Showing papers on "Marangoni effect published in 2017"
TL;DR: In this paper, an improved 3D numerical model is proposed to simulate the heat transfer, fluid flow, solidification and multicomponent mass transport in direct laser deposition of Co-base alloy on steel.
256 citations
TL;DR: In this article, the Runge-Kutta integration scheme is utilized to solve the problem of forced convective heat transfer in a two-phase model of a nanofluid.
212 citations
TL;DR: In this paper, the influence of carbon nanotubes in the Marangoni convection boundary layer flow of viscous fluid has been investigated and convergent series solutions are established for the resulting differential systems.
158 citations
TL;DR: In this paper, the Marangoni flow induced by a temperature-dependent surface-tension gradient near the meniscus line shows negative effects on the deposited crystals and its electrical properties.
Abstract: Low-cost solution-shearing methods are highly desirable for deposition of organic semiconductor crystals over a large area. To enhance the rate of evaporation and deposition, elevated substrate temperature is commonly employed during shearing processes. However, the Marangoni flow induced by a temperature-dependent surface-tension gradient near the meniscus line shows negative effects on the deposited crystals and its electrical properties. In the current study, the Marangoni effect to improve the shearing process of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene for organic field-effect transistor (OFET) applications is utilized and regulated. By modifying the gradient of surface tension with different combinations of solvents, the mass transport of molecules is much more favorable, which largely enhances the deposition rate, reduces organic crystal thickness, enlarges grain sizes, and improves coverage. The average and highest mobility of OFETs can be increased up to 13.7 and 16 cm2 V−1 s−1. This method provides a simple deposition approach on a large scale, which allows to further fabricate large-area circuits, flexible displays, or bioimplantable sensors.
123 citations
TL;DR: Tan et al. as mentioned in this paper investigated the evaporation of mixture droplets in more detail, by successively increasing the mixture complexity from pure water over a binary water-ethanol mixture to the ternary Ouzo mixture (water, ethanol and anise oil).
Abstract: The Greek aperitif Ouzo is not only famous for its specific anise-flavoured taste, but also for its ability to turn from a transparent miscible liquid to a milky-white coloured emulsion when water is added. Recently, it has been shown that this so-called Ouzo effect, i.e. the spontaneous emulsification of oil microdroplets, can also be triggered by the preferential evaporation of ethanol in an evaporating sessile Ouzo drop, leading to an amazingly rich drying process with multiple phase transitions (Tan et al., Proc. Natl Acad. Sci. USA, vol. 113 (31), 2016, pp. 8642-8647). Due to the enhanced evaporation near the contact line, the nucleation of oil droplets starts at the rim which results in an oil ring encircling the drop. Furthermore, the oil droplets are advected through the Ouzo drop by a fast solutal Marangoni flow. In this article, we investigate the evaporation of mixture droplets in more detail, by successively increasing the mixture complexity from pure water over a binary water-ethanol mixture to the ternary Ouzo mixture (water, ethanol and anise oil). In particular, axisymmetric and full three-dimensional finite element method simulations have been performed on these droplets to discuss thermal effects and the complicated flow in the droplet driven by an interplay of preferential evaporation, evaporative cooling and solutal and thermal Marangoni flow. By using image analysis techniques and micro-particle-image-velocimetry measurements, we are able to compare the numerically predicted volume evolutions and velocity fields with experimental data. The Ouzo droplet is furthermore investigated by confocal microscopy. It is shown that the oil ring predominantly emerges due to coalescence.
113 citations
TL;DR: In this article, the effects of many process variables and alloy properties on the structure and properties of additively manufactured parts using four dimensionless numbers were examined using a well-tested three-dimensional transient heat transfer and fluid flow model.
Abstract: The effects of many process variables and alloy properties on the structure and properties of additively manufactured parts are examined using four dimensionless numbers. The structure and properties of components made from 316 Stainless steel, Ti-6Al-4V, and Inconel 718 powders for various dimensionless heat inputs, Peclet numbers, Marangoni numbers, and Fourier numbers are studied. Temperature fields, cooling rates, solidification parameters, lack of fusion defects, and thermal strains are examined using a well-tested three-dimensional transient heat transfer and fluid flow model. The results show that lack of fusion defects in the fabricated parts can be minimized by strengthening interlayer bonding using high values of dimensionless heat input. The formation of harmful intermetallics such as laves phases in Inconel 718 can be suppressed using low heat input that results in a small molten pool, a steep temperature gradient, and a fast cooling rate. Improved interlayer bonding can be achieved at high Ma...
