quasi-2D Geometry Research Articles

The evolution of the demixing morphology in quasi 2D water-lutidine mixtures: a fractal dimension analysis

In this work, the evolution of the morphological properties of the liquid-liquid phase separation process of water-lutidine mixtures in a quasi-2D geometry locally heated by laser absorption is studied at different heating rates by videomicroscopy and complemented by a finite element simulation. Morphological changes of the separated domains are characterised using the fractal dimension and the distribution of domain sizes, capturing the main steps of the process: spinodal decomposition derived from the fluctuations in density at the critical point, separation into a bicontinuous structure, breaking of the structures producing a wide distribution of sizes, and the subsequent growth and coalescence of the demixed regions (domains) to produce a complete separation. A comparison with the simplest case of typical nucleation is made by performing the same analysis at a concentration where critical fluctuations are not present. We conclude that the methodology presented here can be used to characterise the main steps of the phase separation process and distinguish between spinodal decomposition and nucleation.

Investigation of the flow field in a centrifugal rotor through particle image velocimetry

This work aims to investigate the flow field in a centrifugal rotor using two-dimensional Particle Image Velocimetry (PIV). The flow is analyzed at different planes for different rotating speeds and flow rate operations. The assumed rotor is a prototype with equal inlet and outlet blade angles and is built to have a quasi-2D geometry. The PIV measurements were performed using the phase-locking technique, in which 500 samples were obtained from each experiment. Results for pressure, velocity, Reynolds shear stress, and turbulence intensity show that the flow in the centrifugal rotor behaves according to kinematic similarity, even at part-load operation. Moreover, it was observed that the flow could be considered periodic in the rotor channels for the best efficiency point flow rate and at part-load conditions. From the analysis conducted at different planes, differences could be observed in the flow near the hub or shroud for flow rates above 0.75∙QBEP. In general, the velocity magnitude is higher near the hub than near the shroud, while turbulence intensities tend to be higher near the shroud than near the hub. This difference can be attributed to the change in the flow direction from the development pipe to the rotor channels.

Two-Dimensional Investigation of the Fundamentals of OGV Buffeting

The increased demands of compact modern aero engine architectures have highlighted the problem of outlet guide vane (OGV) buffeting in off-design conditions. This structural response to aerodynamic excitations is characterised by increased vibration, risking structural fatigue. Investigations focused on understanding, mitigation and avoidance are therefore of high priority. OGV buffet is a type of transonic buffet caused by unsteady shock movement, but the exact parameters driving it are not fully understood. To try and understand them, this paper examines the buffet of a quasi-2D OGV geometry. Parametric studies of the incidence angle and inlet Mach number were performed. Forcing frequencies for both studies were found to be close to the experimentally detected frequency of vibration in the first bow mode, which demonstrates that buffet is driven by quasi-2D flow features. Increasing the inlet Mach number increased the dominant forcing frequency, whereas increasing the incidence yielded little change. Profiles of unsteady pressure amplitudes were shown to smoothly increase in magnitude with an increasing incidence and inlet Mach number.

A Model to Simulate Gas Dissolution into/through Metals and Its Application to Deuterium in a 316L Steel Chamber with Pb-Li in a Quasi-2D Geometry

Liquid lead-lithium in eutectic proportions (Pb-Li) is a candidate material for Breeding Blankets (BBs) in future Fusion Power Plants (FPP). BB design depends on the diffusivity and Sieverts’ constant (solubility) of tritium in this alloy, but literature reports a large scattering of measurements for these values. A model was developed to address one possible source of this scattering in static experiments, i.e., non-negligible loss of hydrogen gas through steel walls of containers. This model simulates the dissolution of gases into, and their diffusion through, metallic barriers for diffusivity and Sieverts’ constant as inputs. When implemented, it can be used to compute the pressure decrease in a metallic chamber, and comparison of simulated curves with experimental ones allows for estimates of the diffusivity and Sieverts’ constant. This approach was used to estimate these coefficients for deuterium in stainless steel, using experiments performed with a 316L steel chamber from an existing facility (the Vacuum Sieve Tray setup) and simulations in a quasi-2D representation of this chamber. This validated the model, which was then used to simulate the chamber containing Pb-Li, as a means of planning for future experiments.

