1,370 research outputs found
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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
Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces
We report experiments investigating the behaviour of micron-scale fluid
droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes.
The final droplet shape depends on the droplet size relative to that of the
stripes. In particular when the droplet radius is of the same order as the
stripe width, the final shape is determined by the dynamic evolution of the
drop and shows a sensitive dependence on the initial droplet position and
velocity. Numerical solutions of the dynamical equations of motion of the drop
provide a close quantitative match to the experimental results. This proves
helpful in interpreting the data and allows for accurate prediction of fluid
droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui
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
Numerical study of wetting transitions on biomimetic surfaces using a lattice Boltzmann approach with large density ratio
The hydrophobicity of natural surfaces have drawn much attention of scientific communities in recent years. By mimicking natural surfaces, the manufactured biomimetic hydrophobic surfaces have been widely applied to green technologies such as self-cleaning surfaces. Although the theories for wetting and hydrophobicity have been developed, the mechanism of wetting transitions between heterogeneous wetting state and homogeneous wetting state is still not fully clarified. As understanding of wetting transitions is crucial for manufacturing a biomimetic superhydrophobic surface, more fundamental discussions in this area should be carried out. In the present work the wetting transitions are numerically studied using a phase field lattice Boltzmann approach with large density ratio, which should be helpful in understanding the mechanism of wetting transitions. The dynamic wetting transition processes between Cassie-Baxter state and Wenzel state are presented, and the energy barrier and the gravity effect on transition are discussed. It is found that the two wetting transition processes are irreversible for specific inherent contact angles and have different transition routes, the energy barrier exists on an ideally patterned surface and the gravity can be crucial to overcome the energy barrier and trigger the transition
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Numerical simulation of microflow over superhydrophobic surfaces by lattice Boltmann method
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.The superhydrophobicity of a microchannel is determined by not only the wettability of channel wall but also the surface topography. Recent experiments have found that superhrydrophobic surfaces can be achieved by pattering roughness on hydrophobic surfaces. In this paper, the dynamics of two-phase flow in microchannel with different wettability and topography is studied numerically by the lattice Boltzmann method (LBM). The mechanism of drag reduction resulted from the superhydrophobicity is investigated. In
particular, the effect of different rough surfaces on superhydrophobicity is analyzed. It is found that flow behaviours are strongly affected by the wall wettability and topography. The results show that the LBM has a good application prospect in the study of drag reduction in microchannels.The UK Royal Society-NSFC (China) International Joint Project (2009-2011), China NSFC under grant (50920105504), and China Scholarship Council (CSC)
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
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
Generalized Lattice Boltzmann Method with multi-range pseudo-potential
The physical behaviour of a class of mesoscopic models for multiphase flows
is analyzed in details near interfaces. In particular, an extended
pseudo-potential method is developed, which permits to tune the equation of
state and surface tension independently of each other. The spurious velocity
contributions of this extended model are shown to vanish in the limit of high
grid refinement and/or high order isotropy. Higher order schemes to implement
self-consistent forcings are rigorously computed for 2d and 3d models. The
extended scenario developed in this work clarifies the theoretical foundations
of the Shan-Chen methodology for the lattice Boltzmann method and enhances its
applicability and flexibility to the simulation of multiphase flows to density
ratios up to O(100)
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
Interface-Resolving Simulations of Gas-Liquid Two-Phase Flows in Solid Structures of Different Wettability
This PhD study is devoted to numerical investigations of two-phase flows on and through elementary and complex solid structures of varying wettability. The phase-field method is developed and implemented in OpenFOAM®. The numerical method/code is verified by a series of test cases of two-phase flows, and then applied to investigate: (1) droplet wetting on solid surfaces; (2) air bubble rising and interacting with cellular structures and (3) gas-liquid interfacial flows in foam structures
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