1,238 research outputs found
Spreading Dynamics of Nanodrops: A Lattice Boltzmann Study
Spreading of nano-droplets is an interesting and technologically relevant
phenomenon where thermal fluctuations lead to unexpected deviations from
well-known deterministic laws. Here, we apply the newly developed fluctuating
non-ideal lattice Boltzmann method [Gross et al., J. Stat. Mech., P03030
(2011)] for the study of this issue. Confirming the predictions of Davidovich
and coworkers [PRL 95, 244905 (2005)], we provide the first independent
evidence for the existence of an asymptotic, self-similar noise-driven
spreading regime in both two- and three-dimensional geometry. The cross over
from the deterministic Tanner's law, where the drop's base radius grows (in
3D) with time as and the noise dominated regime where is also observed by tuning the strength of thermal noise.Comment: 5 page
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Lattice Boltzmann modelling of droplets on chemically heterogeneous surfaces with large liquid-gas density ratio
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.A lattice Boltzmann method which can simulate droplet dynamics on partial wetting surface with large liquid-gas density ratio is proposed. The interaction between the fluid-fluid interface and the partial wetting wall is typically considered. Using the method, the dynamics of liquid drops on chemically heterogeneous surfaces are numerically simulated. The corresponding mechanisms including droplet spreading, break-up and migration on such surfaces are studied on the basis of droplet shapes, moving contact lines and velocity fields.This work is supported by the UK EPSRC under grant EP/D500125/1
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LBM, a useful tool for mesoscale modelling of single phase and multiphase flow – the variety of applications and approaches at Nottingham
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Giving an overview of Nottingham group’s recent progress on numerical modelling and
approaches in developing and applying the lattice Boltzmann method (LBM), the paper tries to demonstrate that the LBM is a useful tool for mesoscale modelling of single phase and multiphase flow. The variety of applications of the LBM modelling is reported, which include single phase fluid flow and heat transfer around or across rotational cylinder of curved boundary, two-phase flow in mixing layer, electroosmotically driven flow in thin liquid layer, bubbles/drops flow and coalescence in conventional channels and in
microchannels with confined boundary, liquid droplets in gas with relative large density ratio; viscous fingering phenomena of immiscible fluids displacement, and flow in porous media
Wetting gradient induced separation of emulsions: A combined experimental and lattice Boltzmann computer simulation study
Guided motion of emulsions is studied via combined experimental and
theoretical investigations. The focus of the work is on basic issues related to
driving forces generated via a step-wise (abrupt) change in wetting properties
of the substrate along a given spatial direction. Experiments on binary
emulsions unambiguously show that selective wettability of the one of the fluid
components (water in our experiments) with respect to the two different parts
of the substrate is sufficient in order to drive the separation process. These
studies are accompanied by approximate analytic arguments as well as lattice
Boltzmann computer simulations, focusing on effects of a wetting gradient on
internal droplet dynamics as well as its relative strength compared to
volumetric forces driving the fluid flow. These theoretical investigations show
qualitatively different dependence of wetting gradient induced forces on
contact angle and liquid volume in the case of an open substrate as opposed to
a planar channel. In particular, for the parameter range of our experiments,
slit geometry is found to give rise to considerably higher separation forces as
compared to open substrate.Comment: 34 pages, 12 figure
Specialized Inter-Particle Interaction Lbm For Patterned Superhydrophobic Surfaces
SPECIALIZED INTER-PARTICLE INTERACTION LBM FOR PATTERNED SUPERHYDROPHOBIC SURFACES
by
AMAL S. YAGUB
ABSTRACT:
Superhydrophobic surface characteristics are important in many industrial applications, ranging from the textile to the military. It was observed that surfaces fabricated with nano/micro roughness can manipulate the droplet contact angle, thus providing an opportunity to control the droplet wetting characteristics. The Shan and Chen (SC) lattice Boltzmann model (LBM) is a good numerical tool, which holds strong potentials to qualify for simulating droplets wettability. This is due to its realistic nature of droplet contact angle (CA) prediction on flat smooth surfaces. But SC-LBM was not able to replicate the CA on rough surfaces because it lacks a real representation of the physics at work under these conditions. By using a correction factor to influence the interfacial tension within the asperities, the physical forces acting on the droplet at its contact lines were mimicked. This approach allowed the model to replicate some experimentally confirmed Wenzel and Cassie wetting cases. Regular roughness structures with different spacing were used to validate the study using the classical Wenzel and Cassie equations. This work highlights the strength and weakness of the SC model and attempts to qualitatively conform it to the fundamental physics, which causes a change in the droplet apparent contact angle, when placed on nano/micro structured surfaces.
In the second part of this work, the model is used also to analyze the sliding of droplets in contact with flat horizontal surfaces. This part identifies the main factors, which influence the multiphase fluids transport in squared channels. Effects of dimensionless radius, Weber number, Reynolds number and static contact angles are evaluated by calculating the power required for moving single droplets in comparison to the power needed for moving the undisturbed flow in the channel. Guidelines for optimizing the design of such flow are presented.
