Small Separation Distances Research Articles

Particle radial velocity and concentration kernels estimation in isotropic grid turbulence experiments of inertialess particles at small separation distances

Particle radial velocity and concentration kernels estimation in isotropic grid turbulence experiments of inertialess particles at small separation distances

Single-exponential bounds for the smallest singular value of Vandermonde matrices in the sub-Rayleigh regime

Following recent interest by the community, the scaling of the minimal singular value of a Vandermonde matrix with nodes forming clusters on the length scale of Rayleigh distance on the complex unit circle is studied. Using approximation theoretic properties of exponential sums, we show that the decay is only single exponential in the size of the largest cluster, and the bound holds for arbitrary small minimal separation distance. We also obtain a generalization of well-known bounds on the smallest eigenvalue of the generalized prolate matrix in the multi-cluster geometry. Finally, the results are extended to the entire spectrum.

Particle pair statistics of inertial particles at small separation using stereoscopic particle tracking

Particle pair statistics of inertial particles having average Stokes numbers of 2.1 and 14 are measured in isotropic turbulence at a Reynolds number of Reλ = 240. The radial distribution function (RDF) and mean relative approach velocity are obtained at small separation distances using 2-frame stereoscopic particle tracking velocimetry (stereo-PTV). At small separation distance, the RDF varies by an order of magnitude in the range of Stokes numbers investigated. However, the mean relative approach velocity is found to have a weak dependence on Stokes number. The results are shown to have high accuracy when compared to analogous mono-PTV datasets, and can be used to provide a more reliable estimate of the inter-particle collision rate. The main limitation of the measurement is observed at separation distances less than the laser sheet thickness, where the technique tended to underestimate the mean relative approach velocity.

Capillary‐bridge forces between solid particles: Insights from lattice Boltzmann simulations

AbstractLiquid capillary‐bridge formation between solid particles has a critical influence on the rheological properties of granular materials and, in particular, on the efficiency of fluidized bed reactors. The available analytical and semi‐analytical methods have inherent limitations, and often do not cover important aspects, like the presence of non‐axisymmetric bridges. Here, we conduct numerical simulations of the capillary bridge formation between equally and unequally sized solid particles using the lattice Boltzmann method, and provide an assessment of the accuracy of different families of analytical models. We find that some of the models taken into account are shown to perform better than others. However, all of them fail to predict the capillary force for contact angles larger than π/2, where a repulsive capillary force attempts to push the solid particle outward to minimize the surface energy, especially at a small separation distance. We then apply the most suitable model to study the impact of capillary interactions on particle clustering using a coupled lattice Boltzmann and Discrete Element method.

Mechanism of anti-proteins adsorption behavior on superhydrophobic titanium surface

Comprehensively understanding the albumin and fibrinogen adsorption behaviors is of fundamental significance for blood-contacting devices to effectively prevent the proteins adsorption. First, a controllable superhydrophobic surface presented great repellence to different liquids with water contact angle 165.2°, plasma contact angle 154.8° and blood contact angle 152.1°. Secondly, the albumin and fibrinogen adhesion tests indicated that the pure titanium surface was fully covered by the proteins layer while only a small amount of the proteins adhered to the superhydrophobic surface. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) model was built by considering the Lifshitz-van der Waals (LW), the Lewis acid-base (AB) and the electrostatic double-layer (EL) interactions, the positive value of total interaction energy profile suggested that an overall repulsive interaction between the protein molecules and superhydrophobic surface happened at a small separation distance. Finally, the hollow tubes with inner superhydrophobic surface were implanted into carotid artery of rabbits for two weeks. The content of N element measured by EDS mapping provide a new evidence that superhydrophobic surface owns a great resistance of the proteins absorption, and no thrombosis or blood cells adhered to the superhydrophobic surface. This research revealed the mechanism that superhydrophobic modification could effectively prevent the adsorption of plasma proteins, which could provide some theoretical basis to help design the blood-contacting devices with great hemocompatibility.

The influence of surface charge on the coalescence of ice and dust particles in the mesosphere and lower thermosphere

Abstract. Agglomeration of charged ice and dust particles in the mesosphere and lower thermosphere is studied using a classical electrostatic approach, which is extended to capture the induced polarisation of surface charge. Collision outcomes are predicted whilst varying the particle size, charge, dielectric constant, relative kinetic energy, collision geometry and the coefficient of restitution. In addition to Coulomb forces acting on particles of opposite charge, instances of attraction between particles of the same sign of charge are discussed. These attractive forces are governed by the polarisation of surface charge and can be strong at very small separation distances. In the mesosphere and lower thermosphere, these interactions could also contribute to the formation of stable aggregates and contamination of ice particles through collisions with meteoric smoke particles.