Tuning superconductivity in Ge:Ga using Ga+ implantation energy

High-fluence gallium $({\mathrm{Ga}}^{+})$ implantation at medium energies is proven to be an effective tool in forming superconducting (SC) thin films in germanium (Ge). By changing the post-implantation annealing conditions nanocrystalline to single-crystalline Ge matrices have been produced. Irrespective of crystallinity, such processes have mostly led to supersaturated Ge:Ga films where superconductivity is controlled by the extent of coherent coupling between Ga precipitates. Here we use ${\mathrm{Ga}}^{+}$ implantation energy as a means to tailor the spatial distribution and the coupling energy of the Ga precipitates. By systematic structural and magneto-transport studies, we unravel the complex connection between the internal structure of Ge:Ga films and their global SC parameters. At the shallowest implantation depth, we observe the strongest coupling leading to a robust superconductivity that sustains parallel magnetic fields as high as 9.95 T, above the conventional Pauli paramagnetic limit and consistent with a quasi-2D geometry. Further measurements at mK temperatures revealed an anomalous upturn in perpendicular critical field ${\mathrm{B}}_{\ensuremath{\perp}}$ vs temperature whose curvature and thus origin may be tuned between weakly coupled SC arrays and vortex glass states with quenched disorder. This warrants future investigations into Ge:Ga films for applications where tunable disorder is favorable, including test-beds for quantum phase transitions and superinductors in quantum circuits.

Long-lived trimers in a quasi-two-dimensional Fermi system

We consider the problem of three distinguishable fermions confined to a quasi-two-dimensional (quasi-2D) geometry, where there is a strong harmonic potential in one direction. We go beyond previous theoretical work and investigate the three-body bound states (trimers) for the case where the two-body short-range interactions between fermions are unequal. Using the scattering parameters from experiments on ultracold $^{6}$Li atoms, we calculate the trimer spectrum throughout the crossover from two to three dimensions. We find that the deepest Efimov trimer in the $^{6}$Li system is unaffected by realistic quasi-2D confinements, while the first excited trimer smoothly evolves from a 3D-like Efimov trimer to an extended 2D-like trimer as the attractive interactions are decreased. We furthermore compute the excited trimer wave function and quantify the stability of the trimer with respect to three-body recombination by determining the probability that three fermions approach each other at short distances. Our results indicate that the lifetime of the trimer can be enhanced by at least an order of magnitude in the quasi-2D geometry, thus opening the door to realizing long-lived trimers in three-component Fermi gases.

Bifurcations, stability and mode evolution in segregated quasi-2D condensate mixtures

We present new features of low energy Bogoliubov quasiparticle excitations of a two-component Bose–Einstein condensate (TBEC) in quasi-2D geometry at zero temperature using Hartree–Fock–Bogoliubov formalism. We, in particular, consider the TBECs of 133Cs–87Rb and 85Rb–87Rb, and show specific features in the low energy excitation spectrum as a function of the interaction strength. For 85Rb–87Rb TBEC, the appearance of a new zero energy mode is observed. Whereas for 133Cs–87Rb TBEC we report a bifurcation of the slosh mode at the point of transition from miscible to immiscible domain. The lower energy mode, after the bifurcation, goes soft and becomes a new Nambu–Goldstone mode of the system.

