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Wetting and spreading
, 2009
"... Wetting phenomena are ubiquitous in nature and technology. A solid substrate exposed to the environment is almost invariably covered by a layer of fluid material. In this review, the surface forces that lead to wetting are considered, and the equilibrium surface coverage of a substrate in contact wi ..."
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Cited by 26 (0 self)
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Wetting phenomena are ubiquitous in nature and technology. A solid substrate exposed to the environment is almost invariably covered by a layer of fluid material. In this review, the surface forces that lead to wetting are considered, and the equilibrium surface coverage of a substrate in contact with a drop of liquid. Depending on the nature of the surface forces involved, different scenarios for wetting phase transitions are possible; recent progress allows us to relate the critical exponents directly to the nature of the surface forces which lead to the different wetting scenarios. Thermal fluctuation effects, which can be greatly enhanced for wetting of geometrically or chemically structured substrates, and are much stronger in colloidal suspensions, modify the adsorption singularities. Macroscopic descriptions and microscopic theories have been developed to understand and predict wetting behavior relevant to microfluidics and nanofluidics applications. Then the dynamics of wetting is examined. A drop, placed on a substrate which it wets, spreads out to form a film. Conversely, a nonwetted substrate previously covered by a film dewets upon an appropriate change of system parameters. The hydrodynamics of both wetting and dewetting is influenced by the presence of the threephase contact line separating “wet ” regions from those that are either dry or covered by a microscopic film only. Recent theoretical, experimental, and numerical progress in the description of
Finite element framework for describing dynamic wetting phenomena
 Int. J. Numer. Meth. Fluids
"... The finite element simulation of dynamic wetting phenomena, requiring the computation of flow in a domain confined by intersecting a liquid–fluid free surface and a liquid–solid interface, with the threephase contact line moving across the solid, is considered. For this class of flows, different fi ..."
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Cited by 11 (7 self)
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The finite element simulation of dynamic wetting phenomena, requiring the computation of flow in a domain confined by intersecting a liquid–fluid free surface and a liquid–solid interface, with the threephase contact line moving across the solid, is considered. For this class of flows, different finite element method (FEM) implementations have been used in the literature, and in some cases, these produced apparently contradictory results. In the present paper, a robust framework for the FEM simulation of dynamic wetting flows is developed, which, by consistently adhering to the FEM methodology, leaves no room for ad hoc ‘optional’ variations in the numerical handling of these flows. The developed approach makes it possible to conduct a convergence study, assess the spatial resolution required to achieve a preset accuracy and provide the corresponding benchmark calculations. This analysis allows one to identify numerical artefacts, which had previously been interpreted as physical effects, and demonstrates that suppressing numerical errors using a ‘strong ’ implementation of a boundary condition creates bigger and less detectable errors elsewhere in the computational domain. We provide practical recommendations on the spatial resolution required by a numerical scheme for a given set of nondimensional similarity parameters and give a userfriendly stepbystep guide specifying the entire implementation, which allows the reader to easily reproduce all presented results including the benchmark calculations. It is also shown how the developed framework accommodates generalizations of the mathematical model accounting for additional physical effects, such as gradients in surface
Finite Element Simulation of Dynamic Wetting Flows as an Interface Formation Process
, 2012
"... A mathematically challenging model of dynamic wetting as a process of interface formation has been, for the first time, fully incorporated into a numerical code based on the finite element method and applied, as a test case, to the problem of capillary rise. The motivation for this work comes from t ..."