Age-related changes in diffuse optical tomography sensitivity profiles in infancy.

Diffuse optical tomography uses near-infrared light spectroscopy to measure changes in cerebral hemoglobin concentration. Anatomical interpretations of the location that generates the hemodynamic signal requires accurate descriptions of diffuse optical tomography sensitivity to the underlying cortical structures. Such information is limited for pediatric populations because they undergo rapid head and brain development. The present study used photon propagation simulation methods to examine diffuse optical tomography sensitivity profiles in realistic head models among infants ranging from 2 weeks to 24 months with narrow age bins, children (4 and 12 years) and adults (20 to 24 years). The sensitivity profiles changed systematically with the source-detector separation distance. The peak of the sensitivity function in the head was largest at the smallest separation distance and decreased as separation distance increased. The fluence value dissipated more quickly with sampling depth at the shorter source-detector separations than the longer separation distances. There were age-related differences in the shape and variance of sensitivity profiles across a wide range of source-detector separation distances. Our findings have important implications in the design of sensor placement and diffuse optical tomography image reconstruction in (functional) near-infrared light spectroscopy research. Age-appropriate realistic head models should be used to provide anatomical guidance for standalone near-infrared light spectroscopy data in infants.

Multiple ellipsoidal/elliptical inhomogeneities embedded in infinite matrix by equivalent inhomogeneous inclusion method

Traditional equivalent inclusion method provides unreliable predictions of the stress concentrations of two spherical inhomogeneities with small separation distance. This paper determines the stress and strain fields of multiple ellipsoidal/elliptical inhomogeneities by equivalent inhomogeneous inclusion method. Equivalent inhomogeneous inclusion method is an inverse of equivalent inclusion method and substitutes the subdomains of matrix with known strains by equivalent inhomogeneous inclusions. The stress and strain fields of multiple inhomogeneities are decomposed into the superposition of matrix under applied load and each solitary inhomogeneous inclusion with polynomial eigenstrains by the iteration of equivalent inhomogeneous inclusion method. Multiple circular and spherical inhomogeneities are respectively used as examples and examined by the finite element method. The stress concentrations of multiple inhomogeneities with small separation distances are well predicted by equivalent inhomogeneous inclusion method and the accuracies improve with the increase of eigenstrain orders. Equivalent inhomogeneous inclusion method gives more accurate stress predictions than equivalent inclusion method in the problem of two spherical inhomogeneities.

Flickering candle flames and their collective behavior

Oscillation and collective behavior of diffusion flames is a fascinating phenomena. Considering candle bundles with different sizes in variable oxygen concentration, the flickering dynamics of the flames are experimentally and theoretically investigated. Trends for the flickering frequency as a function of the candle number in the bundle and oxygen concentration is revealed for various topologies of the candles packing. The collective behavior of the flames as a function of their separation distance is studied by measuring an appropriate synchronization order parameter and through the common oscillation frequency. In agreement with previous results we find a discontinuous phase transition between an in-phase synchronized state at small separation distance and a counter-phase synchronized state at larger separation distances. A previously used dynamical model is modified in order to accommodate our experimental findings.

Fluid Kinetic Energy Asymptotic Expansion for Two Variable Radii Moving Spherical Bubbles at Small Separation Distance

Two spherical bubbles with changing radii are considered to be moving in ideal fluid along their center-line. The exact expression for the fluid kinetic energy is obtained. The Stokes stream function is expanded in Gegenbauer polynomials in bispherical coordinates. This expansion is used to obtain the exact series for the fluid kinetic energy quadratic form coefficients. The new series are confirmed to be correct by comparison with the known ones. The main advantage of the new kinetic energy form is the possibility to obtain asymptotic expansions at small separation distance between the bubbles. These expansions are obtained and their convergence is analyzed. The results of this work can be used to describe the bubbles approach before the contact and their coalescence in acoustic field.

Bilayer MSe2 and MS2 (M = Mo, W) as a novel drug delivery system for β-lapachone anticancer drug: Quantum chemical study

Transition metal dichalcogenides (TMDCs) considers a promising route in the biomedical applications to deliver drugs. The mechanism of this study is utilized the Bilayer (BL) MSe2 and MS2 to delivery β-lapachone (β-lap) anticancer drug by putting this molecule up on the slabs. Density Functional Theory (DFT) method is applied in the Quantum espresso package to compute various electronic properties. Results demonstrated that the pristine BL MSe2 and MS2 have a semiconductor behavior with indirect electronic band gaps. The complex structures (MSe2/β-lap and MS2/β-lap) have different electronic properties. So, there is a very small electronic band gap with direct transition at the K point for MSe2/ β-lap structure and at K-Γ region for WSe2/β-lap structure. For MoS2/β-lap and WS2/β-lap structures, the electronic band gap also reduced with indirect transition. For the stability of the complex structures, our finding revealed that all these structures became more stable and lower reactive with this anticancer drug due to the total energy is increased compared to the pristine cases. The chemical hardness of the complex structures is a small, which led to the smaller excitation energy required to transfer electrons. Moreover, they have higher ability to transfer electrons. There are smaller separation distance between the valence and conduction bands due to these complex structures have smaller value of the chemical softness. There are strong interaction between BL (MSe2 and MS2) and β-lap anticancer drug because of they have larger (absolute value) of the chemical potential and lower value of electrophilicity. Then, these results stimulate and encourage the experimental researchers to design these new complex structures to deliver β-lap anticancer drug.

