Showing papers on "Marangoni effect published in 2016"
TL;DR: To achieve a uniform deposit in a binary mixture, a small concentration of surfactant and surface-adsorbed polymer is sufficient, which offers a new physicochemical avenue for control of coatings.
Abstract: Surface coatings and patterning technologies are essential for various physicochemical applications. In this Letter, we describe key parameters to achieve uniform particle coatings from binary solutions. First, multiple sequential Marangoni flows, set by solute and surfactant simultaneously, prevent nonuniform particle distributions and continuously mix suspended materials during droplet evaporation. Second, we show the importance of particle-surface interactions that can be established by surface-adsorbed macromolecules. To achieve a uniform deposit in a binary mixture, a small concentration of surfactant and surface-adsorbed polymer (0.05 wt% each) is sufficient, which offers a new physicochemical avenue for control of coatings.
235 citations
TL;DR: In this paper, the particle shape effects on Marangoni convection boundary layer flow of a nanofluid were investigated. But the authors focused on the effect of shape and size of the particles on the surface temperature gradient.
Abstract: Purpose
The purpose of this paper is to study the particle shape effects on Marangoni convection boundary layer flow of a nanofluid. The paper aims to discuss diverse issues befell for the said model.
Design/methodology/approach
The work undertaken is a blend of numerical and analytical studies. Analytical and numerical solutions of nonlinear coupled equations are developed by means of Mathematica package BVPh 2.0 based on the homotopy analysis method.
Findings
The velocity of nanofluid decreases by increasing particle volume friction and similarity parameters. With the increase in particle volume friction and similarity parameter, temperature profile is correspondingly enhanced and decline. The lowest velocity and highest temperature of nanofluid is cause by needle- and disc-shaped particle. Consequence for interface velocity and the surface temperature gradient are perceived by numeric set of results. It is found that the interface velocity is declined by increasing particle volume friction and volume concentration of ethylene glycol in the water. The minimum interface velocity is seen by needle-shaped particle and 30 percent concentrations of ethylene glycol. With increase in volume friction and size of particle, the behaviors of surface temperature gradient are found decreasing and increasing function, respectively. The maximum heat transfer rate at the surface is achieved when we chose sphere nanoparticles and 90 percent concentrations of ethylene glycol as compared to other shapes and concentrations.
Originality/value
This model is investigated for the first time, as the authors know.
137 citations
TL;DR: The role of thermocapillarity as the change of surface tension due to temperature gradient in developing Marangoni flow in liquid films and conclusively bubble and drop actuation is discussed.
Abstract: This paper reviews the past and recent studies on thermocapillarity in relation to microfluidics. The role of thermocapillarity as the change of surface tension due to temperature gradient in developing Marangoni flow in liquid films and conclusively bubble and drop actuation is discussed. The thermocapillary-driven mass transfer (the so-called Benard-Marangoni effect) can be observed in liquid films, reservoirs, bubbles and droplets that are subject to the temperature gradient. Since the contribution of a surface tension-driven flow becomes more prominent when the scale becomes smaller as compared to a pressure-driven flow, microfluidic applications based on thermocapillary effect are gaining attentions recently. The effect of thermocapillarity on the flow pattern inside liquid films is the initial focus of this review. Analysis of the relation between evaporation and thermocapillary instability approves the effect of Marangoni flow on flow field inside the drop and its evaporation rate. The effect of thermocapillary on producing Marangoni flow inside drops and liquid films, leads to actuation of drops and bubbles due to the drag at the interface, mass conservation, and also gravity and buoyancy in vertical motion. This motion can happen inside microchannels with a closed multiphase medium, on the solid substrate as in solid/liquid interaction, or on top of a carrier liquid film in open microfluidic systems. Various thermocapillary-based microfluidic devices have been proposed and developed for different purposes such as actuation, sensing, trapping, sorting, mixing, chemical reaction, and biological assays throughout the years. A list of the thermocapillary based microfluidic devices along with their characteristics, configurations, limitations, and improvements are presented in this review.
131 citations
TL;DR: The light-driven transport of floating liquid marbles is described and it is demonstrated that the anti-Marangoni motion is driven by the free surface deformation, which propels the non-wetting marble against the surface flow and is called the slide effect.
