Showing papers on "Marangoni effect published in 2008"

On the effect of marangoni flow on evaporation rates of heated water drops.

Abstract: In this letter we show that the Marangoni flow contribution to the evaporation rate of small heated water droplets resting on hot substrates is negligible. We compare data of evaporating droplet experiments with numerical results and assess the effect of Marangoni flow and its contribution to the evaporation process. We demonstrate that heat conduction inside these water droplets is sufficient to give an accurate estimate of evaporation rates. Although convection in evaporating water droplets remains an open problem, our aim in this study is to demonstrate that these effects can be neglected in the investigation of evaporation rate evaluation. It is worth noting that the presented results apply to volatile heated drops which might differ from spontaneously evaporating cases.

Marangoni flow-induced self-assembly of hexagonal and stripelike nanoparticle patterns.

Abstract: We have developed a simple Marangoni flow-induced method for self-assembling nanoparticles (NPs) into both hexagonal and stripelike patterns. First, a NPs/ethanol suspension was spread on a slightly nonwettable and a wettable silicon oxide substrate. The Marangoni flow, induced by simultaneous evaporation of ethanol and condensation of water, leads to the formation of the corresponding hexagonal distributed circular NP rings and dotted stripes. The inter-ring spacing and ring size of the hexagonal patterns can be tuned by varying the relative humidity of the N2 stream blown over the slightly nonwettable substrate. Hexagonal patterns of circular NP patches can also be fabricated by lowering the evaporation of the condensed water droplets. On the wettable substrate, complex patterns result when the humidity of the N2 stream changes.

Development of an advanced A-TIG (AA-TIG) welding method by control of Marangoni convection

Abstract: A new type of tungsten inert gas (TIG) welding has been developed, in which an ultra-deep penetration is obtained. In order to control the Marangoni convection induced by the surface tension gradient on the molten pool, He gas containing a small amount of oxidizing gas was used. The effect of the concentration Of O-2 and CO2 in the shielding gas on the weld shape was studied for the bead-on-plate TIG welding of SUS304 stainless under He-O-2 and He-CO2 mixed shielding gases. Because oxygen is a surface active element for stainless steel, the addition of oxygen to the molten pool can control the Marangoni convection from the outward to inward direction on the liquid pool surface. When the oxygen content in the liquid pool is over a critical value, around 70ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the direction of the Marangoni convection. Also, for He-based shielding gas, a high welding current will strengthen both the inward Marangoni convection on the pool surface and the inward electromagnetic convection in the liquid pool. Accordingly, at a welding speed of 0.75 mm/s, the welding current of 160 A and the electrode gap of I mm under the He-0.4%O-2 shielding, the depth/width ratio reaches 1.8, which is much larger for Ar-O-2 shielding gas (0.7). The effects of the welding parameters, such as welding speed and welding current were also systematically investigated. In addition. a double shielding gas method has been developed to prevent any consumption of the tungsten electrode. (c) 2008 Elsevier B.V. All rights reserved.

The mechanism of surfactant effects on drop coalescence

Abstract: We utilize numerical solutions, based on a boundary-integral scheme, to investigate the mechanisms by which surfactant influences the coalescence of a pair of equal size drops that undergo a head-on collision in a biaxial linear flow. It is known that the addition of surfactant inhibits coalescence in the sense that the time required for film drainage to the point of film rupture is significantly increased. Although there is a direct effect on the rate of film drainage due to Marangoni effects within the thin film, we find that an equally important effect is due to the fact that the hydrodynamic force pushing the drops together is increased, hence causing the film to be more strongly deformed into a dimpled configuration that slows the film drainage process.

Virtual microfluidic traps, filters, channels and pumps using Marangoni flows

Abstract: This paper describes how Marangoni flows of various forms can be generated in thin liquid films for the purposes of microfluidic manipulation. Several microfluidic components, including traps, channels, filters and pumps, for manipulating aqueous droplets suspended in a film of oil on blank, unpatterned substrates are demonstrated. These are ‘virtual’ devices because they have no physical structure; they accomplish their function entirely by localized variations in surface tension (Marangoni flows) created in a non-contact manner by heat sources suspended just above the liquid surface. Various flow patterns can be engineered through the geometric design of the heat sources on size scales ranging from 10 to 1000 μm. A point source generates toroidal flows which can be used for droplet merging and mixing. Virtual channels and traps, emulated by linear and annular heat fluxes, respectively, demonstrate nearly 100% size selectivity for droplets ranging from 300 to 1000 μm. A source of heat flux that is parallel to the surface and is triangular with a 10 ◦ taper serves as a linear pump, translating droplets of about the same size at speeds up to 200 μ ms −1 . The paper includes simulations that illuminate the working principle of the devices. Models show that Marangoni flows scale favorably to small length scales. By using microscale thermal devices delivering sharp temperature gradients, it is possible to generate mm s −1 flow velocities with only small increases (<1 ◦ ) in liquid temperature. (Some figures in this article are in colour only in the electronic version)

