2,681 research outputs found
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
Effective slip over partially filled microcavities and its possible failure
Motivated by the emerging applications of liquid-infused surfaces (LIS), we
study the drag reduction and robustness of transverse flows over
two-dimensional microcavities partially filled with an oily lubricant. Using
separate simulations at different scales, characteristic contact line
velocities at the fluid-solid intersection are first extracted from nano-scale
phase field simulations and then applied to micron-scale two-phase flows, thus
introducing a multiscale numerical framework to model the interface
displacement and deformation within the cavities. As we explore the various
effects of the lubricant-to-outer-fluid viscosity ratio
, the capillary number Ca, the static contact
angle , and the filling fraction of the cavity , we find that
the effective slip is most sensitive to the parameter . The effects of
and are generally intertwined, but
weakened if . Moreover, for an initial filling fraction , our results show that the effective slip is nearly independent of the
capillary number, when it is small. Further increasing Ca to about , we identify a possible failure mode, associated
with lubricants draining from the LIS, for . Very viscous lubricants (\eg ),
on the other hand, are immune to such failure due to their generally larger
contact line velocity.Comment: Phys. Rev. Fluids (2018
Influence of viscoelasticity on the nano-micromechanical behavior of latex films and pigmented coatings
Nan0 to microscale deformation behavior of different carboxylated styrene-butadiene co- polymer Latexes were investigated using a commercial nanoindentation device. The latexes dif- fered primarily in their glass transition temperature (Tg). The bulk dynamic rheological properties, as determined from a rheometer, dictate the axismmetric deformation behavior of the latexes. Results from dynamic tests performed on latexes were analyzed using the theories in contact and fracture mechanics. Two theories of linear viscoelastic fracture mechanics (LVEFM) were employed to model the adhesion hysteresis (loading-unloading cycle) curves to obtain meaningful cohesive zone (fracture process zone) parameters and a stress intensity functional (KI(t)) for an entire cycle. The stress intensity functional, extracted from the deformation behavior, is inde- pendent of the loading history and was shown to depend only on the crack propagation velocity, (dddt), for the entire cycle. The quantitative values of stress intensities were then discussed in the light of polymer molecular phenomenonās such as viscous chain desorption. Nanoindetation was developed as a tool for systematically investigating both the bulk as well as the cohesive zone properties of viscoelastic polymers. Effect of plastic deformation on the deformation behavior of high pigment volume concentration (PVC) coatings was also analyzed. Polystyrene plastic pigment, CaC03 and Clay pigments were used to form the coatings layers. High PVC coatings are viscoelastic due to the latex present but also contain air, the third phase, which could explain the plastic deformation if a certain critical yield stress is exceeded. At PVCās greater than 70%, the coatings showed significant plastic (permanent) deformation, which has to be accounted for in modeling the hysteresis curves. The residual plastic deformation was confirmed by imaging the indent over a period of time. Modeling the curves resulted in a compressive yield stress (ĻĪ³) value, which is an important parameter in predicting the calendaring performance of these coatings
Wetting, roughness and hydrodynamic slip
The hydrodynamic slippage at a solid-liquid interface is currently at the
center of our understanding of fluid mechanics. For hundreds of years this
science has relied upon no-slip boundary conditions at the solid-liquid
interface that has been applied successfully to model many macroscopic
experiments, and the state of this interface has played a minor role in
determining the flow. However, the problem is not that simple and has been
revisited recently. Due to the change in the properties of the interface, such
as wettability and roughness, this classical boundary condition could be
violated, leading to a hydrodynamic slip. In this chapter, we review recent
advances in the understanding and expectations for the hydrodynamic boundary
conditions in different situations, by focussing mostly on key papers from past
decade. We highlight mostly the impact of hydrophobicity, roughness, and
especially their combination on the flow properties. In particular, we show
that hydrophobic slippage can be dramatically affected by the presence of
roughness, by inducing novel hydrodynamic phenomena, such as giant interfacial
slip, superfluidity, mixing, and low hydrodynamic drag. Promising directions
for further research are also discussed.Comment: 36 pages, 19 figures. This chapter would be a part of "Nanoscale
liquid interfaces" boo
Effects of pore fluids on quasiāstatic shear modulus caused by poreāscale interfacial phenomena
It is evident from the laboratory experiments that shear moduli of different porous isotropic rocks may show softening behaviour upon saturation. The shear softening means that the shear modulus of dry samples is higher than of saturated samples. Shear softening was observed both at low (seismic) frequencies and high (ultrasonic) frequencies. Shear softening is stronger at seismic frequencies than at ultrasonic frequencies, where the softening is compensated by hardening due to unrelaxed squirt flow. It contradicts to Gassmann's theory suggesting that the relaxed shear modulus of isotropic rock should not depend upon fluid saturation, provided that no chemical reaction between the solid frame and the pore fluid. Several researchers demonstrated that the shear softening effect is reversible during reāsaturation of rock samples, suggesting no permanent chemical reaction between the solid frame and the pore fluid. Therefore, it is extremely difficult to explain this fluidārock interaction mechanism theoretically, because it does not contradict to the assumptions of Gassmann's theory, but contradicts to its conclusions. We argue that the observed shear softening of partiallyāsaturated rocks by different pore fluids is related to poreāscale interfacial phenomena effects, typically neglected by the rock physics models. These interface phenomena effects are dependent on surface tension between immiscible fluids, rock wettability, aperture distribution of microcracks, compressibility of microcracks, porosity of microcracks, elastic properties of rock mineral, fluid saturation, effective stress and wave amplitude. Derived equations allow to estimate effects of pore fluids and saturation on the shear modulus and mechanical strength of rocks.publishedVersio
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