Showing papers on "Surface tension published in 2021"

Multi-layer deposition mechanism in ultra high-frequency pulsed wire arc additive manufacturing (WAAM) of NiTi shape memory alloys

Abstract: Ultra high-frequency pulsed gas tungsten arc welding (UHFP-GTAW)-based Wire Arc Additive Manufacturing (WAAM) was used to fabricate thin walls of NiTi shape memory alloys. The transient heat and fluid flow are critical during fusion-based additive manufacturing, since they impact the as-built microstructure. In this work, a three-dimensional numerical model, which includes the force, surface Gauss heat source and periodic droplet transfer models, was developed to simulate the deposition of 5 layers. The gravity, buoyancy, electromagnetic, surface tension, arc pressure and arc shear stress are considered in the developed force model. An improved free surface tracing method using the volume of fluid (VOF) technique was proposed to more effectively track the free surface of the molten pool with the computational fluid dynamics (CFD) software FLUENT. The multiphysic phenomena associated to the process, namely the temperature and velocity fields of the molten pool, were studied. The model was then validated by experiments. It is revealed that the microstructure of the as-built parts is refined by the UHFP current power which induces significant vibration of the molten pool. These findings lay the foundations for optimizing the WAAM process aiming at fabricating high quality and complex NiTi parts.

Surface Tension of the Oxide Skin of Gallium-Based Liquid Metals.

Abstract: Gallium-based alloys have garnered considerable attention in the scientific community, particularly as they are in an atypical liquid state at and near room temperature. Though physical parameters, such as thermal conductivity, electrical conductivity, viscosity, yield stress, and surface tension, of these alloys are broadly known, the surface tension (surface free energy) of the oxide skin remains intangible due to the high yield stress of the oxide skin. In this article, we propose to employ gradually attenuated vibrations to obtain equilibrium shapes, which are analyzed along the lines of the puddle height method. The surface tension of the oxide skin was determined on quartz glass and liquid metal-phobic diamond coating to be around 350-365 mN/m, thus independent of the substrate surface or employed liquid metal (i.e., eutectic Ga-In (EGaIn) and galinstan). The similarity of the surface tension for different alloys was ascribed to the composition of the oxide skin, which predominantly comprises gallium oxides due to thermodynamic constraints. We envision that this method can also be applied to other liquid metal alloys and liquid metal marble systems facilitating modeling, simulation, and optimization processes.

Interfacial Tension Modulation of Liquid Metal via Electrochemical Oxidation

Abstract: This progress report summarizes recent studies of electrochemical oxidation to modulate the interfacial tension of gallium-based alloys. These alloys, which are liquid at ambient conditions, have the largest interfacial tension of any liquid at room temperature. The ability to modulate the tension offers the possibility to create forces that change the shape and position of the metal. It has been known since the late 1800s that electrocapillarity-the use of potential to modulate the electric double layer on the surface of metals in electrolyte-lowers the interfacial tension of liquid metal. Yet, this phenomenon can only achieve modest changes in interfacial tension since it is limited to potential windows that avoid reactions. A recent discovery suggests that reactions driven by the electrochemical oxidation of gallium alloys cause the interfacial tension to decrease from ~500 mN/m at 0 V to ~0 mN/m at ~0.8 V, a change in tension that goes well beyond what is possible via conventional electrocapillarity or surfactants. The changes in tension are reversible; reductive potentials return the metal back to a state of high interfacial tension. This report aims to summarize key work and introduce beginners to this field by including electrochemistry basics while addressing misconceptions. We discuss applications that utilize modulations in interfacial tension of liquid metal and conclude with remaining opportunities and challenges that need further investigation.

