The hydrodynamic lift velocity of a neutrally buoyant fibre in a simple shear flow near a wall is determined for small, but non-zero, fibre Reynolds number, illustrating the role of non-sphericity in ...lift. The rotational motion and effects of fibre orientation on lift are treated for fibre positions that induce and do not induce solid-body wall contacts. When the fibre does not contact the wall its lift velocity can be obtained in terms of the Stokes flow field by using a generalized reciprocal theorem. The Stokes velocity field is determined using slender-body theory with the no-slip velocity at the wall enforced using the method of images. To leading order the lift velocity at distances large compared with the fibre length and small compared with the Oseen length is found to be
$0.0303\unicodeSTIX{x1D70C}\dot{\unicodeSTIX{x1D6FE}}^{2}l^{2}a/(\unicodeSTIX{x1D707}\ln 2l/a)$
, where
$l$
and
$a$
are the fibre half-length and radius,
$\unicodeSTIX{x1D70C}$
is the density,
$\dot{\unicodeSTIX{x1D6FE}}$
is the shear rate and
$\unicodeSTIX{x1D707}$
is the viscosity of the fluid. When the fibre is close enough to the wall to make solid-body contact during its rotational motion, a process known as pole vaulting coupled with inertially induced changes of fibre orientation determines the lift velocity. The order of magnitude of the lift in this case is larger by a factor of
$l/a$
than when the fibre does not contact the wall and it reaches a maximum of
$0.013\unicodeSTIX{x1D70C}\dot{\unicodeSTIX{x1D6FE}}^{2}l^{3}/(\unicodeSTIX{x1D707}\ln l/a)$
for the case of a highly frictional contact and about half that value for a frictionless contact. These results are used to illustrate how particle shape can contribute to separation methods such as those in microfluidic channels or cross-flow filtration processes.
Collisions in a dilute polydisperse suspension of spheres of negligible inertia interacting through non-continuum hydrodynamics and settling in a slow uniaxial compressional flow are studied. The ...ideal collision rate is evaluated as a function of the relative strength of gravity and uniaxial compressional flow and it deviates significantly from a linear superposition of these driving terms. This non-trivial behaviour is exacerbated by interparticle interactions based on uniformly valid non-continuum hydrodynamics, that capture non-continuum lubrication at small separations and full continuum hydrodynamic interactions at larger separations, retarding collisions driven purely by sedimentation significantly more than those driven purely by the linear flow. While the ideal collision rate is weakly dependent on the orientation of gravity with the axis of compression, the rate including hydrodynamic interactions varies by more than $100\,\%$ with orientation. This dramatic shift can be attributed to complex trajectories driven by interparticle interactions that prevent particle pairs from colliding or enable a circuitous path to collision. These and other important features of the collision process are studied in detail using trajectory analysis at near unity and significantly smaller than unity size ratios of the interacting spheres. For each case analysis is carried for a large range of relative strengths and orientations of gravity to the uniaxial compressional flow, and Knudsen numbers (ratio of mean free path of the media to mean radius).
The collisions in a dilute polydisperse suspension of sub-Kolmogorov spheres with negligible inertia settling in a turbulent flow and interacting through hydrodynamics including continuum breakdown ...on close approach are studied. A statistically significant decrease in ideal collision rate without gravity is resolved via a Lagrangian stochastic velocity-gradient model at Taylor microscale Reynolds number larger than those accessible by current direct numerical simulation capabilities. This arises from the difference between the mean inward velocity and the root-mean-square particle relative velocity. Differential sedimentation, comparable to the turbulent shear relative velocity, but minimally influencing the sampling of the velocity gradient, diminishes the Reynolds number dependence and enhances the ideal collision rate i.e. the rate without interactions. The collision rate is retarded by hydrodynamic interactions between sphere pairs and is governed by non-continuum lubrication as well as full continuum hydrodynamic interactions at larger separations. The collision efficiency (ratio of actual to ideal collision rate) depends on the relative strength of differential sedimentation and turbulent shear, the size ratio of the interacting spheres and the Knudsen number (defined as the ratio of the mean-free path of the gas to the mean radius of the interacting spheres). We develop an analytical approximation to concisely report computed results across the parameter space. This accurate closed form expression could be a critical component in computing the evolution of the size distribution in applications such as water droplets in clouds or commercially valuable products in industrial aggregators.