112 citations
TL;DR: In this article, the authors considered the Marangoni transport of dissipating SWCNT and MWCNT nanofluids under the influence of magnetic force and radiation, and derived numerical solutions of the arising nonlinear problem via Runge-Kutta based shooting approach.
Abstract: Present study addresses the Marangoni transport of dissipating SWCNT and MWCNT nanofluids under the influence of magnetic force and radiation. A novel exponential space dependent heat source is considered. The flow is generated due to a disk with surface tension created by thermal gradient. The partial differential equations system governing the flow of carbon-water nanoliquids and heat transfer through Marangoni convection is established. Subsequent system is reduced to nonlinear ordinary boundary value problem via generalized Karman transformations. Numerical solutions are developed of the arising nonlinear problem via Runge-Kutta based shooting approach. Impacts of embedded parameters are focused on Nusselt number , velocity and heat transport distributions through graphical illustrations. Our simulations figured out that the heat transfer rate increased via Marangoni convection; however it is decayed by applied magnetic force. The temperature of SWCNT-H 2O nanoliquid dominates MWCNT-H2O nanoliquid.
105 citations
TL;DR: Adding a surfactant-like viscous polymer in a droplet could provide a uniform coating of suspended particles, such as cells and various biomaterials, which would be essentially required for droplet assays of biomedical applications.
Abstract: A uniform deposition of the suspended particles in an evaporating droplet is necessary in many research fields. Such deposition is difficult to achieve, because the coffee-ring effect dominates the internal flow in a droplet. The present study adopts a biocompatible, surfactant-like polymer (Polyethylene glycol, PEG) to break the coffee-ring effect and obtain a relatively uniform deposition of the microparticles with yielding multi-ring pattern over a droplet area. Movements of the suspended particles in evaporating droplets and deposition patterns of them on a glass substrate were analyzed with microscopic images and video files. The PEG in the droplets successfully altered the coffee-ring effect because of the surface tension variation, which induced a centripetal Marangoni flow. Balancing these two phenomena apparently generated the Marangoni vortex. For PEG solution droplets, the pinning–depinning process during evaporation was periodically repeated and multiple rings were regularly formed. In conclusion, adding a surfactant-like viscous polymer in a droplet could provide a uniform coating of suspended particles, such as cells and various biomaterials, which would be essentially required for droplet assays of biomedical applications.
105 citations
TL;DR: This work investigates experimentally the evolution of a drop of water and volatile alcohol deposited on a bath of oil: the drop spreads and spontaneously fragments into a myriad of minute droplets whose size strongly depends on the initial concentration of alcohol.
Abstract: Adjusting the wetting properties of water through the addition of a miscible liquid is commonly used in a wide variety of industrial processes involving interfaces. We investigate experimentally the evolution of a drop of water and volatile alcohol deposited on a bath of oil: The drop spreads and spontaneously fragments into a myriad of minute droplets whose size strongly depends on the initial concentration of alcohol. Marangoni flows induced by the evaporation of alcohol play a key role in the overall phenomenon. The intricate coupling of hydrodynamics, wetting, and evaporation is well captured by analytical scaling laws. Our scenario is confirmed by experiments involving other combinations of liquids that also lead to this fascinating phenomenon.
100 citations
TL;DR: In this article, an improved surface tension model is proposed to take into account the influence of sulfur content and temperature at the surface of a melt pool, and the results show that the sulfur redistribution leads to transitional Marangoni flow.
97 citations
TL;DR: In this article, the influence of magnetic field on heat transfer is investigated considering Marangoni convection and Runge-Kutta integration scheme is utilized to solve the problem.
TL;DR: It is found that the Marangoni flow inside the droplet was too small to counteract the capillary flow that deposits CNCs at the edges, resulting in the coffee-ring effect, irrespective of the atmospheric humidity.