Necking, beading, and bulging in soft elastic cylinders

Abstract: Due to surface tension, a beading instability takes place in a long enough fluid column that results in the breakup of the column and the formation of smaller packets with the same overall volume but a smaller surface area. Similarly, a soft elastic cylinder under axial stretching can develop an instability if the surface tension is large enough. This instability occurs when the axial force reaches a maximum with fixed surface tension or the surface tension reaches a maximum with fixed axial force. However, unlike the situation in fluids where the instability develops with a finite wavelength, for a hyperelastic solid cylinder that is subjected to the combined action of surface tension and axial stretching, a linear bifurcation analysis predicts that the critical wavelength is infinite. We show, both theoretically and numerically, that a localized solution can bifurcate sub-critically from the uniform solution, but the character of the resulting bifurcation depends on the loading path. For fixed axial stretch and variable surface tension, the localized solution corresponds to a bulge or a depression, beading or necking, depending on whether the axial stretch is greater than a certain threshold value that is dependent on the material model and is equal to 2 3 when the material is neo-Hookean. At this single threshold value, localized solutions cease to exist and the bifurcation becomes exceptionally supercritical. For either fixed surface tension and variable axial force, or fixed axial force and variable surface tension, the localized solution corresponds to a depression or a bulge, respectively. We explain why the bifurcation diagrams in previous numerical and experimental studies look as if the bifurcation were supercritical although it was not meant to. Our analysis shows that beading in fluids and solids are fundamentally different. Fluid beading resulting from the Plateau–Rayleigh instability follows a supercritical linear instability whereas solid beading in general is a subcritical localized instability akin to phase transition.

Modeling and simulation of 3D geometry prediction and dynamic solidification behavior of Fe-based coatings by laser cladding

Abstract: To accurately predict the three-dimensional (3D) characteristics of the coating and dynamic solidification process during coaxial powder-fed laser cladding, an improved 3D multi-physics finite element model including heat transfer, fluid flow, shielding gas pressure, powder temperature-rise, surface tension, and free surface movement is proposed. The input parameters of the model only depend on the thermophysical properties of the deposited material and the process parameters. Therefore, the model has good universality, and can be flexibly applied in other laser cladding systems or deposited materials. The average width and height error for the experimental and simulated is 6.05% and 4.15%, respectively. The numerical model has good abilities to predict the 3D geometry of the coating. The accuracy of the model can be indeed improved by considering the powder temperature-rise and shielding gas pressure during the modeling process. The dynamic growth process of the coating, spatiotemporal variations of the temperature and temperature gradient were discussed in detail to reveal the solidification mechanism. Finally, the relationship between the crystal characteristic (planar, cellular, columnar, and equiaxed grains) and thermal parameters (the temperature, the temperature gradient, and the solidification rate) was established. The results show that the temperature gradient and undercooling at the solidification interface are significantly responsible for the crystal characteristic, while the grain size is mainly governed by the solidification rate.

From Surface Tension to Molecular Distribution: Modeling Surfactant Adsorption at the Air-Water Interface.

Abstract: Surfactants are centrally important in many scientific and engineering fields and are used for many purposes such as foaming agents and detergents However, many challenges remain in providing a comprehensive understanding of their behavior Here, we provide a brief historical overview of the study of surfactant adsorption at the air-water interface, followed by a discussion of some recent advances in this area from our group The main focus is on incorporating an accurate description of the adsorption layer thickness of surfactant at the air-water interface Surfactants have a wide distribution at the air-water interface, which can have a significant effect on important properties such as the surface excess, surface tension, and surface potential We have developed a modified Poisson-Boltzmann (MPB) model to describe this effect, which we outline here We also address the remaining challenges and future research directions in this area We believe that experimental techniques, modeling, and simulation should be combined to form a holistic picture of surfactant adsorption at the air-water interface

Dynamic Contact Angle Reformulates Pore-Scale Fluid-Fluid Displacement at Ultralow Interfacial Tension

