Anomalous spreading with Marangoni flow on agar gel surfaces

We have experimentally observed anomalous spreading of aqueous alcohol solutions on flat and rough fractal agar gel surfaces. On flat agar gel surfaces, extremely fast spreading [θ(D)(t) ∝ t(-0.92)] that differs from Tanner's law [θ(D)(t) ∝ t(-0.3)] was observed when the liquid contained over 9 wt % of 1-propanol in which strong Marangoni flow was observed as a fluctuation on the liquid surface. However, on fractal gel surfaces, different spreading dynamics [θ(D)(t) ∝ t(-0.58)] were observed, although Marangoni flow still occurred. We found the surface-dependent spreading can be discussed in terms of competition between Marangoni flow and the pinning effect due to surface roughness.     

Spreading of Oil Droplets Containing Surfactants and Pesticides on Water Surface Based on the Marangoni Effect

Oil droplets containing surfactants and pesticides are expected to spread on a water surface, under the Marangoni effect, depending on the surfactant. Pesticides are transported into water through this phenomenon. A high-speed video camera was used to measure the movement of Marangoni ridges. Gas chromatography with an electron capture detector was used to analyze the concentration of the pesticide in water at different times. Oil droplets containing the surfactant and pesticide spread quickly on the water surface by Marangoni flow, forming an oil film and promoting emulsification of the oil–water interface, which enabled even transport of the pesticide into water, where it was then absorbed by weeds. Surfactants can decrease the surface tension of the water subphase after deposition, thereby enhancing the Marangoni effect in pesticide-containing oil droplets. The time and labor required for applying pesticides in rice fields can be greatly reduced by using the Marangoni effect to transport pesticides to the target.     

Measurement of flow visualized on surface of bubble blown in air

Bubbles blown in surfactant (frother) solution reveal surface flows attributed to gravity drainage and opposing Marangoni Effect. A technique is introduced to visualize the flows and estimate trajectory velocity. The flow pattern and velocity depend on frother type (pentanol vs. a polyglycol) and concentration but no correlation with surface tension was found. The relevance of the observations to the action of frother in flotation is briefly discussed.     

Steady Marangoni flow traveling with chemical fronts

When autocatalytic chemical fronts propagate in thin layers of solution in contact with air, they can induce capillary flows due to surface tension gradients across the front (Marangoni flows). We investigate here such an interplay between autocatalytic reactions, diffusion, and Marangoni effects with a theoretical model coupling the incompressible Navier-Stokes equations to a conservation equation for the autocatalytic product concentration in the absence of gravity and for isothermal conditions. The boundary condition at the open liquid/air interface takes the surface activity of this product into account and introduces the solutal Marangoni number M representing the intensity of the coupling between hydrodynamics and reaction-diffusion processes. Positive and negative Marangoni numbers correspond, respectively, to the cases where the product decreases or increases surface tension behind the front. We show that, in both cases, such coupled systems reach an asymptotic dynamics characterized by a steady fluid vortex traveling at a constant speed with the front and deforming it, with, however, an asymmetry between the results for positive and negative M. A parametric study shows that increased propagation speed, front deformation, and possible transient oscillating dynamics occur when the absolute value of M is increased.     

Role of Marangoni instability in fabrication of axially and internally grooved hollow fiber membranes

Hollow fiber membranes (HFMs) are extensively used in different industrial applications. Under some controlled fabrication conditions, axially aligned grooves can be formed on the HFM inner surface during typical immersion precipitation-based phase inversion fabrication processes. Such grooved HFMs are found to be promising for nerve repair and regeneration. The axially aligned grooves appearing on the inner surface of the membrane are considered as hydrodynamic instability patterns. During the immersion precipitation process, a transfer of solvent takes place across the interface between a polymer solution and a nonsolvent. This solvent transfer induces gradients of interfacial tension that are considered to be the driving mechanism for Marangoni instability. The onset of the stationary instability is studied by means of a linear instability theory, and the critical and maximum wavenumbers are determined and discussed in terms of the dimensionless groups characterizing the system: viscosity ratio, diffusivity ratio, Schmidt number, crispation number, adsorption number, Marangoni number, and the polymer bulk concentration. A good agreement is found between the predicted wavelength of the most dangerous wave and the experimental groove width. Consequently, solutal Marangoni instability can explain the groove formation mechanism in HFM fabrication.     

