Showing papers on "Marangoni effect published in 2011"

Pattern formation in drying drops of blood

Abstract: The drying of a drop of human blood exhibits coupled physical mechanisms, such as Marangoni flow, evaporation and wettability. The final stage of a whole blood drop evaporation reveals regular patterns with a good reproducibility for a healthy person. Other experiments on anaemic and hyperlipidaemic people were performed, and different patterns were revealed. The flow motion inside the blood drop is observed and analysed with the use of a digital camera: the influence of the red blood cells motion is revealed at the drop periphery as well as its consequences on the final stage of drying. The mechanisms which lead to the final pattern of the dried blood drops are presented and explained on the basis of fluid mechanics in conjunction with the principles of haematology. The blood drop evaporation process is evidenced to be driven only by Marangoni flow. The same axisymmetric pattern formation is observed, and can be forecast for different blood drop diameters. The evaporation mass flux can be predicted with a good agreement, assuming only the knowledge of the colloids mass concentration.

Surfactant Effects on Bubble Motion and Bubbly Flows

Abstract: Small amounts of surfactant can drastically change bubble behavior. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a nonuniform concentration distribution of surfactant along the bubble surface. In other words, a tangential shear stress appears on the bubble surface due to the surface tension variation caused by the surface concentration distribution, which results in the reduction of the rising velocity of the bubble. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, we review the recent studies related to these interesting behaviors of bubbles caused by the surfactant adsorption/desorption on the bubble surface.

On evaporation of sessile drops with moving contact lines

Abstract: We consider theoretically, computationally and experimentally spontaneous evaporation of water and isopropanol drops on smooth silicon wafers. In contrast to a number of previous works, the solid surface we consider is smooth and therefore the droplets' evolution proceeds without contact line pinning. We develop a theoretical model for evaporation of pure liquid drops that includes Marangoni forces due to the thermal gradients produced by non-uniform evaporation, and heat conduction effects in both liquid and solid phases. The key ingredient in this model is the evaporative flux. We consider two commonly used models: one based on the assumption that the evaporation is limited by the processes originating in the gas (vapour diffusion-limited evaporation), and the other one which assumes that the processes in the liquid are limiting. Our theoretical model allows for implementing evaporative fluxes resulting from both approaches. The required parameters are obtained from physical experiments. We then carry out fully nonlinear time-dependent simulations and compare the results with the experimental ones. Finally, we discuss how the simulation results can be used to predict which of the two theoretical models is appropriate for a particular physical experiment.

Spontaneous motion of a droplet coupled with a chemical wave

Abstract: We propose a framework for the spontaneous motion of a droplet coupled with internal dynamic patterns generated in a reaction-diffusion system. The spatiotemporal order of the chemical reaction gives rise to inhomogeneous surface tension and results in self-propulsion driven by the surrounding flow due to the Marangoni effect. Numerical calculations of internal patterns together with theoretical results of the flow fields at low Reynolds number reproduce well the experimental results obtained using a droplet of the Belousov-Zhabotinsky reaction medium.

Interfacial effects on droplet dynamics in Poiseuille flow

Abstract: Many properties of emulsions arise from interfacial rheology, but a theoretical understanding of the effect of interfacial viscosities on droplet dynamics is lacking. Here we report such a theory, relating to isolated spherical drops in a Poiseuille flow. Stokes flow is assumed in the bulk phases, and a jump in hydrodynamic stress at the interface is balanced by Marangoni and surface viscous forces according to the Boussinesq–Scriven constitutive law. Our model employs a linear equation of state for the surfactant. Our analysis predicts slip, cross-stream migration and droplet-circulation velocities. These results and the corresponding interfacial parameters are separable: e.g., cross-stream migration occurs only if gradients in surfactant concentration are present; slip velocity depends on viscosity contrast and dilatational properties, but not on shear Boussinesq number. This separability allows a new and advantageous means to measure surface viscous and elastic forces directly from the drop interface.