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Cited by 8 (5 self)
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A mathematically challenging model of dynamic wetting as a process of interface formation has been, for the first time, fully incorporated into a numerical code based on the finite element method and applied, as a test case, to the problem of capillary rise. The motivation for this work comes from the fact that, as discovered experimentally more than a decade ago, the key variable in dynamic wetting flows — the dynamic contact angle — depends not just on the velocity of the threephase contact line but on the entire flow field/geometry. Hence, to describe this effect, it becomes necessary to use the mathematical model that has this dependence as its integral part. A new physical effect, termed the ‘hydrodynamic resist to dynamic wetting’, is discovered where the influence of the capillary’s radius on the dynamic contact angle, and hence on the global flow, is computed. The capabilities of the numerical framework are then demonstrated by comparing the results to experiments on the unsteady capillary rise, where excellent agreement is obtained. Practical recommendations on the spatial resolution required by the numerical scheme for a given set of nondimensional similarity parameters are provided, and a comparison to asymptotic results available in limiting cases confirms that the code is converging to the correct solution. The appendix gives a userfriendly stepbystep guide specifying the entire implementation and allowing the reader to easily reproduce all presented results, including the benchmark calculations.
Viscous flow in domains with corners: Numerical artifacts, their origin and removal
 Computer Methods in Applied Mechanics and Engineering
"... a b s t r a c t Viscous flows in domains with boundaries forming twodimensional corners are considered. We examine the case where on each side of the corner the boundary condition for the tangential velocity is formulated in terms of stress. It is shown that computing such flows numerically by str ..."
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Cited by 5 (3 self)
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a b s t r a c t Viscous flows in domains with boundaries forming twodimensional corners are considered. We examine the case where on each side of the corner the boundary condition for the tangential velocity is formulated in terms of stress. It is shown that computing such flows numerically by straightforwardly applying welltested algorithms (and numerical codes based on their use, such as COMSOL Multiphysics) can lead to spurious multivaluedness and meshdependence in the distribution of the fluid's pressure. The origin of this difficulty is that, near a corner formed by smooth parts of the boundary, in addition to the solution of the formulated inhomogeneous problem, there also exists an eigensolution. For obtuse corner angles this eigensolution (a) becomes dominant and (b) has a singular radial derivative of velocity at the corner. Despite the bulk pressure in the eigensolution being constant, when the derivatives of the velocity are singular, numerical errors in the velocities calculation near the corner give rise to pressure spikes, whose magnitude increases as the mesh is refined. A method is developed that uses the knowledge about the eigensolution to remove the artifacts in the pressure distribution. The method is first explained in the simple case of a Stokes flow in a corner region and then generalized for the NavierStokes equations applied to describe steady and unsteady freesurface flows encountered in problems of dynamic wetting.
The Dynamics of Liquid Drops and their Interaction with Solids of Varying Wettabilities
, 2012
"... Microdrop impact and spreading phenomena are explored as an interface formation process using a recently developed computational framework. The accuracy of the results obtained from this framework for the simulation of high deformation freesurface flows is confirmed by a comparison with previous nu ..."
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Cited by 5 (3 self)
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Microdrop impact and spreading phenomena are explored as an interface formation process using a recently developed computational framework. The accuracy of the results obtained from this framework for the simulation of high deformation freesurface flows is confirmed by a comparison with previous numerical studies for the large amplitude oscillations of free liquid drops. Our code’s ability to produce high resolution benchmark calculations for dynamic wetting flows is then demonstrated by simulating microdrop impact and spreading on surfaces of greatly differing wettability. The simulations allow one to see features of the process which go beyond the resolution available to experimental analysis. Strong interfacial effects which are observed at the microfluidic scale are then harnessed by designing surfaces of varying wettability that allow new methods of flow control to be developed.
Transient behavior of the Planarflow Meltspinning process
 In preparation
"... Planarflow melt spinning (PFMS) is a singlestage rapid manufacturing technique for producing thin metal sheets or ribbons. During the processing molten metal flows through a nozzle onto a moving substrate where a puddle is formed. This study focuses on the time dependent behavior of the process a ..."