Cluster validity index for irregular clustering results

Different clustering algorithms with different parameter settings can produce various partitions on the input data. Without the priori knowledge, it is difficult for users to select the proper clustering algorithm and the parameters for the specific data in advance. Therefore, the cluster validity index (CVI) is crucial to help select the best partition that fits the underlying structure of the data. However, most existing CVIs (including some recent ones that designed for complex partitions) have strong assumptions. They only work well for partitions where clusters are spherically distributed, with similar sizes and densities, and with large separation distances. In complicated situations where irregular clustering results (i.e., clustering results having clusters in arbitrary shapes, different sizes and densities, and with small separation distances) exist, they usually fail to find the best fitting partition. Focusing on the insufficiencies of the existing CVIs (designed for hard clustering), this paper presents a new index that helps to find the best partition produced by hard clustering algorithms when irregular clustering results exist. The proposed index uses the density changes inside a cluster, and the density changes from the inner to the inter-cluster regions of the cluster to evaluate its quality. Experiments are implemented on both real and synthetic datasets. 11 other CVIs including some well known as well as recently proposed ones are also tested for comparison. Experimental results are given to demonstrate the effectiveness of the new index.

Motional Dynamical Decoupling for Interferometry with Macroscopic Particles

We extend the concept of dynamical decoupling from spin to mechanical degrees of freedom of macroscopic objects, for application in interferometry. In this manner, the superposition of matter waves can be made resilient to many important sources of noise when these are driven along suitable paths in space. As a concrete implementation, we present the case of levitated (or free falling) nanodiamonds hosting a color center in a magnetic field gradient. We point out that these interferometers are inherently affected by diamagnetic forces, which restrict the separation of the superposed states to distances that scale with the inverse of the magnetic field gradient. Periodic forcing of the mechanical degree of freedom is shown to overcome this limitation, achieving a linear-in-time growth of the separation distance independent of the magnetic field gradient, while simultaneously protecting the coherence of the superposition from environmental perturbations.

Tunable wetting surfaces with interacting cavities via femtosecond laser patterning and wet etching

This paper presents the production of bioinspired slippery glass surfaces with interacting cavities via wet-etching-assisted femtosecond laser fabrication. A femtosecond laser irradiates a glass surface to fabricate microvoid arrays inside the substrate. Then, wet etching is performed to induce microcavities on the sample surface. With laser-induced selective etching, the region below the substrate surface is etched faster, thereby developing microcavities. The microvoid separation distance is found to be important for controlling the contact angle (CA) of the liquid wetting the surface. By choosing an adequate microvoid separation distance and etching time, interacting/interconnected cavities can be successfully fabricated. CAs are expected to be tuned from almost 0° to 137° ± 2.5° based on the cavity separation distance and the processes used (laser patterning, etching, and silanization). These interconnected structures fabricated with small separation distances (e.g., 10 μm) can lock in an infused lubricating liquid and form a stable, inert, slippery interface, known as a slippery liquid-infused porous surface, which acts as a smooth cushion for liquid repellence. Moreover, the infused liquid can significantly increase the transmittance owing to the index matching effect. Such slippery surfaces could be used in several self-cleaning, optical-sensing, and biomedical applications.

Near-Field Thermal Radiation between Two Plates with Sub-10 nm Vacuum Separation.

Radiation greatly exceeding blackbody between two objects separated by microscale distances has attracted great interest. However, challenges in reaching such a small separation between two plates have so far prevented studies below a separation distance of about 25 nm. Here, we report a study of radiation enhancement in the near-field regime of less than 10 nm between two parallel plates. We make use of bulk, rigid plates to approach small separation distances without the adverse snap-in effect, develop embedded temperature sensors to allow near-zero separation, and employ advanced sensing method to level the plates and approach and maintain small separations. Our findings agree with theoretical predictions between parallel surfaces with separations down to 7 nm where an 18000 times enhancement in radiation between two quartz plates is observed. Our method can also be used to explore heat transfer between other materials and can possibly be extended to smaller separation gaps.