Surfactant-driven dynamics of liquid lenses

Abstract: Sessile liquid lenses spreading over a fluid layer, in the presence of Marangoni stresses due to surfactants, show a surprisingly wide range of interesting behaviour ranging from complete spreading of the lens, to spreading followed by retraction, to sustained pulsating oscillations. Models for the spreading process, the effects of surfactant at the moving contact line, sorption kinetics above and below the critical micelle concentration, are all incorporated into the modelling. Numerical results cast light upon the physical processes that drive these phenomena, and the regular oscillatory beating of lenses is shown to occur in specific limits.

Aqueous droplet manipulation by optically induced Marangoni circulation

Abstract: The manipulation of picoliter droplets is demonstrated using optically induced microscale circulatory flows. The circulation results from Marangoni effects induced by optical heating from light patterns created by a computer projector. Manipulation of single droplets and parallel manipulation of multiple droplets are achieved with induced forces of up to 1 nN and an average resolution of 146.5 μm.

Spontaneous mode-selection in the self-propelled motion of a solid/liquid composite driven by interfacial instability.

Abstract: Spontaneous motion of a solid/liquid composite induced by a chemical Marangoni effect, where an oil droplet attached to a solid soap is placed on a water phase, was investigated. The composite exhibits various characteristic motions, such as revolution (orbital motion) and translational motion. The results showed that the mode of this spontaneous motion switches with a change in the size of the solid scrap. The essential features of this mode-switching were reproduced by ordinary differential equations by considering nonlinear friction with proper symmetry.

Tunable Wetting Mechanism of Polypyrrole Surfaces and Low-Voltage Droplet Manipulation via Redox

Abstract: This paper presents the experimental results and analyses on a controlled manipulation of liquid droplets upon local reduction and oxidation (redox) of a smart polymer—dodecylbenzenesulfonate doped polypyrrole (PPy(DBS)). The electrochemically tunable wetting property of PPy(DBS) permitted liquid droplet manipulation at very low voltages (−0.9 to 0.6 V). A dichloromethane (DCM) droplet was flattened upon PPy(DBS) reduction. It was found that the surface tension gradient across the droplet contact line induced Marangoni stress, which caused this deformation. Further observation of PPy(DBS)’s color change upon the redox process confirmed that the surface tension gradient was the driving force for the droplet shape change.

Surface Tension of Liquid Iron as Functions of Oxygen Activity and Temperature

Abstract: Surface tension of liquid iron is an important property for process simulations evaluating Marangoni flow in the melt. In this study, we succeeded to measure accurate surface tension of liquid iron under various oxygen activities and temperatures with overcoming experimental difficulties through the following attempts: (1) To prevent chemical contamination of sample, an oscillating droplet method using an electromagnetic levitator was employed. Surface oscillation frequencies were determined with taking into account sample rotation. (2) To control the oxygen activity of liquid iron, a gas-liquid equilibrium method using CO/CO2 containing gas mixtures was employed. This method enables precise control of oxygen activity with keeping carbon activity low, and therefore, oxygen activity dependence of surface tension was clarified without carbon effect. (3) To measure the surface tension for higher oxygen activities up to the Fe/FeO equilibrium, high-purity iron (99.9972 mass%) was used. Otherwise, minor reactive impurities in the melt such as aluminum are oxidized, which affects the surface tension measurement. Based on the experimental data, the surface tension of liquid iron was expressed as functions of oxygen activity and temperature using the Szyszkowski model. In addition, thermodynamic properties on the oxygen adsorption reaction and structure of the adsorbed oxygen layer on the melt surface were also discussed.

Surface tension gradient driven spreading on aqueous mucin solutions: a possible route to enhanced pulmonary drug delivery.