Abstract: Surfactant flooding is an effective enhanced oil recovery method in which the oil/water interfacial tension (IFT) is reduced to ultralow values (<0.01 mN/m). The microscopic fluid-fluid displacement has been extensively studied at high IFT (>10 mN/m). However, the microscopic displacement dynamics can be significantly different when the IFT is ultralow because the dynamic contact angle increases with the increase of the capillary number. In this study, surfactant flooding was performed and visualized in micromodels to investigate the dynamics of multiphase displacement at ultralow IFT. Although the micromodels used were strongly water-wet, the displacements of oil by surfactant solutions at ultralow IFT appeared as drainage. Furthermore, a macroscopic oil film was left behind on the surface, which indicates that a contact line instability occurred during displacements. The shape of the oil/water meniscus was determined by the balance between viscous forces and capillary forces. The meniscus can be significantly distorted by viscous forces at ultralow IFT. Therefore, the water-wet micromodel exhibits an oil-wet behavior at ultralow IFT, and the displacements of oil by surfactant solutions at ultralow IFT manifested as drainage rather than imbibition. The flow behavior is further complicated by the spontaneous formation of microemulsion during displacement. The microemulsion is mainly formed from the residual oil. The formation of a microemulsion bank made the surfactant solution discontinuous, with transport in the form of droplets in the microemulsion bank and displacement front. The novelty of this work is to reveal the effects of dynamic contact angle on the ultralow IFT displacement.

A fishbone-inspired liquid splitter enables directional droplet transportation and spontaneous separation

Abstract: Despite extensive efforts made in directional manipulation of water and oil microdroplets, challenges still remain in the ultrafast self-transportation and the rapid separation of droplets with different surface tensions due to the lack of driving force. The natural inspiration from fish medullary spines provides a potential way to realize directional oil movement by using an anisotropic surface with micropits. Herein, a fish-spine-like liquid splitter (FSLLS) is developed based on a precise 3D printing and subsequent femtosecond laser structuring. The liquid splitter enables directional transportation of various microdroplets, with the maximum velocity reaching 265.3 mm s−1. More interestingly, it can spontaneously transport liquids with the surface tension ranging from 15 to 48 mN m−1, achieving an outstanding selectivity of specific liquids. Moreover, the lower the surface tension for the liquid in the surface tension range, the faster the transport speed on the FSLLS. This feature can be attributed to the unique micro-pit structure on the FSLLS surface and its oleophilic properties. The combination of ultrafast directional transportation and peculiar selective ability makes the FSLLS achieve the pumping of microdroplets with lower surface tension. As a proof-of-concept, the separation of the mixed droplets with different surface tensions is demonstrated using the FSLLS and the scalable device. This study offers a new bionic design concept for selective microdroplet pumping devices and opens up an avenue for the separation and extraction of oil droplets in the field of multiphase separation, biomedical analysis, microfluidics, etc.

Pinch-off of a surfactant-covered jet

Abstract: Surfactants at fluid interfaces not only lower and cause gradients in surface tension but can induce additional surface rheological effects in response to dilatational and shear deformations. Surface tension and surface viscosities are both functions of surfactant concentration. Measurement of surface tension and determination of its effects on interfacial flows are now well established. Measurement of surface viscosities, however, is notoriously difficult. Consequently, quantitative characterization of their effects in interfacial flows has proven challenging. One reason behind this difficulty is that, with most existing methods of measurement, it is often impossible to isolate the effects of surface viscous stresses from those due to Marangoni stresses. Here, a combined asymptotic and numerical analysis is presented of the pinch-off of a surfactant-covered Newtonian liquid jet. Similarity solutions obtained from slender-jet theory and numerical solutions are presented for jets with and without surface rheological effects. Near pinch-off, it is demonstrated that Marangoni stresses become negligible compared to other forces. The rate of jet thinning is shown to be significantly lowered by surface viscous effects. From analysis of the dynamics near the pinch-off singularity, a simple analytical formula is derived for inferring surface viscosities. Three-dimensional, axisymmetric simulations confirm the validity of the asymptotic analyses but also demonstrate that a thinning jet traverses a number of intermediate regimes before eventually entering the final asymptotic regime.