Abstract: Liquid marbles, that is, liquid drops coated by a hydrophobic powder, do not wet any solid or liquid substrate, making their transport and manipulation both highly desirable and challenging. Herein, we describe the light-driven transport of floating liquid marbles and emphasize a surprising motion behavior. Liquid marbles are deposited on a water solution containing photosensitive surfactants. Irradiation of the solution generates photoreversible Marangoni flows that transport the liquid marbles toward UV light and away from blue light when the thickness of the liquid substrate is large enough (Marangoni regime). Below a critical thickness, the liquid marbles move in the opposite direction to that of the surface flow at a speed increasing with decreasing liquid thickness (anti-Marangoni). We demonstrate that the anti-Marangoni motion is driven by the free surface deformation, which propels the non-wetting marble against the surface flow. We call this behavior "slide effect".
117 citations
TL;DR: In this paper, the effects of substrate temperature, substrate wettability, and particle concentration are experimentally investigated for evaporation of a sessile water droplet containing colloidal particles.
Abstract: Effects of substrate temperature, substrate wettability, and particle concentration are experimentally investigated for evaporation of a sessile water droplet containing colloidal particles. Time-varying droplet shapes and temperature of the liquid–gas interface are measured using high-speed visualization and infrared thermography, respectively. The motion of the particles inside the evaporating droplet is qualitatively visualized by an optical microscope and the profile of the final particle deposit is measured by an optical profilometer. On a nonheated hydrophilic substrate, a ring-like deposit forms after the evaporation, as reported extensively in the literature, while on a heated hydrophilic substrate, a thinner ring with an inner deposit is reported in the present work. The latter is attributed to Marangoni convection, and recorded motion of the particles as well as measured temperature gradient across the liquid–gas interface confirms this hypothesis. The thinning of the ring scales with the substr...
114 citations
TL;DR: In this article, the full 3D Marangoni flow generated by a non-uniform surface tension profile at the interface of a self-propelled spherical emulsion droplet is derived.
Abstract: The Marangoni effect refers to fluid flow induced by a gradient in surface tension at a fluid-fluid interface. We determine the full three-dimensional Marangoni flow generated by a non-uniform surface tension profile at the interface of a self-propelled spherical emulsion droplet. For all flow fields inside, outside, and at the interface of the droplet, we give analytical formulas. We also calculate the droplet velocity vector vD, which describes the swimming kinematics of the droplet, and generalize the squirmer parameter β, which distinguishes between different swimmer types called neutral, pusher, or puller. In the second part of this paper, we present two illustrative examples, where the Marangoni effect is used in active emulsion droplets. First, we demonstrate how micelle adsorption can spontaneously break the isotropic symmetry of an initially surfactant-free emulsion droplet, which then performs directed motion. Second, we think about light-switchable surfactants and laser light to create a patch with a different surfactant type at the droplet interface. Depending on the setup such as the wavelength of the laser light and the surfactant type in the outer bulk fluid, one can either push droplets along unstable trajectories or pull them along straight or oscillatory trajectories regulated by specific parameters. We explore these cases for strongly absorbing and for transparent droplets.
93 citations
TL;DR: The numerical calculations show that the droplet lifetime is affected significantly by the balance between the ability of the surfactant to enhance spreading, suppressing the effect of thermal Marangoni stresses-induced motion, and to hinder the evaporation flux through the reduction of the effective interfacial area of Evaporation.
Abstract: We consider the flow dynamics of a thin evaporating droplet in the presence of an insoluble surfactant and noninteracting particles in the bulk. On the basis of lubrication theory, we derive a set of evolution equations for the film height, the interfacial surfactant, and bulk particle concentrations, taking into account the dependence of liquid viscosity on the local particle concentration. An important ingredient of our model is that it takes into account the fact that the surfactant adsorbed at the interface hinders evaporation. We perform a parametric study to investigate how the presence of surfactants affects the evaporation process as well as the flow dynamics with and without the presence of particles in the bulk. Our numerical calculations show that the droplet lifetime is affected significantly by the balance between the ability of the surfactant to enhance spreading, suppressing the effect of thermal Marangoni stresses-induced motion, and to hinder the evaporation flux through the reduction of ...