Bipolar nanochannels comprising two domains of positively and negatively charged walls along the pore axis are known to rectify current when exposed to an electric potential bias. We find that ...addition of charged nanoparticles can increase rectification considerably, by approximately one order of magnitude. Two bipolar channel geometries are considered here; their behavior is examined at rest and under the influence of a negative bias and a positive bias, respectively. We do so by relying on a molecular-level model of the electrolyte solution in the channels. The large increase in current rectification can be explained by the inherent electric field that charged nanoparticles generate within the channel. This effect is found to be largely dependent on the pore's geometry, its charge distribution, and the sign of the nanoparticles' charge, thereby offering new opportunities for design of engineered nanopore membrane-nanoparticle systems for energy storage.
Charged nanoparticles can significantly increase rectification in bipolar nanopores but this effect is largely dependent on the pore's geometry, its charge distribution, and the sign of the nanoparticles' charge.
Janus, or two-sided, charged membranes offer promise as ionic current rectifiers. In such systems, pores consisting of two regions of opposite charge can be used to generate a current from a gradient ...in salinity. The efficiency of nanoscale Janus pores increases dramatically as their diameter becomes smaller. However, little is known about the underlying transport processes, particularly under experimentally accessible conditions. In this work, we examine the molecular basis for rectification in Janus nanopores using an applied electric field. Molecular simulations with explicit water and ions are used to examine the structure and dynamics of all molecular species in aqueous electrolyte solutions. For several macroscopic observables, the results of such simulations are consistent with experimental observations on asymmetric membranes. Our analysis reveals a number of previously unknown features, including a pronounced local reorientation of water molecules in the pores, and a segregation of ionic species that had not been anticipated by previously reported continuum analyses of Janus pores. Using these insights, a model is proposed for ionic current rectification in which electric leakage at the pore entrance controls net transport.
The evolution of droplets in clouds is studied with focus on the 'size-gap' regime of 15–40 μm radius, where condensation and differential sedimentation are least effective in promoting growth. This ...bottleneck leads to inaccurate growth models and turbulence can potentially rectify disagreement with in-situ cloud measurements. Turbulent shear and differential sedimentation will both drive collisional growth in the ‘size-gap’ and the resulting coupled configurational dynamics is rigorously studied.Droplet inertia will not significantly alter the local collision dynamics as it is weak in typical cloud conditions. However, weak inertia acting over a range of separation scales enhances the concentration of neighbouring drops available for collision. An inertial clustering model is developed which incorporates an inertia-induced drift velocity, relative diffusion due to turbulent shear and acceleration and differential sedimentation. This model is built upon available direct-numerical simulations and theoretical predictions in limiting conditions. It allows predictions over a broad range of particle separations, Stokes numbers, settling velocities and Taylor scale Reynolds numbers Reλ.The inertia-less local collision dynamics of sub-Kolmogorov droplets due to turbulence and gravity is studied for both a frozen linear flow approximation, in line with the classical work by Saffman & Turner, and for a stochastically fluctuating linear flow based on a Lagrangian velocity gradient model. It is found that the ideal collision rate has a significant dependence on Reλ that has not been recognized in previous work. Inclusion of interparticle interactions strongly retards the collision rate. Non-continuum hydrodynamic interactions of droplets in clouds dominates over colloidal forces, deformation, interface mobility, and medium compressibility but has not found extensive treatment in the previous literature. Hence, the collision efficiency, capturing retardation, is calculated over a large parameter space including Knudsen number (Kn), the ratio of mean free path to mean sphere radius, relative size of the interacting spheres, Reλ, and strength of differential sedimentation relative to turbulence. Analytical fits of the collision rate results facilitate their use in drop population models. The steady linear flow approximation facilitated a detailed examination of the complex trajectory evolution that results from the competition of gravity and shear in the presence of non-continuum hydrodynamic interactions.Utilising the collision rate results an evolution study is carried for cloud droplets from condensation controlled sizes of a few micron to differential sedimentation dominated sizes through the ‘size-gap’. For a complete description of cloud droplet dynamics non-collisional components of turbulence, mixing of droplets and water vapour fluctuations, are included. To resolve turbulent intermittency and retain a discrete drop distribution with manageable computational load a Monte Carlo scheme is used. Cloud packets are used to capture multiple realisations of the stochastic turbulent processes. The collection of droplets within each packet represents a unique history of turbulent intensity and water vapour concentration and so different packets represent different regions of the cloud. Turbulent mixing is modelled by moving droplets between different packets. The simulations reveal the strong effect of hydrodynamic interactions and the mean-free path on drop size evolution. Condensation in a uniform environment tends to create a nearly monodisperse drop size distribution making differential sedimentation weak. It is shown that turbulent shear and clustering as well as water vapour fluctuations play important roles in producing polydispersity and allowing droplet growth through the size gap.