Universal nanodroplet branches from confining the Ouzo effect

We report the self-organization of universal branching patterns of oil nanodroplets under the Ouzo effect [Vitale S, Katz J (2003) Langmuir 19:4105-4110]-a phenomenon in which spontaneous droplet formation occurs upon dilution of an organic solution of oil with water. The mixing of the organic and aqueous phases is confined under a quasi-2D geometry. In a manner analogous to the ramification of ground stream networks [Devauchelle O, Petroff AP, Seybold HF, Rothman DH (2012) Proc Natl Acad Sci USA 109: 20832-20836 and Cohen Y, et al. (2015) Proc Natl Acad Sci USA 112:14132-14137] but on a scale 10 orders of magnitude smaller, the angles between the droplet branches are seen to exhibit remarkable universality, with a value around 74° ± 2°, independent of the various control parameters of the process. Numerical simulations reveal that these nanodroplet branching patterns are governed by the interplay between the local concentration gradient, diffusion, and collective interactions. We further demonstrate the ability of the local concentration gradient to drive autonomous motion of colloidal particles in the highly confined space, and the possibility of using the nucleated nanodroplets for nanoextraction of a hydrophobic solute. The understanding obtained from this work provides a basis for quantitatively understanding the complex dynamical aspects associated with the Ouzo effect. We expect that this will facilitate improved control in nanodroplet formation for many applications, spanning from the preparation of pharmaceutical polymeric carriers, to the formulation of cosmetics and insecticides, to the fabrication of nanostructured materials, to the concentration and separation of trace analytes in liquid-liquid microextraction.

Pattern formation in pulsed gas-solid fluidized beds – The role of granular solid mechanics

Under certain conditions, gas-solid fluidized beds are known to develop a structured flow of bubbles when exposed to periodically pulsating air flows. In quasi-two-dimensional beds, periodically rising bubbles form a triangular tessellation in the vertical plane. Bubble nucleation sites at the distributor plate alternate during each cycle. This pattern sets an excellent benchmark for fundamental studies of fluidization. Notably, most common Eulerian descriptions of granular flow do not yet capture this interplay between solid mechanics and fluid-solid momentum exchange, which we show to be instrumental to the dynamic rearrangement of bubbles in a pulsed bed. We report the first successful CFD simulations of structured bubble flows in a deep, quasi-2D geometry using a Eulerian-Lagrangian CFD-DEM framework. Numerical results are in quantitative agreement with experiments. The simulated dynamics reveal that the patterns emerge from the transition of the granular collective behavior between solid-like and fluid-like, which is an outcome of dynamical coupling between gas and particles. The simulated results point out the essential role of solid frictional stresses on inducing and maintaining the formation of bubble patterns. This underscores the value of investigating pulsation-induced patterns as a prime manifestation of the mesoscopic physics underpinning fluidization, and highlights the direction for improving current practices.

Exact two-body solutions and quantum defect theory of two-dimensional dipolar quantum gas

In this paper, we provide the two-body exact solutions of the two-dimensional (2D) Schrödinger equation with isotropic interactions. An analytic quantum defect theory is constructed based on these solutions and it is applied to investigate the scattering properties as well as two-body bound states of an ultracold polar molecules confined in a quasi-2D geometry. Interestingly, we find that for the attractive case, the scattering resonance happens simultaneously in all partial waves, which has not been observed in other systems. The effect of this feature on the scattering phase shift across such resonances is also illustrated.

A molecular dynamics study of heterogeneous nucleation at grain boundaries during solid-state phase transformations

The study of solid-state nucleation through experiment is often limited by its tiny length range and short time scale. In this study, molecular dynamics (MD) simulations in a quasi 2D geometry are used to study the process of a BCC ferrite phase in pure Fe nucleating at a grain boundary (GB) in an FCC austenite polycrystalline system. In the MD simulations the critical nucleus can be identified, the bulk free energy difference between FCC and BCC is known for the Fe interatomic potential used and all relevant interface and GB energies are computed using a Gibbs–Cahn formulation. For nucleation events that exhibited low energy facets completely contained within the parent FCC phase, the results agreed well with predictions from classical nucleation theory (CNT) in terms of both the size and shape of the critical nucleus. For systems where the emerging nucleus contains facets that cross the GB plane the agreement with classical theory is less convincing and the observed nucleus does not exhibit parallel facets as predicted from the Winterbottom construction. The latter nucleation case involves a so-called pucker mechanism of the FCC grain boundary to accommodate the emerging nucleus and the effect of GB puckering on the incubation time is discussed.