Marangoni effects on evaporative lithographic patterning of colloidal films

Abstract: We investigate the effects of Marangoni stresses on the evaporative lithographic patterning of colloidal films (Harris, D. J.; Hu, H.; Conrad, J. C.; Lewis, J. A. Phys. Rev. Lett. 2007, 98 (14), 148301). Films are dried beneath a mask that induces periodically varying regions of free and hindered evaporation. Direct imaging reveals that silica microspheres suspended within an organic solvent exhibit recirculating flows induced by temperature and surface tension gradients that arise during drying. The films display remarkable pattern formation with a majority of the particles deposited in the masked regions. Above a critical colloid volume fraction, recirculating flows are suppressed, leading to particle deposition in unmasked regions of high evaporative flux.

Massive amplification of surface-induced transport at superhydrophobic surfaces.

Abstract: We study electro- and diffusio-osmosis of aqueous electrolytes at superhydrophobic surfaces by means of computer simulation and hydrodynamic theory. We demonstrate that the diffusio-osmotic flow at superhydrophobic surfaces can be amplified by more than 3 orders of magnitude relative to flow in channels with a zero interfacial slip. By contrast, little enhancement is observed at these surfaces for electro-osmotic flow. This amplification for diffusio-osmosis is due to the combined effects of enhanced slip and ion surface depletion or excess at the air-water interfaces on superhydrophobic surfaces. This effect is interpreted in terms of capillary driven Marangoni motion. A practical microfluidic pumping device is sketched on the basis of the slip-enhanced diffusio-osmosis at a superhydrophobic surface.

The effects of surfactant on the multiscale structure of bubbly flows.

Abstract: It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble’s rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1 mm size can be much smaller than that on a clean bubble.

Colloidal attraction induced by a temperature gradient

Abstract: Colloidal crystals are of extreme importance for applied research, such as photonic crystals technology, and for fundamental studies in statistical mechanics. Long range attractive interactions, such as capillary forces, can drive the spontaneous assembly of such mesoscopic ordered structures. However long range attractive forces are very rare in the colloidal realm. Here we report a novel strong and long ranged attraction induced by a thermal gradient in the presence of a wall. Switching on and off the thermal gradient we can rapidly and reversibly form stable hexagonal 2D crystals. We show that the observed attraction is hydrodynamic in nature and arises from thermal induced slip flow on particle surfaces. We used optical tweezers to directly measure the force law and compare it to an analytic prediction based on Stokes flow driven by Marangoni forces.

Investigations of the Marangoni effect on the regular structures in heated wavy liquid films

Abstract: Formation and development of quasi-regular metastable structures within laminar-wavy falling films were studied. These structures emerge within the residual layer between large waves and could be one reason for the break up of the falling film. The temperature field of the film surface was visualised using IR-thermography. The film thickness was obtained from point measurements with the chromatic confocal imaging method and converted into a film thickness field, based on a quasi-steady assumption and IR thermography images. The thermo-capillary nature (Marangoni effect) of the regular structures was proven experimentally.

Three-dimensional flow instabilities in a thermocapillary-driven cavity.

Abstract: A linear stability analysis of the buoyant-thermocapillary flow in open rectangular cavities with aspect ratios in the range $\ensuremath{\Gamma}=1.2$ to 8 is carried out for Prandtl number $\mathrm{Pr}=10$ and conditions of previous experiments. The results are in very good agreement with most available experimental data. The energy transfer between the basic and the perturbation flow reveals that buoyancy is not directly instrumental in the instabilities. For aspect ratios less than about three a stationary three-dimensional cellular flow arises. The instability relies on the lift-up mechanism operating in the shear layer below the free surface and it is aided by weak Marangoni forces. For larger aspect ratios Marangoni effects play a more significant role. While plane hydrothermal waves may appear a certain distance away from the hot wall for sufficiently large aspect ratios, the instability at intermediate aspect ratios is strongly influenced by the local nonparallel basic-flow structure.

Coupled laser molecular trapping, cluster assembly, and deposition fed by laser-induced Marangoni convection.