Bipolar nanochannels comprising two domains of positively and negatively charged walls along the pore axis are known to rectify current when exposed to an electric potential bias. We find that ...addition of charged nanoparticles can increase rectification considerably, by approximately one order of magnitude. Two bipolar channel geometries are considered here; their behavior is examined at rest and under the influence of a negative bias and a positive bias, respectively. We do so by relying on a molecular-level model of the electrolyte solution in the channels. The large increase in current rectification can be explained by the inherent electric field that charged nanoparticles generate within the channel. This effect is found to be largely dependent on the pore's geometry, its charge distribution, and the sign of the nanoparticles' charge, thereby offering new opportunities for design of engineered nanopore membrane-nanoparticle systems for energy storage.
Janus --or two-sided, charged membranes offer promise as ionic current rectifiers. In such systems, pores consisting of two regions of opposite charge can be used to generate a current from a ...gradient in salinity. The efficiency of \textcolor{black}{nanoscale} Janus pores increases dramatically as their diameter becomes smaller. However, little is known about the underlying transport processes\textcolor{black}{, particularly under experimentally accessible conditions}. In this work, we examine the molecular basis for rectification in Janus nanopores using an applied electric field. Molecular simulations with explicit water and ions are used to examine the structure and dynamics of all molecular species in aqueous electrolyte solutions. \textcolor{black}{For several macroscopic observables, the results of such simulations are consistent with experimental observations on asymmetric membranes. Our analysis reveals a number of previously unknown features, including a pronounced local re-orientation of water molecules in the pores, and a segregation of ionic species that has not been anticipated by previously reported continuum analyses of Janus pores. Using these insights, a model is proposed for ionic current rectification in which electric leakage at pore entrance controls net transport.
Large-scale transcriptome sequencing efforts have vastly expanded the catalog of long non-coding RNAs (lncRNAs) with varying evolutionary conservation, lineage expression, and cancer specificity. ...Here, we functionally characterize a novel ultraconserved lncRNA, THOR (ENSG00000226856), which exhibits expression exclusively in testis and a broad range of human cancers. THOR knockdown and overexpression in multiple cell lines and animal models alters cell or tumor growth supporting an oncogenic role. We discovered a conserved interaction of THOR with IGF2BP1 and show that THOR contributes to the mRNA stabilization activities of IGF2BP1. Notably, transgenic THOR knockout produced fertilization defects in zebrafish and also conferred a resistance to melanoma onset. Likewise, ectopic expression of human THOR in zebrafish accelerated the onset of melanoma. THOR represents a novel class of functionally important cancer/testis lncRNAs whose structure and function have undergone positive evolutionary selection.
Display omitted
•THOR is a conserved lncRNA expressed in cancers and in normal testis•THOR functions via a conserved interaction with IGF2BP1, stabilizing its targets•Oncogenicity of THOR is corroborated in a zebrafish model
An ultraconserved lncRNA promotes oncogenesis.
PDF