An Aerodynamic Assessment of Vehicle-Side Wall Interaction using Numerical Simulations

The effects of induced pressure loads from a realistic vehicle onto the surface of a road-side wall are numerically investigated. Parameters such as vehicle speeds, vehicle-wall separation distances and the effects of inclined walls are examined to numerically characterize the vehicle-wall interactions. Aerodynamic characteristics such as the drag, lift, side forces and pressure coefficients are analyzed on the vehicle to provide a basis for comparison between each of the aforementioned variations. Our results demonstrate that a smaller separation distance between vehicle and wall enhances the pressure induced on both the wall and car which is found to be consistent with the experimental data published previously. We find that the presence of a wall in close proximity to the passing vehicle unfavourably influences the induced pressure on the side-wall and abruptly increases the drag, lift and side forces experienced by the vehicle. For a vertical side-wall, from a wall separation point of view, a separation distance of 1.35 normalized by the height of the vehicle tends to retrieve the cars’ original drag and lift value. In addition, our results demonstrate that a wall inclined to the ground favourably influences the aerodynamic characteristics of the vehicle compared to its vertical counterpart.

Flow-structure interactions of two parallel inverted flags with small separation distances in a water tunnel

In this study, the dynamics and flow fields of two parallel inverted flags are investigated using particle image velocimetry technology. The separation distance between two flags is less than two times the length of the flag, and the length ratio of these two flags is considered in the investigation. The results show that for the dynamic behaviours of two identical flags with a larger separation distance, the anti-phase and in-phase modes occur successively in the periodic oscillation as the flow velocity increases. The anti-phase and in-phase oscillations occur according to the formation position of the low-pressure and recirculation areas at different flow velocities. Moreover, a novel coupled flapping mode is observed at smaller separation distances: the contact anti-phase flapping mode, in which one flag oscillates with a large symmetric amplitude, and the other flag oscillates with a single-side large amplitude. As the separation distance further decreases, the in-phase mode appears for a larger range of flow velocity values, to avoid contact for the largest possible amplitude oscillation. Finally, as the length ratio decreases to 0.75, the oscillation frequency of the shorter flag becomes twice that of the longer flag, causing the in-phase and anti-phase oscillations to occur simultaneously in one cycle (i.e., the multi-phase flapping state). Interestingly, the two flags oscillate out of phase in the flapping apart process to avoid contact at a higher flow velocity. In general, the lower amplitude of the longer flag and two contact flags relative to that of an isolated flag clearly indicates the importance of two equal-length and non-contact flags for energy harvesting.

Detailed heat transfer measurements of impinging swirling and non-swirling jet arrays emitted from grooved orifice plate

An innovative type of heat exchanger with heat transfer benefits from jet impingements is proposed. The increase of surface area to volume ratio for the heat exchanger requires the minimization of the separation distance for the impinging jets, which motivates the present study to comparatively examine the heat transfer performances between the non-swirling and swirling impinging jet arrays emitted from the orifice plate with vertical and horizontal grooves at small separation distances less than one orifice diameter. As demonstrated by the selective experimental results, either the grooves on the orifice plate or the swirling jets can effectively alleviate the unfavorable crossflow effect to boost the heat trasnfer rates at small separation distances. After analyzing the isolated and coupled Rej and separation-distance effects on the local and area-averaged heat transfer properties for the non-swirling and swirling impinging jet arrays, two sets of physically consistent heat transfer and friction factor correlations are devised for practical applications.

Capillary forces on wet particles with a liquid bridge transition from convex to concave

The transition of a liquid bridge profile from convex to concave and the associated capillary forces are experimentally studied via particle-particle and particle-plane pairs. The results demonstrate that a convex liquid bridge appears at a relatively large water volume and small separation distance, where the capillary force remains approximately constant. As the separation distance increases, the liquid bridge is stretched from convex to concave, and the capillary force initially doesn't change much but increases to the maximum value and then decreases gradually. By combining analytical analysis and experimental data, a framework is proposed for predicting the evolution of liquid bridge profiles. Additionally, a capillary force model that is applicable to both convex and concave liquid bridges is formulated. The applicability of this force model to multiple-particle systems is demonstrated by comparing DEM predictions of repose angles of sandpile and hopper flow discharge rates with experimental measurements at various water contents.

Experimental Evidence for Algebraic Double-Layer Forces.

According to conventional wisdom, electric double-layer forces normally decay exponentially with separation distance. Here, we present experimental evidence of algebraically decaying double-layer interactions. We show that algebraic interactions arise in both strongly overlapping as well as counterion-only regimes, albeit the evidence is less clear for the former regime. In both of these cases, the disjoining pressure profile assumes an inverse square distance dependence. At small separation distances, another algebraic regime is recovered. In this regime, the pressure decays as the inverse of separation distance.