Abstract: Surface tension gradient driven, or "Marangoni", flow can be used to move exogenous fluid, either surfactant dispersions or drug carrying formulations, through the lung. In this paper, we investigate the spreading of aqueous solutions of water-soluble surfactants over entangled, aqueous mucin solutions that mimic the airway surface liquid of the lung. We measure the movement of the formulation by incorporating dyes into the formulation while we measure surface flows of the mucin solution subphase using tracer particles. Surface tension forces and/or Marangoni stresses initiate a convective spreading flow over this rheologically complex subphase. As expected, when the concentration of surfactant is reduced until its surface tension is above that of the mucin solution, the convective spreading does not occur. The convective spreading front moves ahead of the drop containing the formulation. Convective spreading ends with the solution confined to a well-defined static area which must be governed by a surface tension balance. Further motion of the spread solution progresses by much slower diffusive processes. Spreading behaviors are qualitatively similar for formulations based on anionic, cationic, or nonionic surfactants, containing either hydrophilic or hydrophobic dyes, on mucin as well as on other entangled aqueous polymer solution subphases. This independence of qualitative spreading behaviors from the chemistry of the surfactant and subphase indicates that there is little chemical interaction between the formulation and the subphase during the spreading process. The spreading and final solution distributions are controlled by capillary and hydrodynamic phenomena and not by specific chemical interactions among the components of the system. It is suggested that capillary forces and Marangoni flows driven by soluble surfactants may thereby enhance the uniformity of drug delivery to diseased lungs.

Self-organization of core-shell and core-shell-corona structures in small liquid droplets

Abstract: It was recently discovered that core-shell and core-shell-corona microstructures in immiscible liquid alloys, which were previously obtained only in outer space, can be fabricated under gravity condition on earth using conventional gas atomization. The origin was attributed solely to Marangoni motion driven by temperature-dependence of interfacial energy. We found in this letter, with the aid of computer simulation, that coupled processes of spinodal decomposition, decomposition-induced fluid flow, collision and collision-induced-collision among second-phase droplets all play critical rules at different stages in the formation of these structures. Their contributions relative to the Marangoni effect are analyzed as function of system size.

Confined thermocapillary motion of a three-dimensional deformable drop

Abstract: In this paper, simulations are performed of the thermocapillary motion of three-dimensional and axisymmetric drops in a confined apparatus. The refined level-set grid method is used to track the interface and resolve very small deformations. We compare our results to theoretically predicted thermocapillary migration velocities of drops and to experimentally measured migration velocities in microgravity experiments. The motivation of the present work is to address four important questions surrounding thermocapillary migration. These are as follows. (1) What is the impact of initial conditions on both the initial transient and steady state drop behavior? (2) What is the impact of the domain geometry on drop behavior? (3) Do drops deform for intermediate Marangoni numbers and are those deformations axisymmetric? (4) Can the assumption of constant temperature fluid properties be used when simulating physical experiments? To answer the first question, we explore the parameter space of initial drop temperature distribution and drop holding time. We find that in lower Marangoni number regimes, the drop rapidly settles to a quasisteady state. For larger Marangoni numbers, the initial conditions dominate the drop behavior. To address the second and third questions, we look at the spatial distribution of tangential temperature gradients on the surface of the drop as well as drop deformations and migration velocities. The domain geometry induces nonaxisymmetric deformations and temperature distributions. The results of several axisymmetric runs with realistic physical properties are examined to answer the fourth question. It is found that the variation of material properties influences the drop migration behavior in a nontrivial way.

Numerical simulation of the Marangoni effect on transient mass transfer from single moving deformable drops

Abstract: A level set approach was adopted in numerical simulation of interphase mass transfer from a deformable drop moving in a continuous immiscible liquid, and the simulation results on Marangoni effect were presented with respect to three experimental runs in the methyl isobutyl ketone-acetic acid-water system. Experiments showed that when the solute concentration was sufficiently high, the Marangoni effect would occur with the interphase mass transfer enhanced. Numerical results indicated that the mass-transfer coefficient with Marangoni effect was larger than that without Marangoni effect and stronger Marangoni effect made the drop deform more easily. The predictions were qualitatively in accord with the experimental data. Numerical simulation revealed well the transient flow structure of Marangoni effect. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 2670-2683, 2011