Abstract: Cellulose nanocrystals (CNCs), ribbonlike crystalline nanoparticles, are a biobased material that can be a great alternative to obtaining films with tunable optical properties. Iridescent and light-diffracting films can be readily obtained via the drying of a suspension of these cellulose nanocrystals. The characteristics of the particle deposition process together with the self-assembly in the precluding suspension has a direct effect on the optical properties of the obtained films. Particle deposition onto a substrate is affected by the flow dynamics inside sessile droplets and usually yields a ring-shaped deposition pattern commonly referred to as the coffee-ring effect. We set out to measure and describe the drying kinetics under different conditions. We found that the Marangoni flow inside the droplet was too small to counteract the capillary flow that deposits CNCs at the edges, resulting in the coffee-ring effect, irrespective of the atmospheric humidity. By varying the amount of ethanol in the atmosphere, we were able to find a balance between (1) colloidal stability in the droplet, which is reduced by ethanol diffusion into the droplet, and (2) increasing Marangoni flow relative to capillary flow inside the droplet by changing the droplet surface tension. We could thus make iridescent films with a uniform thickness.
01 Feb 2017
TL;DR: A mathematical model for the drying of sessile droplets, based on the lubrication approximation, to binary mixture droplets is extended, relevant for e.g. inkjet printing applications, where ink consisting of several components are used.
Abstract: We extended a mathematical model for the drying of sessile droplets, based on the lubrication approximation, to binary mixture droplets. This extension is relevant for e.g. inkjet printing applications, where ink consisting of several components are used. The extension involves the generalization of an established vapor diffusion-limited evaporation model to multi-component mixtures. The different volatilities of the liquid components generate a composition gradient at the liquid-air interface. The model takes the composition-dependence of the mass density, viscosity, surface tension, mutual diffusion coefficient and thermodynamic activities into account. This leads to a variety of effects ranging from solutal Marangoni flow over deviations from the typical spherical cap shape to an entrapped residual amount of the more volatile component at later stages of the drying. These aspects are discussed in detail on the basis of the numerical results for water-glycerol and water-ethanol droplets. The results show good agreement with experimental findings. Finally, the accuracy of the lubrication approximation is assessed by comparison with a finite element method.
TL;DR: In this article, the authors investigated the Marangoni contraction of sessile drops of a binary mixture of a volatile and a nonvolatile liquid and showed a previously unknown universal power-law relation between the apparent contact angle and the relative undersaturation of the ambient atmosphere.
Abstract: The Marangoni contraction of sessile drops of a binary mixture of a volatile and a nonvolatile liquid has been investigated experimentally and theoretically. The origin of the contraction is the locally inhomogeneous evaporation rate of sessile drops. This leads to surface tension gradients and thus to a Marangoni flow. Simulations show that the interplay of Marangoni flow, capillary flow, diffusive transport, and evaporative losses can establish a quasistationary drop profile with an apparent nonzero contact angle even if both liquid components individually wet the substrate completely. Experiments with different solvents, initial mass fractions, and gaseous environments reveal a previously unknown universal power-law relation between the apparent contact angle and the relative undersaturation of the ambient atmosphere: θapp ∼ (RHeq – RH)1/3. This experimentally observed power law is in quantitative agreement with simulation results. The exponent can also be inferred from a scaling analysis of the hydrod...
TL;DR: In this article, the authors developed a theoretical model to predict the finite spreading time and length scales, the Marangoni-driven convection flow speed, and the finite timescale to establish the quasi-steady state for the solutal MCF.