In last part of work, the sliding of droplets on sloped surfaces with and without roughness is numerically investigated. The Shan and Chen (SC) Lattice Boltzmann model (LBM) is used to analyze the effect of pinning on the movement of droplets placed on sloped surfaces. The model is checked for conformance with the Furmidge equation which applies to tilted unstructured surfaces. It is shown that a droplet sliding on a perfectly smooth surface requires very minimal slope angle and that pinning due to the inhomogeneous nature of manufactured smooth surfaces is the key factor in determining the minimal slope angle. The model is also used on sloped rough surfaces to check the effects of roughness on the movement of single droplets. The numerical outcomes are compared with published experimental results for validation and a dimensionless number is suggested for quantifying the degree of pinning needed to control the behavior of sliding droplets on sloped surfaces
Wetting of anisotropic sinusoidal surfaces - experimental and numerical study of directional spreading
Directional wettability, i.e. variation of wetting properties depending on the surface orientation, can be achieved by anisotropic surface texturing. A new high precision process can produce homogeneous sinusoidal surfaces (in particular parallel grooves) at the micro-scale, with a nano-scale residual roughness five orders of magnitude smaller than the texture features. Static wetting experiments have shown that this pattern, even with a very small aspect ratio, can induce a strong variation of contact angle depending on the direction of observation. A comparison with numerical simulations (using Surface Evolver software) shows good agreement and could be used to predict the fluid-solid interaction and droplet behaviour on textured surfaces. Two primary mechanisms of directional spreading of water droplets on textured stainless steel surface have been identified. The first one is the mechanical barrier created by the textured surface peaks, this limits spreading in perpendicular direction to the surface anisotropy. The second one is the capillary action inside the sinusoidal grooves accelerating spreading along the grooves. Spreading has been shown to depend strongly on the history of wetting and internal drop dynamics
Dynamical density functional theory for the evaporation of droplets of nanoparticle suspension
We develop a lattice gas model for the drying of droplets of a nanoparticle
suspension on a planar surface, using dynamical density functional theory
(DDFT) to describe the time evolution of the solvent and nanoparticle density
profiles. The DDFT assumes a diffusive dynamics but does not include the
advective hydrodynamics of the solvent, so the model is relevant to highly
viscous or near to equilibrium systems. Nonetheless, we see an equivalent of
the coffee-ring stain effect, but in the present model it occurs for
thermodynamic rather the fluid-mechanical reasons. The model incorporates the
effect of phase separation and vertical density variations within the droplet
and the consequence of these on the nanoparticle deposition pattern on the
surface. We show how to include the effect of slip or no-slip at the surface
and how this is related to the receding contact angle. We also determine how
the equilibrium contact angle depends on the microscopic interaction
parameters.Comment: 35 pages, 10 figure
Numerical Study of Cloud-Sized Droplet Impact and Freezing on Superhydrophobic Surfaces
In-flight icing is a serious meteorological hazard caused by supercooled cloud particles (with an average size of 20–50 µm) that turn into ice as an immediate consequence of impact with an aircraft, and it poses a serious risk to the safety of the aircraft and its passengers. Anti-icing surface treatment is a potential solution to mitigate ice accretion and maintain optimal flying conditions. Superhydrophobic coatings inspired by nature (e.g., lotus leaf) have attracted much attention in recent years due to their excellent water repellent properties. These coatings have been extensively applied on various substrates for self-cleaning, anti-fogging, and anti-corrosive applications. The performance of these coatings depends on the chemical composition and their rough hierarchical surface morphology composed of micron and sub-micron-sized structures. Recently, there has been an increased interest to fabricate superhydrophobic coatings that can repel droplets of cloud-relevant sizes (20–50 µm) before they freeze to the surface in practical flight conditions (i.e., icephobic surfaces).
The main goal of this work was to numerically model the hydrodynamic and thermal behaviour of cloud-sized droplets on superhydrophobic surfaces when interacting with micron-sized surface features. Consequently, by correlating the hydrophobicity and the icephobicity of the surface, we found viable solutions to counteract icing and to prevent ice accumulation on critical aerodynamic surfaces. For this purpose, we developed a computational model to analyze the hydrodynamics of the impact of the micro-droplet on a micro-structured superhydrophobic surface under room temperature and freezing (including rapid-cooling and supercooling) conditions. All coding and implementations were carried out in the OpenFOAM platform, which is a collection of open-source C++ libraries for computational continuum mechanics and CFD analysis. Superhydrophobic surfaces were directly modelled as a series of fine, micro-structured arrays with defined cross sections and patterns. Surface chemistry was included in the simulations using a dynamic contact angle model that describes well the hydrodynamics of a micro-droplet on rough surfaces. A multi-region transient solver for incompressible, laminar, multi-phase flow of non-isothermal, non-Newtonian fluids with conjugate heat transfer boundary conditions between solid and fluid regions was developed to simulate both the dynamics of the micro-droplet impact on the substrate and the associated heat transfer inside the droplet and the solid bulk simultaneously. In addition, a phase change (freezing) model was added to capture the onset of ice formation and freezing front of the liquid micro-droplet. The computational model was validated using experimental data reported in the literature. In addition, an analytical model was derived using the balance of energy before impact and at the maximum spreading stage, which we found to be in good agreement with the data obtained from simulations.
Since aluminum (Al) is the base material used in aerospace industries, the thermo-physical properties of aluminum were extensively used in our simulations. Comparing laser-patterned aluminum substrates with a ceramic base composite material that has a low thermal diffusivity (such as titanium-dioxide), we showed that the onset of icing was significantly delayed on the ceramic-based substrate, as the droplet detached before freezing to the surface. Finally, a freezing model for the supercooled water droplet based on classical nucleation theory was developed. The model is an approximation for a supercooled droplet of the recalescence step, which was assumed to be initiated by heterogeneous nucleation from the substrate. This research extended our knowledge about the hydrodynamic and freezing mechanisms of a micro-droplet on superhydrophobic surfaces. The developed solvers can serve as a design tool to engineer the roughness and thermo-physical properties of superhydrophobic coatings to prevent the freezing of cloud-sized droplets in practical flight conditions
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