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Cited by 4 (2 self)
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Planarflow melt spinning (PFMS) is a singlestage rapid manufacturing technique for producing thin metal sheets or ribbons. During the processing molten metal flows through a nozzle onto a moving substrate where a puddle is formed. This study focuses on the time dependent behavior of the process and looks at the puddle dynamics during the cast. Understanding how the issues of heat transfer, fluid flow and contacting dynamics influence the quality of the cast ribbon are the primary focus of the study. The commercial acceptance of PFMS requires ribbons to be cast with good quality (e.g. uniform thickness). There are a large range scales that are relevant to the ribbon product. Thickness variations occur on the macroscale over the length of the cast (50 m). There is a steady decrease in ribbon thickness over the length of the cast. There is also a periodic variation on the length scale of the wheel circumference (23 m). Steady mass, momentum and energy balances are used to understand these long lengthscale thickness variations. There are also thickness variations that occur on smaller scales, cm to mm.
Thermal Transport Behavior of a Liquid Plug Moving Inside a Dry Capillary Tube
 Heat Pipe Science and Technology, Accepted, DOI: 10.1615/HeatPipeScieTech.2013006563
, 2013
"... ABSTRACT: Computational simulation of flow and heattransfer characteristics of a single isolated liquid plug (of glycerin and water, respectively) flowing in a round capillary tube (internal diameter, D = 2.0 mm), so as to understand its local thermohydrodynamic transport behavior, is reported. B ..."
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Cited by 2 (1 self)
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ABSTRACT: Computational simulation of flow and heattransfer characteristics of a single isolated liquid plug (of glycerin and water, respectively) flowing in a round capillary tube (internal diameter, D = 2.0 mm), so as to understand its local thermohydrodynamic transport behavior, is reported. Both, constant heat flux and constanttemperature boundary conditions have been considered at the tube wall. When an isolated liquid plug moves in a capillary tube, there is a difference in the advancing and receding dynamic contact angles of the two menisci, respectively. This has been considered in the simulations. The linearized simplification of HoffmanTanner’s law is used to model the variation in the two respective apparent dynamic contact angles with the velocity of the liquid plug (i.e., Ca = µU/σ). Simulations are carried out for a range of Capillary numbers and lengths of liquid plugs (L/D ratio). It has been found that variation in dynamic contact angle leads to an enhanced local and average heattransfer coefficient in the moving liquid plug; the local fluid circulation being affected by menisci deformation. In addition, as the length of the liquid plug is increased, the heattransfer coefficient decreases and finally shows the asymptotic transport behavior of Poiseuille flow. Other than the flow Capillary number, the fluid Prandtl number and the L/D ratio of the liquid plug, strongly affect the local thermo hydrodynamics.
The impact of inkjet droplets on a paperlike structure. Fluid Dyn.
 droplet and spray/wall interactions 181 Kalantari,
, 2011
"... Abstract: Inkjet technology has been recognized as one of the most successful and promising microsystem technologies. The wide application areas of printer heads and the increasing demand of high quality prints are making ink consumption and print seethrough important topics in the inkjet technol ..."
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Abstract: Inkjet technology has been recognized as one of the most successful and promising microsystem technologies. The wide application areas of printer heads and the increasing demand of high quality prints are making ink consumption and print seethrough important topics in the inkjet technology. In the present study we investigate numerically the impact of ink droplets onto a porous material that mimics the paper structure. The mathematical framework is based on a free energy formulation, coupling the CahnHilliard and Navier Stokes equations, for the modelling of the twophase flow. The case studied here consists of a multiphase flow of airliquid along with the interaction between a solid structure and an interface. In order to characterize the multiphase flow characteristics, we investigate the effects of surface tension and surface wettability on the penetration depth and spreading into the paperlike structure.
Frictional Properties of Automatic Transmission Fluids: Part 2: Origins of FrictionSliding Speed Behaviour
"... A wet clutch consists of a series of alternating friction material and steel separator discs immersed in transmission fluid and its effective performance depends on the friction between these pairs of discs. In order to engage and disengage efficiently, ..."
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Cited by 1 (1 self)
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A wet clutch consists of a series of alternating friction material and steel separator discs immersed in transmission fluid and its effective performance depends on the friction between these pairs of discs. In order to engage and disengage efficiently,