity and heterogeneous distribution of active sites. Finally, challenges and future perspectives in this promising field were identified.     

Tomographic reconstruction of shock layer flows

The tomographic reconstruction of supersonic flows faces two challenges. Firstly, techniques used in the past, such as the direct Fourier method (DFM) (Gottlieb and Gustafsson in On the direct Fourier method for computer tomography, 1998; Morton in Tomographic imaging of supersonic flows, 1995) or various backprojection (Kak and Slaney in Principles of computerized tomographic imaging, vol. 33 in Classics in Applied Mathematics, 2001) techniques, have only been able to reconstruct areas of the flow which are upstream of any opaque objects, such as a model. Secondly, shock waves create sharp discontinuities in flow properties, which can be difficult to reconstruct both in position and in magnitude with limited data. This paper will present a reconstruction method, matrix inversion using ray-tracing and least squares conjugate gradient (MI-RLS), which uses geometric ray-tracing and a sparse matrix iterative solver (Paige and Saunders in ACM Trans. Math. Softw. 8(1):43–71, 1982) to overcome both of these challenges. It will be shown, through testing with a phantom object described in tomographic literature, that the results compare favourably to those produced by the DFM technique. Finally, the method will be used to reconstruct three-dimensional density fields from interferometric shock layer images, with good resolution (Faletič in Tomographic reconstruction of shock layer flows, 2005). This paper was based on work that was presented at the 3rd International Symposium on Interdisciplinary Shock Wave Research, Canberra, Australia, March 1–3, 2006.     

Two-breather solutions for the class I infinitely extended nonlinear Schrödinger equation and their special cases

Nonlinear Dynamics - We derive the two-breather solution of the class I infinitely extended nonlinear Schrödinger equation. We present a general form of this multi-parameter solution that...     

Hard sphere correlation functions in the Percus-Yevick approximation

A reformulation of the Ornstein-Zernike equation permits rapid and simple numerical calculation of the total correlation functions for single and multi-component systems of hard spheres. Such correlation functions permit the application of sophisticated statistical mechanical perturbation theories to more complicated systems than has been possible heretofore.     

Combining 2D synchrosqueezed wave packet transform with optimization for crystal image analysis

We develop a variational optimization method for crystal analysis in atomic resolution images, which uses information from a 2D synchrosqueezed transform (SST) as input. The synchrosqueezed transform is applied to extract initial information from atomic crystal images: crystal defects, rotations and the gradient of elastic deformation. The deformation gradient estimate is then improved outside the identified defect region via a variational approach, to obtain more robust results agreeing better with the physical constraints. The variational model is optimized by a nonlinear projected conjugate gradient method. Both examples of images from computer simulations and imaging experiments are analyzed, with results demonstrating the effectiveness of the proposed method.     

Seeing the Unseen: Experimental Observation of Magnetic Anapole State Inside a High-Index Dielectric Particle

The existence of non-radiating electromagnetic sources attracts much attention in photonic community and gives rise to extensive discussions of various applications in lasing, medical imaging, sensing, and nonlinear optics. In this article, the existence of magnetic anapole states (or magnetic-type non-radiating sources) characterized by a suppressed magnetic dipole radiation in a dielectric cylindrical particle is theoretically predicted and experimentally demonstrated. The specific features of the magnetic anapole state under ideal conditions are identified, followed by a demonstration of how their existence can be detected in practical structures. The concept is valid in various frequency bands from visible range for nanoparticles to microwave range for millimeter size objects. The experimental study is performed in microwave frequency range which allows not only to measure the far-field (scattered field) characteristics, but also to probe the peculiar field profile directly inside the dielectric particle. The experimental results agree well with the analytical ones and pave the way to detect and identify nontrivial different-type anapole states.     

The three body interaction energy between optically active molecules

An account is given of a steady-state molecular dynamics method for the calculation of the shear viscosity of a dense fluid. The method is applied to the case of a system described by a Lennard-Jones potential with parameters appropriate to argon. Satisfactory agreement with experimental data is found over a range of temperature and density for which the viscosity varies by a factor of approximately 8. The results are interpreted in terms of a modified hard-sphere model and it is shown that the Lennard-Jones fluid obeys a Stokes-type relation in which the effective molecular diameter is close to that deduced from equilibrium structural properties.     

Versatile wideband balanced detector for quantum optical homodyne tomography

We present a comprehensive theory and an easy to follow method for the design and construction of a wideband homodyne detector for time-domain quantum measurements. We show how one can evaluate the performance of a detector in a specific time-domain experiment based on the electronic spectral characteristic of that detector. We then present and characterize a high-performance detector constructed using inexpensive, commercially available components such as low-noise high-speed operational amplifiers and high-bandwidth photodiodes. Our detector shows linear behavior up to a level of over 13 dB clearance between shot noise and electronic noise, in the range from DC to 100 MHz. The detector can be used for measuring quantum optical field quadratures both in the continuous-wave and pulsed regimes with standard commercial mode-locked lasers.     

Optimal Lyapunov quantum control of two-level systems: Convergence and extended techniques

Taking a two-level system as an example, we show that a strong control field may enhance the efficiency of optimal Lyapunov quantum control but could decrease its control fidelity. A relationship between the strength of the control field and the control fidelity is established. An extended technique, which combines free evolution and external control, is proposed to improve the control fidelity. We analytically demonstrate that the extended technique can be used to design a control law for steering a two-level system exactly to one predetermined eigenstate of the free Hamiltonian. In such a way, the convergence of the extended optimal Lyapunov quantum control can be guaranteed.     

Biomimetic propulsion under random heaving conditions,using active pitch control

Marine mammals travel long distances by utilizing and transforming wave energy to thrust through proper control of their caudal fin. On the other hand, manmade ships traveling in a wavy sea store large amounts of wave energy in the form of kinetic energy for heaving, pitching, rolling and other ship motions. A natural way to extract this energy and transform it to useful propulsive thrust is by using a biomimetic wing. The aim of this paper is to show how an actively pitched biomimetic wing could achieve this goal when it performs a random heaving motion. More specifically, we consider a biomimetic wing traveling with a given translational velocity in an infinitely extended fluid and performing a random heaving motion with a given energy spectrum which corresponds to a given sea state. A formula is invented by which the instantaneous pitch angle of the wing is determined using the heaving data of the current and past time steps. Simulations are then performed for a biomimetic wing at different heave energy spectra, using an indirect Source-Doublet 3-D–BEM, together with a time stepping algorithm capable to track the random motion of the wing. A nonlinear pressure type Kutta condition is applied at the trailing edge of the wing. With a mollifier-based filtering technique, the 3-D unsteady rollup pattern created by the random motion of the wing is calculated without any simplifying assumptions regarding its geometry. Calculated unsteady forces, moments and useful power, show that the proposed active pitch control always results in thrust producing motions, with significant propulsive power production and considerable beneficial stabilizing action to ship motions. Calculation of the power required to set the pitch angle prove it to be a very small percentage of the useful power and thus making the practical application of the device very tractable.