Abstract: The way two liquids interact depends on how miscible they are. A remarkable phenomenon involving two miscible liquids is now reported: placing a drop of isopropanol on a water surface results in a Marangoni flow, and a static lens in the middle. Mixing and spreading of different liquids are omnipresent in nature, life and technology, such as oil pollution on the sea1,2, estuaries3, food processing4, cosmetic and beverage industries5,6, lab-on-a-chip devices7, and polymer processing8. However, the mixing and spreading mechanisms for miscible liquids remain poorly characterized. Here, we show that a fully soluble liquid drop deposited on a liquid surface remains as a static lens without immediately spreading and mixing, and simultaneously a Marangoni-driven convective flow is generated, which are counterintuitive results when two liquids have different surface tensions. To understand the dynamics, we develop a theoretical model to predict the finite spreading time and length scales, the Marangoni-driven convection flow speed, and the finite timescale to establish the quasi-steady state for the Marangoni flow. The fundamental understanding of this solutal Marangoni flow may enable driving bulk flows and constructing an effective drug delivery and surface cleaning approach without causing surface contamination by immiscible chemical species.
TL;DR: Both natural and Marangoni convection induced by plasmonic heating of Au NPs are covered, which revealed that a temperature derivative of surface tension at the bubble surface is a key factor to control the fluid convection.
Abstract: Gold nanoparticles (Au NPs) efficiently convert incident light into heat under the resonant conditions of localized surface plasmon. Controlling mass transfer through plasmonic heating of Au NPs has potential applications such as manipulation and fabrication within a small space. Here, we describe the formation of stationary microbubbles and subsequent fluid convection induced by CW laser heating of Au NPs in water. Stationary bubbles of about 1–20 μm in diameter were produced by irradiating individual Au NPs with a CW laser. Spatial profiles and velocity distribution of fluid convection around the microbubbles were visualized by the wide-field fluorescence imaging of tracer nanospheres. To evaluate the bubble-induced convection, numerical simulations were performed on the basis of general heat diffusion and Navier–Stokes equations. A comparison between the experimental and computational results revealed that a temperature derivative of surface tension at the bubble surface is a key factor to control the fluid convection. Temperature differences of a few Kelvin at the bubble surface resulted in convective velocities ranging from 102 to 103 μm s−1. The convective velocity gradually increased with increasing bubble diameter. This article covers both natural and Marangoni convection induced by plasmonic heating of Au NPs.
TL;DR: In this article, the effect of the laser scanning speed and laser power on the thermodynamic behavior of the molten pool was investigated numerically, and it was shown that the temperature gradient and the resultant surface tension gradient between the center and the edge of the pool increase with decreasing the scanning speed or increasing the laser power, thereby intensifying the Marangoni flow and attendant turbulence within the pool.
Abstract: A three-dimensional model was developed to simulate the radiation heat transfer in the AlSi10Mg packed bed. The volume of fluid method (VOF) was used to capture the free surface during selective laser melting (SLM). A randomly packed powder bed was obtained using discrete element method (DEM) in Particle Flow Code (PFC). The proposed model has demonstrated a high potential to simulate the selective laser melting process (SLM) with high accuracy. In this paper, the effect of the laser scanning speed and laser power on the thermodynamic behavior of the molten pool was investigated numerically. The results show that the temperature gradient and the resultant surface tension gradient between the center and the edge of the molten pool increase with decreasing the scanning speed or increasing the laser power, thereby intensifying the Marangoni flow and attendant turbulence within the molten pool. However, at a relatively high scanning speed, a significant instability may be generated in the molten pool. The perturbation and instability in the molten pool during SLM may result in an irregular shaped track.
TL;DR: In this paper, a three-dimensional numerical model was developed to investigate effects of laser scanning speed, laser power, and hatch spacing on the thermodynamic behaviors of the molten pool during selective laser melting of AlSi10Mg powder.
Abstract: A three-dimensional numerical model was developed to investigate effects of laser scanning speed, laser power, and hatch spacing on the thermodynamic behaviors of the molten pool during selective laser melting of AlSi10Mg powder. A randomly distributed packed powder bed was achieved using discrete element method (DEM). The powder bed can be treated as a porous media with interconnected voids in the simulation. A good agreement between numerical results and experimental results establish the validity of adopted method. The numerical results show that the Marangoni flow within the molten pool was significantly affected by the processing parameters. An intense Marangoni flow leads to a perturbation within the molten pool. In addition, a relatively high scanning speed tends to cause melt instability. The perturbation or the instability within the molten pool results in the formation of pores during SLM, which have a direct influence on the densification level.