92 citations
TL;DR: This work demonstrates an integrated liquid cooling system by utilizing a small droplet of liquid metal Galinstan, which is placed over the hot spot, and facilitates the rapid cooling of localized hot spots.
Abstract: The continued miniaturization of electronic components demands integrated liquid cooling systems with minimized external connections and fabrication costs that can be implanted very close to localized hot spots. This might be challenging for existing liquid cooling systems because most of them rely on external pumps, connecting tubes, and microfabricated heat sinks. Here, we demonstrate an integrated liquid cooling system by utilizing a small droplet of liquid metal Galinstan, which is placed over the hot spot. Energizing the liquid metal droplet with a square wave signal creates a surface tension gradient across the droplet, which induces Marangoni flow over the surface of droplet. This produces a high flow rate of coolant medium through the cooling channel, enabling a “soft” pump. At the same time, the high thermal conductivity of liquid metal extends the heat transfer surface and facilitates the dissipation of heat, enabling a “soft” heat sink. This facilitates the rapid cooling of localized hot spots,...
92 citations
TL;DR: In this paper, a mesoscopic model has been established to investigate the thermodynamic mechanisms and densification behavior of nickel-based superalloy during additive manufacturing/three-dimensional (3D) printing (AM/3DP) by numerical simulation, using a finite volume method.
Abstract: A mesoscopic model has been established to investigate the thermodynamic mechanisms and densification behavior of nickel-based superalloy during additive manufacturing/three-dimensional (3D) printing (AM/3DP) by numerical simulation, using a finite volume method (FVM). The influence of the applied linear energy density (LED) on dimensions of the molten pool, thermodynamic mechanisms within the pool, bubbles migration and resultant densification behavior of AM/3DP-processed superalloy has been discussed. It reveals that the center of the molten pool slightly shifts with a lagging of 4 μm towards the center of the moving laser beam. The Marangoni convection, which has various flow patterns, plays a crucial role in intensifying the convective heat and mass transfer, which is responsible for the bubbles migration and densification behavior of AM/3DP-processed parts. At an optimized LED of 221.5 J/m, the outward convection favors the numerous bubbles to escape from the molten pool easily and the resultant considerably high relative density of 98.9 % is achieved. However, as the applied LED further increases over 249.5 J/m, the convection pattern is apparently intensified with the formation of vortexes and the bubbles tend to be entrapped by the rotating flow within the molten pool, resulting in a large amount of residual porosity and a sharp reduction in densification of the superalloy. The change rules of the relative density and the corresponding distribution of porosity obtained by experiments are in accordance with the simulation results.
91 citations
TL;DR: In this paper, the authors investigated the Marangoni boundary layer flow and heat transfer of copper-water nanofluid driven by exponential temperature with radiation effects and found that the solid volume fraction and nanoparticle shape have significant impacts on the thermal conductivity and sphere nanoparticle has better enhancement on heat transfer than other nanoparticle shapes.
89 citations
TL;DR: This work uses light to generate Marangoni surface stresses resulting in flow patterns that accumulate particles at predefined positions that allow particles regardless of their size or surface properties to be patterned in model suspensions as well as in complex, real-world formulations such as commercial coffee.
Abstract: Controlled particle deposition on surfaces is crucial for both exploiting collective properties of particles and their integration into devices. Most available methods depend on intrinsic properties of either the substrate or the particles to be deposited making them difficult to apply to complex, naturally occurring or industrial formulations. Here we describe a new strategy to pattern particles from an evaporating drop, regardless of inherent particle characteristics and suspension composition. We use light to generate Marangoni surface stresses resulting in flow patterns that accumulate particles at predefined positions. Using projected images, we generate a broad variety of complex patterns, including multiple spots, lines and letters. Strikingly, this method, which we call evaporative optical Marangoni assembly (eOMA), allows us to pattern particles regardless of their size or surface properties, in model suspensions as well as in complex, real-world formulations such as commercial coffee.
TL;DR: In this article, the influence of the velocity profile and temperature distribution for the present nanofluids was investigated under effect of the involved parameters, and the exact solutions of the resulted equations were solved using a new approach via Laplace transform.
TL;DR: It is shown theoretically that near a fluid-fluid interface a single active colloidal particle generating, e.g., chemicals or a temperature gradient experiences an effective force of hydrodynamic origin that can dominate the dynamics of the particle as compared to Brownian motion, dispersion forces, or self-phoretic effects.
