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We adopt a thermodynamically consistent multi-phase, multi-component phase-field model to investigate the morphological evolution of peritectic transition in carbon steel though 2-D ...and 3-D simulations. By using phase-field method, we rationalize the peritectic solidification in both 2-D and 3-D simulations under different liquid supersaturations as well as on the δ particle with distinct microstructures. Through the comparison between 2-D and 3-D simulation results, we clarify the reason for the different growth rate of γ phase in two and three dimensions. In 3-D simulation, we observe the unequal growth rate of γ phase in radial and axis directions. In addition, a novel measurement method is proposed to determine the dynamic contact angle. We anticipate that the simulation results can be applied to interpret the isothermal peritectic transition with a liquid supersaturation in alloys.
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•Systematic measurements of the apparent dynamic contact angle of dilute polymer solution drops are presented.•Polymer concentrations cover almost entirely the dilute regime.•Drop ...impacts onto commercial PTFE surfaces are investigated at different Weber numbers.
The dynamic contact angle of dilute polymer solution drops impacting on a hydrophobic PTFE surface is studied experimentally by high-speed imaging, for polymer concentrations ranging between 0ppm and 400ppm, and impact Weber numbers between 15 and 115. Images with spatial resolution of 16.4μm/pixel were captured at 8000fps to resolve the short time-scale dynamics. Results show that the receding contact angle of dilute polymer solution drops is significantly smaller than in case of drops of pure water, suggesting that the receding contact line encounter a higher resistance to its displacement. The minimum contact angle decreases with respect to both increasing polymer concentration and increasing Weber number. At lower Weber numbers, contact angle oscillations were observed, which can be related to the liquid flow to and from the rim during drop retraction.
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Wilhelmy plate method does not consider viscous force in the force balance equation to measure the dynamic contact angle and this results in a significant error in the measurement. ...Differences between the results obtained by optical method and Wilhelmy plate method indicate the importance of viscous force in the force balance equation. A theoretical viscous model is proposed, which must be considered in the force balance equation in Wilhelmy plate method to increase the accuracy of the dynamic contact angle measurement, especially for the case of highly viscous liquids and for experiments at large speeds of the plate, which can lead to large shear rate along the surface of the plate.
•A finite force is identified for the no-slip hydrodynamic solution of the MCL.•Microscopic dynamic contact angle is modeled using a new dynamic Young’s equation.•MCL force and angle are a function ...of viscosity, velocity, and surface tension only.
In theoretical analyses of the moving contact line, an infinite force along the solid wall has been reported based off the non-integrable stress along a single interface. In this investigation we demonstrate that the stress singularity is integrable and results in a finite force at the moving contact line if the contact line is treated as a one-dimensional manifold and all three interfaces that make up the moving contact line are taken into consideration. This is due to the dipole nature of the vorticity and pressure distribution around the moving contact line. Mathematically, this finite force is determined by summing all the forces that act over an infinitesimally small cylindrical control volume that encloses the entire moving contact line. With this finite force, we propose a new dynamic Young’s equation for microscopic dynamic contact angle that is a function of known parameters only, specifically the interface velocity, surface tension, and fluid viscosity. We combine our model with Cox’s model for apparent dynamic contact angle and find good agreement with published dynamic contact angle measurements.
Surface micro- and nanostructural modifications of dental and orthopedic implants have shown promising in vitro, in vivo and clinical results. Surface wettability has also been suggested to play an ...important role in osteoblast differentiation and osseointegration. However, the available techniques to measure surface wettability are not reliable on clinically relevant, rough surfaces. Furthermore, how the differentiation state of osteoblast lineage cells impacts their response to micro/nanostructured surfaces, and the role of wettability on this response, remain unclear. In the current study, surface wettability analyses (optical sessile drop analysis, environmental scanning electron microscopic analysis and the Wilhelmy technique) indicated hydrophobic static responses for deposited water droplets on microrough and micro/nanostructured specimens, while hydrophilic responses were observed with dynamic analyses of micro/nanostructured specimens. The maturation and local factor production of human immature osteoblast-like MG63 cells was synergistically influenced by nanostructures superimposed onto microrough titanium (Ti) surfaces. In contrast, human mesenchymal stem cells cultured on micro/nanostructured surfaces in the absence of exogenous soluble factors exhibited less robust osteoblastic differentiation and local factor production compared to cultures on unmodified microroughened Ti. Our results support previous observations using Ti6Al4V surfaces showing that recognition of surface nanostructures and subsequent cell response is dependent on the differentiation state of osteoblast lineage cells. The results also indicate that this effect may be partly modulated by surface wettability. These findings support the conclusion that the successful osseointegration of an implant depends on contributions from osteoblast lineage cells at different stages of osteoblast commitment.