Abstract: A coupled mechanism for molecular aggregation in a thin water solution film by laser-tweezers is suggested based on (i) simulation of light intensity distribution and (ii) order of magnitude analysis of heat and mass transport induced by Marangoni convection. The analysis suggests that the laser induced temperature distribution develops within 1 ms and Marangoni convection flow commences within 0.01–1 s, which increases by 1–2 orders of magnitude the mass transfer of dissolved molecules into the laser focus where they are trapped and aggregate by attractive van der Waals forces. This mechanism, considered for the particular case of polymer assembly, suggests that it can also be successfully applied for assembling other types of clusters and molecular aggregates from solutions.

Influence of insoluble surfactant on the deformation and breakup of a bubble or thread in a viscous fluid

Abstract: The influence of surfactant on the breakup of a prestretched bubble in a quiescent viscous surrounding is studied by a combination of direct numerical simulation and the solution of a long-wave asymptotic model. The direct numerical simulations describe the evolution toward breakup of an inviscid bubble, while the effects of small but non-zero interior viscosity are readily included in the long-wave model for a fluid thread in the Stokes flow limit. The direct numerical simulations use a specific but realizable and representative initial bubble shape to compare the evolution toward breakup of a clean or surfactant-free bubble and a bubble that is coated with insoluble surfactant. A distinguishing feature of the evolution in the presence of surfactant is the interruption of bubble breakup by formation of a slender quasi-steady thread of the interior fluid. This forms because the decrease in surface area causes a decrease in the surface tension and capillary pressure, until at a small but non-zero radius, equilibrium occurs between the capillary pressure and interior fluid pressure. The long-wave asymptotic model, for a thread with periodic boundary conditions, explains the principal mechanism of the slender thread's formation and confirms, for example, the relatively minor role played by the Marangoni stress. The large-time evolution of the slender thread and the precise location of its breakup are, however, influenced by effects such as the Marangoni stress and surface diffusion of surfactant. © 2008 Cambridge University Press.

Experimental study of the free surface deformation due to thermal convection in liquid bridges

Abstract: Optical imaging was used to measure the free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges of 5-cSt silicone oil. We obtained the free surface position averaged over time in both the steady and oscillatory regimes. The deviation of the free surface contour from the corresponding equilibrium shape was determined with an uncertainty of about 2 μm. This deviation grew linearly with the applied temperature difference with a proportionality coefficient depending on the liquid bridge volume at equilibrium. Shrinkage at the upper part of the liquid bridge was slightly greater than bulging at the lower with the sum of the maximum deviations at both parts being about 30 μm near the onset of oscillations. This sum, normalized with the radius of the supporting disks, was of the same order of magnitude as the Capillary number. We observed the influence of thermal expansion, surface tension variation over the free surface, and fluid motion separately. The local mean curvature was also calculated and compared with its value at equilibrium, showing that the hydrodynamic effects were important.

Numerical study on the shear-induced lift force acting on a spherical bubble in aqueous surfactant solutions

Abstract: A single bubble motion in aqueous surfactant solutions is discussed in this paper. We focus on the change of the lift force acting on a bubble in a linear shear flow under the condition that the bubble surface is contaminated by surfactant adsorption which leads the Marangoni effect. With an increase of Langmuir number which corresponds to a decrease of desorption rate constant of surfactant, the lift force acting on a spherical bubble decreases from the value of a clean bubble to near zero. This reduction is significantly related to a nonaxisymmetric distribution of pressure on the surface. Comparing the present results with those of our previous simulations using an axisymmetric stagnant cap model, the lift coefficients in the present simulations show larger values than those of the stagnant cap model. This is due to a nonaxisymmetric distribution of surface concentration. This asymmetry of the distribution enhances the asymmetry of the surface pressure distribution, which ends up the larger shear-induced lift force than that of the axisymmetric stagnant cap model.

Thermocapillary migration of nondeformable drops

Abstract: In this paper, we present a numerical study on the thermocapillary migration of drops. The Navier–Stokes equations coupled with the energy conservation equation are solved by the finite-difference front-tracking scheme. The axisymmetric model is adopted in our simulations, and the drops are assumed to be perfectly spherical and nondeformable. The benchmark simulation starts from the classical initial condition with a uniform temperature gradient. The detailed discussions and physical explanations of migration phenomena are presented for the different values of (1) the Marangoni numbers and Reynolds numbers of continuous phases and drops and (2) the ratios of drop densities and specific heats to those of continuous phases. It is found that fairly large Marangoni numbers may lead to fluctuations in drop velocities at the beginning part of simulations. Finally, we also discuss the influence of initial conditions on the thermocapillary migrations.