Abstract: We show theoretically that near a fluid-fluid interface a single active colloidal particle generating, e.g., chemicals or a temperature gradient experiences an effective force of hydrodynamic origin. This force is due to the fluid flow driven by Marangoni stresses induced by the activity of the particle; it decays very slowly with the distance from the interface, and can be attractive or repulsive depending on how the activity modifies the surface tension. We show that, for typical systems, this interaction can dominate the dynamics of the particle as compared to Brownian motion, dispersion forces, or self-phoretic effects. In the attractive case, the interaction promotes the self-assembly of particles into a crystal-like monolayer at the interface.
TL;DR: In this paper, the force on a probe at a viscous, compressible interface, laden with soluble surfactant that equilibrates on a finite time scale, is calculated.
Abstract: We calculate here the force on a probe at a viscous, compressible interface, laden with soluble surfactant that equilibrates on a finite time scale. The motion of the probe through the interface drives variations in the surfactant concentration at the interface that in turn leads to a Marangoni flow that contributes to the force on the probe. We demonstrate that the Marangoni force on the probe depends non-trivially on the surface shear and dilatational viscosities of the interface indicating the difficulty in extracting these material properties from force measurements at compressible interfaces.
TL;DR: In this paper, the authors studied the boiling crisis and evaporation of droplets of ethanol water solution on a horizontal ceramic heating surface and derived the dependence of the vapor layer height on wall overheating at boiling crisis at Leidenfrost temperature manifold.
TL;DR: In this article, the Hartmann effect due to externally applied magnetic fields was investigated by numerical simulation for partial penetration welding of aluminium, and the results showed that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag.
TL;DR: Three distinct flow regimes have been revealed in the evaporation of a water-based binary mixture droplet suspended with alumina nanoparticles, and the relative weightings of Regimes I and II determine the motion of the nanoparticles and the formation of the final drying pattern.
Abstract: The flow regimes and the deposition pattern have been investigated by changing the ethanol concentration in a water-based binary mixture droplet suspended with alumina nanoparticles. To visualize the flow patterns, Particle Image Velocimetry (PIV) has been applied in the binary liquid droplet containing the fluorescent microspheres. Three distinct flow regimes have been revealed in the evaporation. In Regime I, the vortices and chaotic flows are found to carry the particles to the liquid-vapor interface and to promote the formation of particle aggregation. The aggregates move inwards in Regime II as induced by the Marangoni flow along the droplet free surface. Regime III is dominated by the drying of the left water and the capillary flow driving particles radially outward is observed. The relative weightings of Regimes I and II, which are enhanced with an increasing load of ethanol, determine the motion of the nanoparticles and the formation of the final drying pattern.
Posted Content•
TL;DR: A first-order model corroborates the liquid-gas interface temperature measurements and variation in the measured ring profile with the substrate temperature, and proposes a regime map for predicting three types of deposits-namely, ring, thin ring with inner deposit, and inner deposit-for varying substrate temperature.
Abstract: Effects of substrate temperature, substrate wettability and particles concentration are experimentally investigated for evaporation of a sessile water droplet containing colloidal particles. Time-varying droplet shapes and temperature of the liquid-gas interface are measured using high-speed visualization and infrared thermography, respectively. The motion of the particles inside the evaporating droplet is qualitatively visualized by an optical microscope and profile of final particle deposit is measured by an optical profilometer. On a non-heated hydrophilic substrate, a ring-like deposit forms after the evaporation, as reported extensively in the literature; while on a heated hydrophilic substrate, a thinner ring with an inner deposit is reported in the present work. The latter is attributed to Marangoni convection and recorded motion of the particles as well as measured temperature gradient across the liquid-gas interface confirms this hypothesis. The thinning of the ring scales with the substrate temperature and is reasoned to stronger Marangoni convection at larger substrate temperature. In case of a non-heated hydrophobic substrate, an inner deposit forms due to very early depinning of the contact line. On the other hand, in case of a heated hydrophobic substrate, the substrate heating as well as larger particle concentration helps in the pinning of the contact line, which results in a thin ring with an inner deposit. We propose a regime map for predicting three types of deposits namely, ring, thin ring with inner deposit and inner deposit - for varying substrate temperature, substrate wettability and particles concentration. A first-order model corroborates the liquid-gas interface temperature measurements and variation in the measured ring profile with the substrate temperature.