TL;DR: In this paper, an effective liquid conductivity approach has been developed to describe the convective transport modes existing within the melt pool in powder bed additive manufacturing processes and a modified Bond number was calculated by comparing surface tension forces with viscous forces within the melting pool region.
Abstract: An effective liquid conductivity approach has been developed to describe the convective transport modes existing within the melt pool in powder bed additive manufacturing processes. A first principles approach is introduced to derive an effective conductive transport mode that encompasses conduction and advection within the melt pool. A modified Bond number was calculated by comparing surface tension forces with viscous forces within the melt pool region. It was determined, due to the small size scale of melt pools in powder bed processes, that the surface tension gradient driven flow, or the Marangoni effect, is the dominant mass transport phenomenon within the melt pool. Validation was conducted by comparing simulation melt pool widths and depths against experimental measurements for Inconel 718 built at beam powers of 150 W, 200 W and 300 W and a scan speed of 200 mm/s. By introducing the effective liquid conductivity, simulated melt pool widths were up to 50% closer to experimental widths and simulated melt pool depths were up to 80% closer to experimental measurements. Analytic temperature profiles and melt pool dimensions are compared between Ti6Al4V, Stainless Steel 316L, Aluminum 7075 and Inconel 718 built with similar process parameters, while including effective liquid conductivity. The reasons for differences in temperature and melt pool geometry are discussed.
TL;DR: It is shown that surface viscosities play a critical role to explain the accumulation of surfactant in the satellite droplet and that surface tension variation was limited to solutocapillarity and Marangoni stresses.
Abstract: We examine both theoretically and experimentally the breakup of a pendant drop loaded with an insoluble surfactant. The experiments show that a significant amount of surfactant is trapped in the resulting satellite droplet. This result contradicts previous theoretical predictions, where the effects of surface tension variation were limited to solutocapillarity and Marangoni stresses. We solve numerically the hydrodynamic equations, including not only those effects but also those of surface shear and dilatational viscosities. We show that surface viscosities play a critical role to explain the accumulation of surfactant in the satellite droplet.
TL;DR: The evaporation of sessile multi-component droplets is modeled with an axisymmetic finite element method and results are compared with corresponding results of aubrication theory model, showing that the application of lubrication theory can cause considerable errors.
TL;DR: Rapid Marangoni flows around a water vapor microbubble (WVMB) is investigated using the thermoplasmonic effect of a gold nanoisland film (GNF) and is expected to be useful not only for microfluidic mixing but also for fundamental research on viscous flow induced by a single stokeslet.
Abstract: Rapid Marangoni flows around a water vapor microbubble (WVMB) is investigated using the thermoplasmonic effect of a gold nanoisland film (GNF). By focusing a laser onto the GNF, a stable WVMB with a diameter of ~10 μm is generated in degassed water, while an air bubble generated in non-degassed water is larger than 40 μm. Under continuous heating, the WVMB involves significantly rapid Marangoni flow. This flow is well-described by a stokeslet sat ~10 μm above the surface of GNF, from which the maximum flow speed around the WVMB is estimated to exceed 1 m/s. This rapid flow generation is attributed to the small bubble size, over which the temperature is graded, and the superheat at the bubble surface in contact with the GNF. It is expected to be useful not only for microfluidic mixing but also for fundamental research on viscous flow induced by a single stokeslet.
TL;DR: A layered flow pattern is formed because of the out-of-equilibrium phase behavior at high volumetric flow rates where advection is much faster than the diffusive interfacial mass transfer and transverse mixing, which promote equilibrium behavior.
Abstract: The dynamic behavior of microemulsion-forming water–oil–amphiphiles mixtures is investigated in a 2.5D micromodel. The equilibrium phase behavior of such mixtures is well-understood in terms of macroscopic phase transitions. However, what is less understood and where experimental data are lacking is the coupling between the phase change and the bulk flow. Herein, we study the flow of an aqueous surfactant solution–oil mixture in porous media and analyze the dependence of phase formation and spatial phase configurations on the bulk flow rate. We find that a microemulsion forms instantaneously as a boundary layer at the initial surface of contact between the surfactant solution and oil. The boundary layer is temporally continuous because of the imposed convection. In addition to the imposed flow, we observe spontaneous pulsed Marangoni flows that drag the microemulsion and surfactant solution into the oil stream, forming large (macro)emulsion droplets. The formation of the microemulsion phase at the interfa...
