Bofeng Bai

Mechanisms of Molecular Permeation through Nanoporous Graphene Membranes

We present an investigation of molecular permeation of gases through nanoporous graphene membranes via molecular dynamics simulations; four different gases are investigated, namely helium, hydrogen, nitrogen, and methane. We show that in addition to the direct (gas-kinetic) flux of molecules crossing from the bulk phase on one side of the graphene to the bulk phase on the other side, for gases that adsorb onto the graphene, significant contribution to the flux across the membrane comes from a surface mechanism by which molecules cross after being adsorbed onto the graphene surface. Our results quantify the relative contribution of the bulk and surface mechanisms and show that the direct flux can be described reasonably accurately using kinetic theory, provided the latter is appropriately modified assuming steric molecule-pore interactions, with gas molecules behaving as hard spheres of known kinetic diameters. The surface flux is negligible for gases that do not adsorb onto graphene (e.g., He and H2), while for gases that adsorb (e.g., CH4 and N2) it can be on the order of the direct flux or larger. Our results identify a nanopore geometry that is permeable to hydrogen and helium, is significantly less permeable to nitrogen, and is essentially impermeable to methane, thus validating previous suggestions that nanoporous graphene membranes can be used for gas separation. We also show that molecular permeation is strongly affected by pore functionalization; this observation may be sufficient to explain the large discrepancy between simulated and experimentally measured transport rates through nanoporous graphene membranes.

NUMERICAL SIMULATION OF CELL ADHESION AND DETACHMENT IN MICROFLUIDICS

Inspired by the complex biophysical processes of cell adhesion and detachment under blood flow in vivo, numerous novel microfluidic devices have been developed to manipulate, capture, and separate bio-particles for various applications, such as cell analysis and cell enumeration. However, the underlying physical mechanisms are yet unclear, which has limited the further development of microfluidic devices and point-of-care (POC) systems. Mathematical modeling is an enabling tool to study the physical mechanisms of biological processes for its relative simplicity, low cost, and high efficiency. Recent development in computation technology for multiphase flow simulation enables the theoretical study of the complex flow processes of cell adhesion and detachment in microfluidics. Various mathematical methods (e.g., front tracking method, level set method, volume of fluid (VOF) method, fluid–solid interaction method, and particulate modeling method) have been developed to investigate the effects of cell properties (i.e., cell membrane, cytoplasma, and nucleus), flow conditions, and microchannel structures on cell adhesion and detachment in microfluidic channels. In this paper, with focus on our own simulation results, we review these methods and compare their advantages and disadvantages for cell adhesion/detachment modeling. The mathematical approaches discussed here would allow us to study microfluidics for cell capture and separation, and to develop more effective POC devices for disease diagnostics.

Shear-rate dependent effective thermal conductivity of H2O+SiO2 nanofluids

Effective thermal conductivity (ETC) of water-based silicon dioxide nanofluids in shear flow fields (flow shear rate range was 0–820 1/s) was measured using a rotating Couette apparatus. The results show that the ETC of the nanofluids in shear flow fields is significantly higher than that in static states. For the flow shear rates lower than a critical value (infinite-shear rate), the ETC asymptotically increases with increasing the flow shear rate; for the flow shear rates higher than the critical value, the ETC displays a plateau value (infinite-shear thermal conductivity). The increase of the ETC with shear rate is more obvious as increase the nanoparticle diameter and the nanoparticle volume fraction. For 16 different measured nanofluids, the infinite-shear rates vary from 445.0 to 712.1 1/s, while the infinite-shear thermal conductivities increase by 9%–17% comparing with the zero-shear thermal conductivities. The conventional ETC prediction correlation proposed for the suspensions containing micro-sized particles is not suitable for the nanofluids qualitatively and quantitatively. Finally, an exponential correlation is proposed based on our measured data to predict the ETC of nanofluids considering the effects of flow shear rate, nanoparticle diameter, and nanoparticle volume fraction.

Numerical Study on Turbulent Mixing of Spray Droplets in Crossflow

A three-dimensional model for turbulent gas-droplet two-phase flow is developed to obtain a systematical understanding of the mixing process of centrifugal spray jet and turbulent crossflow in a horizontal tube. The turbulent dispersion and droplet-wall interaction are considered for modeling the dispersed phase in Lagrangian frame. The effects of influencing factors on the mixing process, including water/gas mass flow ratio, spray angle, droplet diameter, axial injection angle of spray nozzle and tangential injection angle of spray nozzle, are examined. Two indices, the degree of mixedness and the dimensionless droplet concentration, are proposed to assess themixing performance of the spraydroplets in the crossflow ina complementaryway.The simulation results reveal that adding droplets into the crossflow imposes significant impact on the crossflow and in turn the affected crossflow produces considerable effect on dispersion and aggregation of the droplets. It is also found that the optimummixing of droplets and crossflowcanbe achievedwith the conditions that thewater/gasmassflow ratio is 1:1, the spray angle is near 90 , the droplet diameter is larger, the axial injection angle of spray nozzle is 90 , and the tangential injection angle of spray nozzle is 0 .