01 Mar 2016
TL;DR: Line printing offers a feasible approach to remove the pixel well structure which is widely used to confine the ink-jet printed solution and forms a flat film surface.
Abstract: Line printing offers a feasible approach to remove the pixel well structure which is widely used to confine the ink-jet printed solution. In the study, a uniform line is printed by an ink-jet printer. To achieve a uniform surface profile of the printed line, 10 vol% low-volatile solvent DMA (3,4-Dimethylanisole) is mixed with high-volatile solvent Pxy (p-xylene) as the solvent. After a solution-processable small molecule is dissolved, the surface tension of DMA solution becomes lower than that of Pxy solution, which creates an inward Marangoni flow during the solvent evaporation. The inward Marangoni flow balances out the outward capillary flow, thereby forming a flat film surface. The line width of the printed line depends on the contact angle of the solution on the hole injection layer.
TL;DR: In this paper, the hydrodynamic prerequisites for self-sustained locomotion are reviewed and two examples to realize those conditions for emulsion droplets, i.e. droplets stabilized by a surfactant layer in a surrounding immiscible liquid.
Abstract: Self-propelled droplets are a special kind of self-propelled matter that are easily fabricated by standard microfluidic tools and locomote for a certain time without external sources of energy. The typical driving mechanism is a Marangoni flow due to gradients in the interfacial energy on the droplet interface. In this article we review the hydrodynamic prerequisites for self-sustained locomotion and present two examples to realize those conditions for emulsion droplets, i.e. droplets stabilized by a surfactant layer in a surrounding immiscible liquid. One possibility to achieve self-propelled motion relies on chemical reactions affecting the surface active properties of the surfactant molecules. The other relies on micellar solubilization of the droplet phase into the surrounding liquid phase. Remarkable cruising ranges can be achieved in both cases and the relative insensitivity to their own ‘exhausts’ allows to additionally study collective phenomena.
TL;DR: In this article, the coalescence of two water drops, one of which contains surfactant, was carried out in surrounding silicone oils of various viscosities, and it was found that, in all the studied cases, the outward motion of the liquid neck follows the power scaling law with exponent ∼ 0.5 with the pre-factor dependent upon the surfactants concentration and viscosity of outer phase.
TL;DR: In this article, the authors report on experimental observations/visualization of thermocapillary or Marangoni flows in a pure water drop via infrared thermography, which were induced by imposing a temperature gradient on the drop by locally heating the substrate directly below the center with a laser.
Abstract: We report on experimental observations/visualization of thermocapillary or Marangoni flows in a pure water drop via infrared thermography The Marangoni flows were induced by imposing a temperature gradient on the drop by locally heating the substrate directly below the center with a laser Evidently, a temperature gradient along the liquid-air interface of ca 25 °C was required for the Marangoni flows to be initiated as twin vortices and a subsequent gradient of ca 15 °C to maintain them The vortices exhibited an oscillatory behavior where they merged and split in order for the drop to compensate for the non-uniform heating and cooling The origin of these patterns was identified by comparing the dimensionless Marangoni and Rayleigh numbers, which showed the dominance of the Marangoni convection This fact was further supported by a second set of experiments where the same flow patterns were observed when the drop was inverted (pendant drop)
TL;DR: In this article, a 3D numerical simulation of transport phenomena using volume of fluid method is conducted for multiple-layer single track laser additive manufacturing (LAM), which predicts the temperature and fluid flow velocity distributions, transient variation of the melt pool fluid boundary shape and remelting, and solidified build geometry during deposition of successive LAM layers.