Two-state Bogoliubov theory of a molecular Bose gas

We present an analytic Bogoliubov description of a BEC of polar molecules trapped in a quasi-2D geometry and interacting via internal state-dependent dipole-dipole interactions. We derive the mean-field ground-state energy functional, and we derive analytic expressions for the dispersion relations, Bogoliubov amplitudes, and dynamic structure factors. This method can be applied to any homogeneous, two-component system with linear coupling, and direct, momentum-dependent interactions. The properties of the mean-field ground state, including polarization and stability, are investigated, and we identify three distinct instabilities: a density-wave rotonization that occurs when the gas is fully polarized, a spin-wave rotonization that occurs near zero polarization, and a mixed instability at intermediate fields. These instabilities are clarified by means of the real-space density-density correlation functions, which characterize the spontaneous fluctuations of the ground state, and the momentum-space structure factors, which characterize the response of the system to external perturbations. We find that the gas is susceptible to both density-wave and spin-wave response in the polarized limit but only a spin-wave response in the zero-polarization limit. These results are relevant for experiments with rigid rotor molecules such as RbCs, $\Lambda$-doublet molecules such as ThO that have an anomalously small zero-field splitting, and doublet-$\Sigma$ molecules such as SrF where two low-lying opposite-parity states can be tuned to zero splitting by an external magnetic field.

Suppression and Revival of Weak Localization through Control of Time-Reversal Symmetry

We report on the observation of suppression and revival of coherent backscattering of ultracold atoms launched in an optical disorder in a quasi-2D geometry and submitted to a short dephasing pulse, as proposed by Micklitz, Müller, and Altland [Phys. Rev. B 91, 064203 (2015)]. This observation demonstrates a novel and general method to study weak localization by manipulating time reversal symmetry in disordered systems. In future experiments, this scheme could be extended to investigate higher order localization processes at the heart of Anderson (strong) localization.

Quasi-two-dimensional Fermi gases at finite temperatures

We consider a Fermi gas with short-range attractive interactions that is confined along one direction by a tight harmonic potential. For this quasi-two-dimensional (quasi-2D) Fermi gas, we compute the pressure equation of state, radio frequency spectrum, and the superfluid critical temperature $T_c$ using a mean-field theory that accounts for all the energy levels of the harmonic confinement. Our calculation for $T_c$ provides a natural generalization of the Thouless criterion to the quasi-2D geometry, and it correctly reduces to the 3D expression derived from the local density approximation in the limit where the confinement frequency $\omega_z \to 0$. Furthermore, our results suggest that $T_c$ can be enhanced by relaxing the confinement and perturbing away from the 2D limit.

Effective description of the interaction between anisotropic viscous fingers

We study patterns formed by viscous fingering in a rectangular network of microfluidic channels. Due to the strong anisotropy of such a system, the emerging patterns have a form of thin needle-like fingers, which interact with each other, competing for an available flow. We develop an upscaled description of this system in which only the fingers are tracked and the effective interactions between them are introduced, mediated through the evolving pressure field. Due to the quasi-2d geometry of the system, this is conveniently accomplished using conformal mapping techniques. A complex two-phase flow problem is thus reduced to a much simpler task of tracking evolving shapes in a 2d complex plane. This description, although simplified, turns out to capture all the key features of the system's dynamics and allows for the effective prediction of the resulting growth patterns.

BCS-BEC crossover in a quasi-two-dimensional Fermi gas

We consider a two-component gas of fermionic atoms confined to a quasi-two-dimensional (quasi-2D) geometry by a harmonic trapping potential in the transverse direction. We construct a mean field theory of the BCS-BEC crossover at zero temperature that allows us to extrapolate to an infinite number of transverse harmonic oscillator levels. In the extreme BEC limit, where the confinement length exceeds the dimer size, we recover 3D dimers confined to 2D with weak repulsive interactions. However, even when the interactions are weak and the Fermi energy is less than the confinement frequency, we find that the higher transverse levels can substantially modify fermion pairing. We argue that recent experiments on pairing in quasi-2D Fermi gases [Y. Zhang et al., Phys. Rev. Lett. 108, 235302 (2012)] have already observed the effects of higher transverse levels.