In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority ...of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas–liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.
The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an “adhesion-like” force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid–air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.
The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (θadv=10°–70°) and hydrophobic (θadv=105°–120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.
It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We<˜80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading.
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•Numerical prediction of low viscous droplet dynamics after impact on a solid substrate.•The inclusion of an adhesion force at the contact line of the droplet is proposed.•Dynamic contact angle is derived from simulation and not set as a boundary condition.•Good agreement with experiments for a wide range of Weber numbers is achieved.•Good agreement with experiments for hydrophobic and hydrophilic surfaces is achieved.
•Dynamic wetting is studied on surfaces with 3D roughness by GPU-LBM.•The 3D roughness morphology effects are investigated.•The overall surfaces remain hydrophilic and Wenzel state at low Ca, ...different from 2D simulations.
Dynamic wetting on rough surface is important in imbibition processes in micro- and nano- channels. Due to high computational costs, the previous studies by direct numerical simulations mostly adopted two-dimensional (2D) simplifications of surface with roughness. The cutting-edge advances in high performance computing make it possible to conduct three-dimensional (3D) direct simulations. This work presents 3D simulations on channel flows with roughness on walls via the GPU-based multiphase lattice Boltzmann method (LBM). The effects from 3D roughness morphology are investigated. In contrast to the wettability alternation in 2D rough ones, the 3D rough hydrophilic surfaces remain hydrophilic and Wenzel state at even low capillary numbers (Ca). The apparent contact angle follows the hydrodynamic model on rough surfaces except for a more sensitivity to Ca. The results also indicate that the 2D simplifications overestimate the pressure drop for rough channels, while underestimate over 20% for smooth channels. Therefore, it is crucial to consider 3D structures of surface roughness for dynamic wetting in real channels.
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A comprehensive study of molten alloy capillary flow in the wetting/non-wetting wedge-tee configuration is presented. The horizontal Al2O3 substrate is non-wetting, while the vertical, AlMn alloy ...AA3003, is wetting. The braze alloy is a composite of an aluminum‑silicon alloy (Al10Si) with potassium fluoro-aluminate flux embedded. The purpose for selecting such a wetting/non-wetting configuration is for repair and construction in space by means of brazing. When capillary forces are not opposed by gravity, the spreading of a liquid braze may be controlled by a non-wetting surface.
Specifically, we study: (i) the evolution of the free surface shape of a large melt mass in the gravity field, (ii) kinetics of advancing/receding triple lines and dynamic contact angles, and (iii) microstructure of the resolidified molten alloy on the substrates.
The receding contact line exhibits a sudden withdrawal towards the corner, then a long-time stagnation before final equilibrium in the corner. In contrast to the typical monotonic evolution of the advancing contact angle, the receding contact angle features an anomalous non-monotonic behavior. The microstructure of the re-solidified sample features a phase macrosegregation.
Characterizing the contact line dynamics on solid walls is often a crucial problem encountered in the simulation of complex interfacial unsteady flows, such as drop impacts on solid surfaces. In this ...work, a new model is proposed to reproduce the contact line dynamics in a simple but effective way, based on introducing in the momentum equation a force term proportional to the deviation of the calculated contact angle from the value predicted by a dynamic model that takes into account wettability hysteresis. The model has been implemented in a volume of fluid (VOF) method and is applied to the simulation of drop impacts leading to deposition outcomes, although it could be extended to other interface tracking methods and is also applicable to more complex drop impacts involving fingering and splashing. Numerous tests have been performed to evaluate the accuracy and robustness of the proposed model over a wide range of Reynolds and Weber numbers. The results substantially improve those obtained by imposing only the contact angle as a boundary condition at the contact line, and satisfactorily predict a variety of experimental results from the literature for very different impact and wettability conditions.
•A new force model for accurate reproduction of contact line dynamics is proposed.•Force based on deviation of the computed contact angle from a dynamic model.•Isosurface extraction to measure the contact angle and locate the contact line.•Experimental and numerical validation of the model in drop deposition VOF simulations.
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