Superspreading mechanisms: An overview

Abstract: An aqueous solution of trisiloxane-ethoxylate surfactants (superspreaders) has fascinating surface properties that promote rapid spreading over hydrophobic substrates and efficiently reduce the surface tension at the air/solution interface to 21–22 mN/m. Superspreaders have a variety of commercial and industrial applications, and can be used as adjuvants, surface modifiers for fabrics, cleaners and much more. Since their discovery in the 1960s, and despite their significant technological applications, the phenomenon that drives superspreading is still not well understood and is under continuous discussion. The goal of the presented review is to discuss and analyze the data presented in the literature and then to elucidate the concepts and mechanisms to explain what drives the fast rate of spreading. Two concepts are presented (and then excluded) for elucidating the understanding of the fast spreading rate over hydrophobic surfaces: the first concept concludes that the spreading is driven by the contact angle dynamics due to the reduction in the surface tension and/or interfacial tension of the solution/substrate leading to a decreased contact angle during spreading and the value of the spreading coefficient S ≥ 0, while the second concept attempts to show that the spreading is driven by the Marangoni flow over a stretching surface of a spreading drop or at the precursor film. However, neither the spreading coefficient, S ≥ 0, nor the Marangoni flow satisfactorily explains why the rate of spreading vs. the degree of surface wettability has a maximum. This review will help readers gain insight on superspreading and stimulate researchers to explore the superspreading phenomenon for novel applications.

Non-isobaric Marangoni boundary layer flow for Cu, Al2O3 and TiO2 nanoparticles in a water based fluid

Abstract: In this paper, a non-isobaric Marangoni boundary layer flow that can be formed along the interface of immiscible nanofluids in surface driven flows due to an imposed temperature gradient, is considered. The solution is determined using a similarity solution for both the momentum and energy equations and assuming developing boundary layer flow along the interface of the immiscible nanofluids. The resulting system of nonlinear ordinary differential equations is solved numerically using the shooting method along with the Runge-Kutta-Fehlberg method. Numerical results are obtained for the interface velocity, the surface temperature gradient as well as the velocity and temperature profiles for some values of the governing parameters, namely the nanoparticle volume fraction φ (0≤φ≤0.2) and the constant exponent β. Three different types of nanoparticles, namely Cu, Al2O3 and TiO2 are considered by using water-based fluid with Prandtl number Pr =6.2. It was found that nanoparticles with low thermal conductivity, TiO2, have better enhancement on heat transfer compared to Al2O3 and Cu. The results also indicate that dual solutions exist when β<0.5. The paper complements also the work by Golia and Viviani (Meccanica 21:200–204, 1986) concerning the dual solutions in the case of adverse pressure gradient.

Novel pattern forming states for Marangoni convection in volatile binary liquids

Abstract: We describe experiments on Marangoni convection in thin evaporating liquid films. The films are binary mixtures of ethanol and water exposed to the ambient room air during all experimental runs. These experiments exhibit a variety of different, often novel, patterns, depending on the concentration (weight fraction) c of ethanol. Among these are mobile circular convective patterns, which have not been previously observed, to our knowledge. The convective patterns evolve due to the evaporation of both the solvent and the solute, and their size increases substantially with the initial concentration c. The patterns reported here differ from those found in binary mixtures of NaCl and water, where only water evaporates.

Drops settling in sharp stratification with and without Marangoni effects

Abstract: We present numerical simulations of drops settling in a layered ambient fluid. We focus on nearly spherical drops with Reynolds numbers of order 10. The ambient is composed of miscible fluids, with the top layer lighter than the lower one, representing fluid stratified through temperature or salinity variations, while the drop itself is heavier than both layers. The surface tension between the ambient and the drop may or may not be different for each layer. Such a system can be used to model oil droplets settling or rising in the ocean. When surface tension is uniform, the drop slows down significantly as it encounters the transition region, due to entrained fluid from the upper layer, before accelerating again in the lower layer. We characterize this effect in terms of the sharpness of the transition, and the drop's Reynolds number. When the upper and lower surface tensions are not matched, the drop may either suddenly accelerate through the transition region if the lower surface tension is less than the upper one, or be prevented from crossing into the lower layer if the lower surface tension is larger than the upper one. We focus on the drop's speed across the transition, and determine the conditions under which a drop may remain suspended at the transition region.