Phase diagram of octapod-shaped nanocrystals in a quasi-two-dimensional planar geometry

Recently, we reported the formation of crystalline monolayers consisting of octapod-shaped nanocrystals (so-called octapods) that had arranged in a square-lattice geometry through drop deposition and fast evaporation on a substrate [W. Qi, J. de Graaf, F. Qiao, S. Marras, L. Manna, and M. Dijkstra, Nano Lett. 12, 5299 (2012)]. In this paper we give a more in-depth exposition on the Monte Carlo simulations in a quasi-two-dimensional (quasi-2D) geometry, by which we modelled the experimentally observed crystal structure formation. Using a simulation model for the octapods consisting of four hard interpenetrating spherocylinders, we considered the effect of the pod length-to-diameter ratio on the phase behavior and we constructed the full phase diagram. The methods we applied to establish the nature of the phase transitions between the various phases are discussed in detail. We also considered the possible existence of a Kosterlitz-Thouless-type phase transition between the isotropic liquid and hexagonal rotator phase for certain pod length-to-diameter ratios. Our methods may prove instrumental in guiding future simulation studies of similar anisotropic nanoparticles in confined geometries and monolayers.

Quasicondensation reexamined

We study in detail the effect of quasicondensation. We show that this effect is strictly related to dimensionality of the system. It is present in one dimensional systems independently of interactions – exists in repulsive, attractive or in non-interacting Bose gas in some range of temperatures below characteristic temperature of the quantum degeneracy. Based on this observation we analyze the quasicondensation in terms of a ratio of the two largest eigenvalues of the single particle density matrix for the ideal gas. We show that in the thermodynamic limit in higher dimensions the second largest eigenvalue vanishes (as compared to the first one) with total number of particles as ≃ N−γ whereas goes to zero only logarithmically in one dimension. We also study the effect of quasicondensation for various geometries of the system: from quasi-1D elongated one, through spherically symmetric 3D case to quasi-2D pancake-like geometry.

Bound States in a Quasi-Two-Dimensional Fermi Gas

We consider the problem of N identical fermions of mass m(↑) and one distinguishable particle of mass m(↓) interacting via short-range interactions in a confined quasi-two-dimensional (quasi-2D) geometry. For N=2 and mass ratios m(↑)/m(↓)<13.6, we find non-Efimov trimers that smoothly evolve from 2D to 3D. In the limit of strong 2D confinement, we show that the energy of the N+1 system can be approximated by an effective two-channel model. We use this approximation to solve the 3+1 problem and we find that a bound tetramer can exist for mass ratios m(↑)/m(↓) as low as 5 for strong confinement, thus providing the first example of a universal, non-Efimov tetramer involving three identical fermions.

Core/Shell Colloidal Semiconductor Nanoplatelets

We have recently synthesized atomically flat semiconductor colloidal nanoplatelets with quasi 2D geometry. Here, we show that core/shell nanoplatelets can be obtained with a 2D geometry that is conserved. The epitaxial growth of the shell semiconductor is performed at room temperature. We report the detailed synthesis of CdSe/CdS and CdSe/CdZnS structures with different shell thicknesses. The shell growth is characterized both spectroscopically and structurally. In particular, the core/shell structure appears very clearly on high-resolution, high-angle annular dark-field transmission electron microscope images, thanks to the difference of atomic density between the core and the shell. When the nanoplatelets stand on their edge, we can precisely count the number of atomic planes forming the core and the shell. This provides a direct measurement, with atomic precision, of the core nanoplatelets thickness. The constraints exerted by the shell growth on the core is analyzed using global phase analysis. The core/shell nanoplatelets we obtained have narrow emission spectra with full-width at half-maximum close to 20 nm, and quantum yield that can reach 60%.