Interaction of three-dimensional hydrodynamic and thermocapillary instabilities in film flows

Abstract: We study three-dimensional wave patterns on the surface of a film flowing down a uniformly heated wall. Our starting point is a model of four evolution equations for the film thickness h , the interfacial temperature theta , and the streamwise and spanwise flow rates, q and p , respectively, obtained by combining a gradient expansion with a weighted residual projection. This model is shown to be robust and accurate in describing the competition between hydrodynamic waves and thermocapillary Marangoni effects for a wide range of parameters. For small Reynolds numbers, i.e., in the "drag-gravity regime," we observe regularly spaced rivulets aligned with the flow and preventing the development of hydrodynamic waves. The wavelength of the developed rivulet structures is found to closely match the one of the most amplified mode predicted by linear theory. For larger Reynolds numbers, i.e., in the "drag-inertia regime," the situation is similar to the isothermal case and no rivulets are observed. Between these two regimes we observe a complex behavior for the hydrodynamic and thermocapillary modes with the presence of rivulets channeling quasi-two-dimensional waves of larger amplitude and phase speed than those observed in isothermal conditions, leading possibly to solitarylike waves. Two subregions are identified depending on the topology of the rivulet structures that can be either "ridgelike" or "groovelike." A regime map is further proposed that highlights the influence of the Reynolds and the Marangoni numbers on the rivulet structures. Interestingly, this map is found to be related to the variations of amplitude and speed of the two-dimensional solitary-wave solutions of the model. Finally, the heat transfer enhancement due to the increase of interfacial area in the presence of rivulet structures is shown to be significant.

Numerical Simulation of Direct Metal Laser Sintering of Single-Component Powder on Top of Sintered Layers

Abstract: A three-dimensional model describing melting and resolidification of direct metal laser sintering of loose powders on top of sintered layers with a moving Gaussian laser beam is developed. Natural convection in the liquid pool driven by buoyancy and Marangoni effects is taken into account. A temperature transforming model is employed to model melting and resolidification in the laser sintering process. The continuity, momentum, and energy equations are solved using a finite volume method. The effects of dominant processing parameters including number of the existing sintered layers underneath, laser scanning velocity, and initial porosity on the sintering process are investigated.

Modeling evaporation of sessile drops with moving contact lines

Abstract: Evaporating thin films and drops are present in numerous natural situations and applications of technical importance. Coated liquid films, for example, are often left to dry by evaporation. Residual films whose thickness may vary from millimetric in the case of paints to nanometric for photoresist films in semiconductor applications are often desired in a uniform state. However, various kinds of instabilities, many driven by evaporation-related mechanisms, often occur. Evaporative sessile drops are perhaps even more interesting since nonuniform drop thickness and the presence of contact lines separating liquid, gas, and solid phases lead to additional effects, such as a possibility of nonuniform evaporation along the liquid-gas interface, temperature gradients, and related Marangoni effects. These effects are crucial in various problems, such as the so-called coffee-stain phenomenon involving the deposition of solid particles dissolved in the liquid close to a contact line 1 and its numerous applications, such as analysis of DNA microarrays 2. Despite its apparent simplicity, the problem of an evaporating drop on a thermally conducting solid substrate involves a number of physical processes, including mass and energy transfer between the three phases, diffusion, and/or convection of vapor in the gas phase, coupled with complex physics in the vicinity of a contact line. So-called “2-sided” models include processes in both the liquid and gas phases, but lead to a mathematical formulation of significant complexity, even when solid-phase and contact-line issues are not considered 3. As we discuss below, various simplifications are based on estimating the importance of the relevant physical processes and lead eventually to models which concentrate only on one of the phases gas or liquid. The estimates, however, involve quantities which are often not precisely known. In this paper, we demonstrate that various assumptions, commonly used in the literature, lead to models which can produce qualitatively different results. The difference in the theoretical results suggests experimental measurements which can be used to decide on which model is appropriate for a particular physical problem. This will allow for a direct comparison between these models including temperature profiles at the evaporating interface. The complete 2-sided model can be simplified by realizing that thermal conductivity and viscosity of vapor are small compared to the liquid ones. In addition, assuming that the gas phase is convection free, one reduces the 2-sided model to the so-called “1.5-sided” model, which includes the processes in the liquid and the diffusion of vapor in the surrounding gas 4. An estimate of a typical diffusion time scale, td=l 2 /D, involves the relevant thickness of the gas phase, l, and the diffusion constant for, e.g., water vapor D