Abstract: A three-dimensional (3D) numerical simulation of transport phenomena using volume of fluid method is conducted for multiple-layer single track laser additive manufacturing (LAM). It predicts the temperature and fluid flow velocity distributions, transient variation of the melt pool fluid boundary shape and remelting, and solidified build geometry during deposition of successive LAM layers. The prediction showed reasonable accuracy in predicting peak temperature and deposit geometry. The prediction error of peak temperature is less than 2.5%, and prediction error of deposit height and width are less than 12%. Correlations between dimensionless process/material parameters (Pe, Pr, and Ma) and melt pool 3D fluid flow patterns and liquid boundary shape were studied. The analysis showed the hemispherical melt pool free surface in LAM causes the mechanisms that determine melt pool liquid-solid boundary shape to be different from melt pools formed on a flat surface. Although the maximum surface velocity decreased from 8.59 cm s−1 on the first layer to 5.06 cm s−1 on the fifth layer, the outward Marangoni flow is redirected from outward to downward due to the increase of surface curvature from 39.5° on the first layer to 75.9° on the fifth layer. Consequently, the penetration into the solid substrate at the outward edges becomes deeper and the pool bottom becomes more convex. These detailed physical insights provided by process simulations facilitate prediction of localized dimensional variations in LAM builds.
01 Jan 2016
TL;DR: In this article, the authors propose equations of motion for the dynamics of liquid films of surfactant suspensions that consist of a general gradient dynamics framework based on an underlying energy functional, which includes wetting energy, surface energy of the free interface (constant contribution plus an entropic term) and bulk mixing entropy.
Abstract: In this paper we propose equations of motion for the dynamics of liquid films of surfactant suspensions that consist of a general gradient dynamics framework based on an underlying energy functional. This extends the gradient dynamics approach to dissipative non-equilibrium thin film systems with several variables, and casts their dynamic equations into a form that reproduces Onsager's reciprocity relations. We first discuss the general form of gradient dynamics models for an arbitrary number of fields and discuss simple well-known examples with one or two fields. Next, we develop the gradient dynamics (three field) model for a thin liquid film covered by soluble surfactant and discuss how it automatically results in consistent convective (driven by pressure gradients, Marangoni forces and Korteweg stresses), diffusive, adsorption/desorption, and evaporation fluxes. We then show that in the dilute limit, the model reduces to the well-known hydrodynamic form that includes Marangoni fluxes due to a linear equation of state. In this case the energy functional incorporates wetting energy, surface energy of the free interface (constant contribution plus an entropic term) and bulk mixing entropy. Subsequently, as an example, we show how various extensions of the energy functional result in consistent dynamical models that account for nonlinear equations of state, concentration-dependent wettability and surfactant and film bulk decomposition phase transitions. We conclude with a discussion of further possible extensions towards systems with micelles, surfactant adsorption at the solid substrate and bioactive behaviour.
TL;DR: In this article, the authors present a study of the inkjet printing of tantalum-oxide-based dielectric structures on indium-tinoxide-coated glass.
Abstract: We present a study of the inkjet printing of tantalum-oxide-based dielectric structures on indium-tin-oxide-coated glass. Ta-Al-Si-alkoxide-based ink formulations with 2-methoxyethanol (2MOE) as the main solvent and a highly viscous glycerol (GLY) or 1,3-propanediol (PD) co-solvent exhibit the optimal values of viscosity and surface tension for piezoelectric inkjet printing, and show good jetting performance. However, the drying of the printed structures results in a pronounced “coffee-stain” effect. We relate this phenomenon to the much higher volatility of 2MOE than either of the viscous solvents and the dominant evaporation of the former immediately after the ink has been printed on the substrate. Consequently, the recirculating Marangoni solvent flow exists only at the onset of drying and ceases to exist once the 2MOE has completely evaporated from the drying feature. The combination of all three solvents appears to prolong the duration of the Marangoni flow, as suggested by differential scanning calorimetry, resulting in an improved uniformity of dried structures. By adjusting the solvent composition we could tailor the topology of deposits and print 45 nm-thick, flat and uniform capacitors with the performance (good dielectric properties of er ∼ 15, tan(δ) ∼ 0.034 at 100 kHz and a low leakage current density of 2.4 × 10−7 A cm−2 at 200 kV cm−1) comparable to spin-coated films.
TL;DR: A reduced-order model is formulated that includes advective transport within the droplet for prediction of organic liquid droplet evaporation on a nonwetting substrate and it is confirmed that the predicted temperature differential across the height of the droplets matches experiments.