DIRECT NUMERICAL SIMULATION OF DETACHMENT OF SINGLE CAPTURED LEUKOCYTE UNDER DIFFERENT FLOW CONDITIONS

Antibody-based cell isolation using microfluidics finds widespread applications in disease diagnostics and treatment monitoring at point of care (POC) for global health. However, the lack of knowledge on underlying mechanisms of cell capture greatly limits their developments. To address this, in this study, we developed a mathematical model using a direct numerical simulation for the detachment of single leukocyte captured on a functionalized surface in a rectangular microchannel under different flow conditions. The captured leukocyte was modeled as a simple liquid drop and its deformation was tracked using a level set method. The kinetic adhesion model was used to calculate the adhesion force and analyze the detachment of single captured leukocyte. The results demonstrate that the detachment of single captured leukocyte was dependent on both the magnitude of flow rate and flow acceleration, while the latter provides more significant effects. Pressure gradient was found to represent as another critical factor promoting leukocyte detachment besides shear stress. Cytoplasmic viscosity plays a much more important role in the deformation and detachment of captured leukocyte than cortex tension. Besides, better deformability (represented as lower cytoplasmic viscosity) noteworthy accelerates leukocyte detachment. The model presented here provides an enabling tool to clarify the interaction of target cells with functional surface and could help for developing more effective POC devices for global health.

Mixing of Hollow-Cone Spray with a Confined Crossflow in Rectangular Duct

This paper presents the experimental results on the mixing of a hollow-cone spray with a confined crossflow. The experiments were carried out inside a rectangular duct (95×95u2009u2009mm in the cross section) at the ambient temperature and pressure with different spray and crossflow conditions, different injection angles, and different nozzle numbers (single and double). The criteria number, J, which can well evaluate the spray-crossflow mixing effect was defined. A better mixing can be achieved with proper J. The upper and bottom counter-rotating vortex pair structures were observed to dominate the mixing and strongly influence the mixing behaviors. For the double nozzle, more counter-rotating vortex pair structures are caused, and their sizes and strength become smaller than that of the single nozzle, which is beneficial for mixing. The spray direction along the crossflow also contributed to the mixing enhancement. The mixing optimization is realized with proper J and spray injection angles.

Single-jet Spray Mixing with a Confined Crossflow

Abstract In order to achieve uniform mixing between spray droplets and crossflow, cold-model experiment of a hollow-cone water spray in an air crossflow is investigated via a numerical simulation. The simulation cases are designed by using the orthogonal design method. The Eulerian-Lagrangian formulation is employed for modeling the droplets-crossflow two-phase flow while the realizable k -ɛ turbulence model is used to describe the turbulence. A new index, mixedness quality, is proposed to assess the overall mixing of the droplets in the crossflow. The simulation results demonstrate that the counter-rotating vortex pair (CVP) imposes a more significant impact on the spatial distribution than on the size distribution of the droplets. Pairs of CVP with smaller scales are preferable for achieving a better mixing. The influencing factors are listed in the following order in terms of the degree of their impact from the greatest to the least: the Sauter diameter of the initial droplets, the mixing tube diameter, the spray angle, the velocity of the inlet crossflow, and the vertical velocity of the initial droplets. A moderate droplet diameter, a smaller tube diameter, a moderate spray angle, a greater crossflow velocity and a moderate vertical velocity of the droplet are favorable for achieving a higher mixedness quality of the jet spray in a confined crossflow.

EFFECT OF FLOW ACCELERATION ON DEFORMATION AND ADHESION DYNAMICS OF CAPTURED CELLS

Cell deformation and adhesion under shear flows play an important role in both cell migration in vivo and capture based microfluidic devices in vitro. Adhesion dynamics of captured cell (e.g., firm adhesion, cell rolling and cell detachment) under steady shear flows have been studied extensively. However, cell adhesion under accelerating flows is common both in vivo and in vitro, and dynamics of cell adhesion under accelerating flows remains unknown. As such, we used a mathematical model based on the front tracking method and investigated the effect of flow acceleration on deformation and adhesion dynamics of captured cells, including cell deformation index, cell shape evolution, the velocities of cell center, contact time and wall shear stress for cell rolling and detachment by using a series of parameter values for leukocyte. The results showed that the cell presented three dynamics states (i.e., firm adhesion, rolling and detachment) with increasing wall shear stress under uniform flows. Wall shear stresses were 1.12 Pa for firm adhesion and detachment, respectively. The wall shear stresses were at the range 1.48–1.63 Pa (higher than 1.12 Pa) when cell left the bottom surface of the channel under flow accelerations (a = 0.975–1.625 m/s2). The minimum of deformation index under accelerating flow was smaller than that under uniform flow. In conclusion, the flow acceleration promotes the deformation and adhesion of captured cells. These findings could further the understanding of cell migration in vivo and promote the development of capture based microfluidic devices in vitro.

Numerical Simulation of Multi Species Mass Transfer in a SOFC Electrodes Layer Using Lattice Boltzmann Method

A two-dimensional multi-component Lattice Boltzmann (LB) model based on kinetic theory for gas mixtures combined with a representative elementary volume (REV) scale LB algorithm based on the Brinkman equation for flows in porous media is developed to simulate the mass transport in the porous anode and cathode of SOFC. The concentration overpotential is calculated and compared with that obtained by the extended Fick’s Model (FM), the Dusty Gas Model (DGM), and the Stefan Maxwell Model (SMM), as well as the experimental results. It is concluded that LB method is a much more accurate method for the simulation of mass transfer within fuel cell electrodes. Moreover, the effects of different electrode geometrical and operating parameters on concentration polarization are also investigated.Copyright