TL;DR: In this article, a comprehensive review of theoretical and experimental studies of several configurations involving liquid-gas interfaces in contact with heated solid substrates which can be either dry or covered with micro- or nanoscale films is provided.
TL;DR: The results suggest that the helical motion of the CLC droplets is driven by chiral couplings between the Marangoni flow and rotational motion via the helicals director field of CLC Droplets.
Abstract: We report the first experimental realization of a chiral artificial microswimmer exhibiting helical motion without any external fields. We discovered that a cholesteric liquid crystal (CLC) droplet with a helical director field swims in a helical path driven by the Marangoni flow in an aqueous surfactant solution. We also showed that the handedness of the helical path is reversed when that of the CLC droplet is reversed by replacing the chiral dopant with the enantiomer. In contrast, nematic liquid crystal (NLC) droplets exhibited ballistic motions. These results suggest that the helical motion of the CLC droplets is driven by chiral couplings between the Marangoni flow and rotational motion via the helical director field of CLC droplets.
TL;DR: In this article, a two phase dusty liquid model is considered and the role of physical parameters are focused in momentum and heat transport distributions, where stretching variables are utilized to transform the prevailing partial differential system into a nonlinear ordinary differential system, which is then solved numerically via Runge-Kutta-Fehlberg approach coupled with a shooting technique.
TL;DR: In this paper, the vector potential-vorticity formalism and finite volume method were used respectively to formulate and solve the governing equations and calculations were performed for different Marangoni numbers and different nanoparticles concentrations.
Abstract: Heat transfer, fluid flow and entropy generation due to combined buoyancy and thermocapillary forces in a 3D differentially heated enclosure containing Al2O3 nanofluid are carried out for different Marangoni numbers and different nanoparticles concentrations. The vector potential-vorticity formalism and the finite volume method are used respectively to formulate and to solve the governing equations and calculations were performed for Marangoni number from −103 to 103, volume fraction of nanoparticles from 0 to 0.2 and for a fixed Rayleigh number at 105. An intensification of the flow and an increase in heat transfer and of total entropy generation occur with the increase in nanoparticles volume fraction for all Marangoni numbers.
TL;DR: In this paper, the exponential space dependent heat source (ESHS) process is utilized to explore the thermal transport characteristics of Marangoni convective flow in a Cu-H 2 O nanoliquid due to an infinite disk.
Abstract: The exponential space dependent heat source (ESHS) process is utilized to explore the thermal transport characteristics of Marangoni convective flow in a Cu-H 2 O nanoliquid due to an infinite disk. Flow is driven by linear temperature. Five distinct nanoparticle shapes such as sphere, tetrahedron, column, hexahedron and lamina are accounted. Impacts of Joule heating, radiation and viscous dissipation are also retained. Hamilton-Crosser’s expression is employed to deploy effective thermal conductivity of nanoliquid. Multi degree partial differential equations system is reduced by Karman transformations and then solved via shooting method. It is figured out that the heat transfer rate is enhanced for stronger Marangoni convection and nanoparticle volume fraction. Also, shape of the nanoparticles significantly affects the flow fields.
TL;DR: It is shown that the droplet motion can be induced even in the absence of the Marangoni effect due to the gradient of the evaporation rate, which explains the previously observed attraction-repulsion-chasing behaviors of evaporating droplets.
Abstract: Evaporating droplets are known to show complex motion that has conventionally been explained by the Marangoni effect (flow induced by the gradient of surface tension). Here, we show that the droplet motion can be induced even in the absence of the Marangoni effect due to the gradient of the evaporation rate. We derive an equation for the velocity of a droplet subject to the nonuniform evaporation rate and nonuniform surface tension placed on an inert substrate, where the wettability is uniform and unchanged. The equation explains the previously observed attraction-repulsion-chasing behaviors of evaporating droplets.
TL;DR: In this paper, a two-phase lattice Boltzmann method was developed to simulate axisymmetric thermocapil- lary flows by an improved color-gradient model.