A Two-Field Formulation for Surfactant Transport within the Algebraic Volume of Fluid Method

Surfactant transport plays an important role in many technical processes and industrial applications such as chemical reactors, microfluidics, printing and coating technology. High fidelity numerical simulations of two-phase flow phenomena reveal rich insights into the flow dynamics, heat, mass and species transport. In the present study, a two-field formulation for surfactant transport within the algebraic volume of fluid method is presented. The slight diffuse nature of representing the interface in the algebraic volume of fluid method is utilized to track the concentration of surfactant at the interface as a volumetric concentration. Transport of insoluble and soluble surfactants is investigated by tracking two different concentrations of the surfactant, one within the bulk of the liquid and the other one at the interface. These two transport equations are in turn coupled by source terms considering the ad-/desorption processes at a liquid-gas interface. Appropriate boundary conditions at a solid-fluid interface are formulated to ensure surfactant conservation, while also enabling to study the ad-/desorption processes at a solid-fluid interface. The developed numerical method is verified by comparing the numerical simulations with well-known analytical and numerical reference solutions. The presented numerical methodology offers a seamless integration of surfactant transport into the algebraic volume of fluid method, where the latter has many advantages such as volume conservation and an inherent ability of handling large interface deformations and topological changes

Growth of large-area single- and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces

We report graphene films composed mostly of one or two layers of graphene grown by controlled carbon precipitation on the surface of polycrystalline Ni thin films during atmospheric chemical vapor deposition(CVD). Controlling both the methane concentration during CVD and the substrate cooling rate during graphene growth can significantly improve the thickness uniformity. As a result, one- or two- layer graphene regions occupy up to 87% of the film area. Single layer coverage accounts for 5-11% of the overall film. These regions expand across multiple grain boundaries of the underlying polycrystalline Ni film. The number density of sites with multilayer graphene/graphite (>2 layers) is reduced as the cooling rate decreases. These films can also be transferred to other substrates and their sizes are only limited by the sizes of the Ni film and the CVD chamber. Here, we demonstrate the formation of films as large as 1 in2. These findings represent an important step towards the fabrication of large-scale high-quality graphene samples

Adomian decomposition method simulation of Von Kármán swirling bioconvection nanofluid flow

The study reveals analytically on the 3-dimensional viscous time-dependent gyrotactic bioconvection in swirling nanofluid flow past from a rotating disk. It is known that the deformation of the disk is along the radial direction. In addition to that Stefan blowing is considered. The Buongiorno nanofluid model is taken care of assuming the fluid to be dilute and we find Brownian motion and thermophoresis have dominant role on nanoscale unit. The primitive mass conservation equation, radial, tangential and axial momentum, heat, nano-particle concentration and micro-organism density function are developed in a cylindrical polar coordinate system with appropriate wall (disk surface) and free stream boundary conditions. This highly nonlinear, strongly coupled system of unsteady partial differential equations is normalized with the classical Von Kármán and other transformations to render the boundary value problem into an ordinary differential system. The emerging 11th order system features an extensive range of dimensionless flow parameters i.e. disk stretching rate, Brownian motion, thermophoresis, bioconvection Lewis number, unsteadiness parameter, ordinary Lewis number, Prandtl number, mass convective Biot number, Péclet number and Stefan blowing parameter. Solutions of the system are obtained with developed semi-analytical technique i.e. Adomian decomposition method. Validation of the said problem is also conducted with earlier literature computed by Runge-Kutta shooting technique

Challenges in nanoscale physics of wetting phenomena

We describe the aims and content of this issue

Editorial: Challenges in nanoscale physics of wetting phenomena

We describe the aims and content of this issue

A hydrodynamic model for subcooled liquid jet impingement at the Leidenfrost condition

Stable film boiling occurs in the stagnation region of an impinging subcooled liquid jet during quenching of very hot steel plates. During film boiling the liquid is separated from the surface of the plate by a continuous vapor layer. The minimum surface temperature required to support film boiling is referred to as the Leidenfrost temperature. The present work is devoted to the development of a theoretical model for determination of vapor layer thickness, wall heat flux and wall superheat at the Leidenfrost condition. The model is developed for the jet impingement region with a strong transverse pressure gradient, i.e. within the stagnation and the acceleration regions. For convenience, the entire stagnation and acceleration region is referred to as the stagnation region in this article. Due to the pressure gradient in the impingement region, both the vapor and the liquid flow outwards from the stagnation point. In the current model, it is assumed that the Leidenfrost condition corresponds to zero shear stress at the vapor–liquid interface in the entire stagnation region. Our analytical model is developed for both planar and circular jets, assuming that the entire stagnation region satisfies this condition. The model is based on a solution of the momentum equation in the vapor layer, and the energy equation in the liquid. For a planar jet, the predicted vapor layer thickness is in good agreement with the experimental data of Bogdanic et al. [1]. A vapor film thickness of 8 ± 2 μm for stable film boiling close to the Leidenfrost state has been measured, while the current model predicts a film thickness of 6.31 μm at the stagnation point for the same conditions. The wall heat flux is under-predicted by about 5–47% compared to the experimental data available in the literature, while the wall superheat is under-predicted by up to 70%. In the present analysis, the entire stagnation region has been considered to be at the Leidenfrost condition, which is unrealistic for experiments. In the published experiments, no spatially resolved heat transfer and wall temperature measurements were performed. It is possible that in the experiments transition and film boiling might occur simultaneously in the stagnation region at the minimum heat flux condition. Hence, the wall superheat estimations in this study deviate more than the heat flux estimations. Accurate experimental data are required to validate the model over a wider range of parameters

Experimental Investigation of Dynamics and Atomization of a Liquid Film Flowing over a Spinning Disk

One of the most commonly used methods of liquid atomization is the rotary atomization. In this process a radially spreading thin liquid film is created on a surface of a rotating disk, due to centrifugal forces. The liquid flows over the disk edge and disintegrates. The liquid film is mostly wavy. The radially propagating waves induce fluctuations resulting in an expansion of the drop size distribution after atomization. An experimental apparatus has been built to investigate the effect of the film dynamics on the atomization process. A water jet is impinging at the center of a rotating disk made of stainless steel. The local instantaneous film thickness is measured using a confocal chromatic sensoring technique. The drop sizes are determined using the shadowgraphy method. The film flow on the rotating disk has been investigated in a wide range of parameters. The strongly wavy structure of the film flow has been observed for all sets of parameters. The development of waves depends on the nozzle-to-disk distance. The radial distribution of the time-averaged film thickness over the disk surface agrees fairly well with the correlations found in the literature. First results of the drop size distribution show a bimodal distribution for low liquid mass flow rates