Abstract: We quantitatively characterize the flow field inside organic liquid droplets evaporating on a nonwetting substrate. A mushroom-structured surface yields the desired nonwetting behavior with methanol droplets, while use of a cooled substrate (5–15 °C) slows the rate of evaporation to allow quasi-static particle image velocimetry. Visualization reveals a toroidal vortex within the droplet that is characteristic of surface tension-driven flow; we demonstrate by means of a scaling analysis that this recirculating flow is Marangoni convection. The velocities in the droplet are on the order of 10–45 mm/s. Thus, unlike in the case of evaporation on wetting substrates where Marangoni convection can be ignored for the purpose of estimating the evaporation rate, advection due to the surface tension-driven flow plays a dominant role in the heat transfer within an evaporating droplet on a nonwetting substrate because of the large height-to-radius aspect ratio of the droplet. We formulate a reduced-order model that in...
Journal Article•
TL;DR: In this article, the surface tension of a free liquid film is lowered by an amount Delta sigma over a size a by chemical or thermal contamination, and the spot diffuses (within a time a(2)/D, with D the diffusion coefficient of the pollutant in the substrate), and then an inhomogeneous outward interstitial flow which digs the film within a time tau 0 similar to root rho ha( 2)/Delta sigma with rho the density of the liquid.
Abstract: We call thick those films for which the disjoining pressure and thermal fluctuations are ineffective. Water films with thickness h in the 1-100 m m range are thick, but are also known, paradoxically, to nucleate holes spontaneously. We have uncovered a mechanism solving the paradox, relying on the extreme sensitivity of the film to surface tension inhomogeneities. The surface tension of a free liquid film is lowered by an amount Delta sigma over a size a by chemical or thermal contamination. At the same time this spot diffuses (within a time a(2)/D, with D the diffusion coefficient of the pollutant in the substrate), the Marangoni stress Delta sigma/a induces an inhomogeneous outward interstitial flow which digs the film within a time tau 0 similar to root rho ha(2)/Delta sigma with rho the density of the liquid. When the Peclet number Pe = a(2)/D tau(0) is larger than unity, the liquid substrate motion reinforces the surface tension gradient, triggering a self-sustained instability insensitive to diffusional regularisation. Several experimental illustrations of the phenomenon are given, both qualitative and quantitative, including a precise study of the first instants of the unstable dynamics made by controlled perturbations of a Savart sheet at large Pe.
TL;DR: In this article, controllable self-assembly of diphenylalanine (FF) in an evaporative dewetting solution is reported, where fluid mechanical dimensionless numbers, namely Rayleigh, Marangoni, and capillary numbers, are introduced to control the interaction between the solution and FF molecules in the selfassembly process.
Abstract: Self-assembled peptide nanostructures have unique physical and biological properties and promising applications in electrical devices and functional molecular recognition. Although solution-based peptide molecules can self-assemble into different morphologies, it is challenging to control the self-assembly process. Herein, controllable self-assembly of diphenylalanine (FF) in an evaporative dewetting solution is reported. The fluid mechanical dimensionless numbers, namely Rayleigh, Marangoni, and capillary numbers, are introduced to control the interaction between the solution and FF molecules in the self-assembly process. The difference in the film thickness reflects the effects of Rayleigh and Marangoni convection, and the water vapor flow rate reveals the role of viscous fingering in the emergence of aligned FF flakes. By employing dewetting, various FF self-assembled patterns, like concentric and spokelike, and morphologies, like strips and hexagonal tubes/rods, can be produced, and there are no signi...
TL;DR: In this paper, the role of turbulence in laser melting of a steel alloy with surface active elements was investigated by using direct numerical simulations (DNS) and the results revealed the presence of two competing vortices driven by thermocapillary forces towards a local surface tension maximum.
TL;DR: In this paper, the influence of the contact angle on the flow field developed inside a nanofluid droplet consisting of a mixture of water and carbon nanotubes (CNT) is investigated.
Abstract: The heat transfer characteristics of liquid droplets are influenced by the hydrophobicity of the surfaces. Fluid properties and surface energy play important roles in heat transfer assessment. In the present study, the influence of the contact angle on the flow field developed inside a nanofluid droplet consisting of a mixture of water and carbon nanotubes (CNT) is investigated. Flow field and heat transfer characteristics are simulated numerically in line with the experimental conditions. It is found that the flow velocity predicted numerically is in good agreement with the experimental data. Nusselt and Bond numbers increase at large contact angles and Marangoni force dominates over buoyancy force.