Regimes of thermocapillary migration of droplets under partial wetting conditions

Abstract: We study the thermocapillary migration of two-dimensional droplets of partially wetting liquids on a non-uniform heated substrate. An equation for the thickness profile of the droplet is derived by employing lubrication approximations. The model includes the effect of a non-zero contact angle introduced through a disjoining― conjoining pressure term. Instead of assuming a fixed shape for the droplet, as in previous works, here we allow the droplet to change its profile with time. We identify and describe three different regimes of behaviour. For small contact angles, the droplet spreads into a long film profile with a capillary ridge near the leading edge, a behaviour that resembles the experiments on Marangoni films reported by Ludviksson & Lightfoot (Am. Inst. Chem. Eng. J., vol. 17, 1971, pp. 1166). For large contact angles, the droplet moves as a single entity, weakly distorted from its static shape. This regime is the usual one reported in experiments on thermocapillary migration of droplets. We also show some intriguing morphologies that appear in the transition between these two regimes. The occurrence of these three regimes and their dependence on various parameters is analysed.

Dynamics of a falling film with solutal Marangoni effect.

Abstract: We investigate the dynamics of a thin liquid film on an inclined planar substrate in the presence of an insoluble surfactant on its free surface. We consider both the linear and nonlinear regimes. The linear regime is examined through the Orr-Sommerfeld eigenvalue problem of the full Navier-Stokes and concentration equations and wall and free-surface boundary conditions. The nonlinear regime is investigated through two different models. The first one is obtained from the classical long-wave expansion and the second one through an integral-boundary-layer approximation combined with a simple Galerkin projection. Although accurate close to the instability threshold, the first model fails to describe the dynamics of the system far from criticality. On the other hand, the second model not only captures accurately the behavior close to the instability threshold, but is also valid far from criticality. Analytical and numerical results on the role of the surfactant on the free-surface dynamics are presented. In the linear regime, the Marangoni stresses induced by the surfactant reduce the domain of instability for the base flow. In the nonlinear regime, the system evolves into solitary pulses for both the free surface and surfactant concentration. The amplitude and velocity of these pulses decrease as the Marangoni effect becomes stronger.

Calculation of surface tension and surface phase transition line in binary Ga-Tl system

Abstract: The surface phase transition (SPT) line has been calculated at the Ga-rich side of the binary Ga–Tl system. The liquid alloys at a given temperature can be divided into the following four different regions as a function of Tl-content of the alloy: (i) at very low Tl-content one bulk Ga-rich alloy is obtained not covered by any nano-layer, (ii) at a somewhat higher Tl-content one bulk Ga-rich liquid phase is obtained, covered by a Tl-rich nano-layer, (iii) at medium Tl-content two bulk liquids are obtained, (iv) at high Tl-content one Tl-rich liquid is obtained with no nano-layer on it. An improved version of the Ga–Tl phase diagram is offered, showing the equilibrium SPT line and the region of the Ga-rich bulk liquid, covered by a Tl-rich nano-layer. The presence of the latter changes some physical properties of the system. Particularly, the temperature coefficient of the surface tension changes from a low negative value to a high positive value when increasing the Tl-composition of the alloy and the SPT line is crossed. The position of the SPT line is essential to control Marangoni convection in liquid monotectic alloys.

Marangoni convection and weld shape variations in He–CO 2 shielded gas tungsten arc welding on SUS304 stainless steel

Abstract: Bead-on-plate GTA welding (gas tungsten arc welding) on a SUS304 substrate is carried out to investigate the effect of carbon dioxide gas in the helium base shielding on the oxygen content in the weld pool and the weld shape variations. Experimental results show that small addition of carbon dioxide to the shielding gas can precisely adjust the weld metal oxygen content and change the weld shape from wide shallow type to narrow deep one when the weld pool oxygen content is over the critical value, which is from 68 to 82 ppm, due to the Marangoni convection reversal from the outward to inward mode on the pool surface. The weld depth/width ratio increases two times suddenly when the carbon dioxide content in the torch gas is over 0.4 or 0.2% for 1 mm or 3 mm arc length, respectively. The GTA weld shape depends to a large extent on the pattern and magnitude of the Marangoni convection on the pool surface, which is influenced by the active element oxygen content in the SUS304 pool, temperature coefficient of the surface tension (dσ/dT), and the temperature gradient on the pool surface (dT/dr, r is the radius of the weld pool surface). Changing the welding parameters will alter the temperature distribution and gradient on the pool surface, and thus, affect the magnitude of the Marangoni convection and the final weld shape.