Numerical simulation of Marangoni effects of single drops induced by interphase mass transfer in liquid-liquid extraction systems by the level set method

The mathematical model of mass transfer-induced Marangoni effect is formulated. The drop surface evolution is captured by the level set method, in which the interface is represented by the embedded set of zero level of a scalar distance function defined in the whole computational domain. Numerical simulation of the Marangoni effect induced by interphase mass transfer to/from deformable single drops in unsteady motion in liquid-liquid extraction systems is performed in a Eulerian axisymmetric reference frame. The occurrence and development of the Marangoni effect are simulated, and the re- sults are in good agreement with the classical theoretical analysis and previous simulation.     

Marangoni flow of Ag nanoparticles from the fluid-fluid interface

Fluid flow is observed when a volume of passivated Ag nanoparticles suspended in chloroform is mixed with a water/ethanol (v/v) mixture containing acidified 11-mercaptoundecanoic acid. Following mechanical agitation, Ag nanoparticles embedded in a film are driven from the organic-aqueous interface. A reddish-brown colored film, verified by transmission electron microscopy to contain uniformly dispersed Ag nanoparticles, is observed to spontaneously climb the interior surface of an ordinary, laboratory glass vial. This phenomenon is recorded by a digital video recorder, and a measurement of the distance traveled by the film front versus time is extracted. Surface (interfacial) tension gradients due to surfactant concentration, temperature, and electrostatic potential across immiscible fluids are known to drive interface motion; this well-known phenomenon is termed Marangoni flow or the Marangoni effect. Experimental results are presented that show the observed mass transfer is dependent on an acid surfactant concentration and on the volume fraction of water in the aqueous phase, consistent with fluid flow induced by interfacial tension gradients. In addition, an effective desorption rate constant for the Marangoni flow is measured in the range of approximately 0.01 to approximately 1 s(-1) from a fit to the relative film front distance traveled versus time data. The fit is based on a time-dependent expression for the surface (interface) excess for desorption kinetics. Such flow suggests that purposeful creation of interfacial tension gradients may aid in the transfer of 2- and 3-dimensional assemblies, made with nanostructures at the liquid-liquid interface, to solid surfaces.     

Analysis of the effects of Marangoni stresses on the microflow in an evaporating sessile droplet

We study the effects of Marangoni stresses on the flow in an evaporating sessile droplet, by extending a lubrication analysis and a finite element solution of the flow field in a drying droplet, developed earlier. The temperature distribution within the droplet is obtained from a solution of Laplace's equation, where quasi-steadiness and neglect of convection terms in the heat equation can be justified for small, slowly evaporating droplets. The evaporation flux and temperature profiles along the droplet surface are approximated by simple analytical forms and used as boundary conditions to obtain an axisymmetric analytical flow field from the lubrication theory for relatively flat droplets. A finite element algorithm is also developed to solve simultaneously the vapor concentration, and the thermal and flow fields in the droplet, which shows that the lubrication solution with the Marangoni stress is accurate for contact angles as high as 40 degrees. From our analysis, we find that surfactant contamination, at a surface concentration as small as 300 molecules/microm(2), can almost entirely suppress the Marangoni flow in the evaporating droplet.     

Thermal Marangoni Convection of a Fluid Film Coating a Deformable Membrane

The thermal Marangoni instability of a fluid film coating a deformable membrane has been investigated by taking into account the deformation of the fluid free surface. Numerical calculations for different thermal boundary conditions are presented. The prestressed membrane is supposed to be very thin and therefore its behavior is similar to that of an isothermal fluid free surface with a surface tension but with a different mechanical boundary condition; that is, the fluid should stick on its surface and thus the fluid velocity is zero. An important assumption is that the membrane has no temperature dependence and therefore that only one Marangoni number exists for the free surface of the fluid. Numerical results are presented for stationary and oscillatory thermocapillary instability in both the sinuous and the varicose modes. It is shown that membrane deformation has important implications on the Marangoni instability of the fluid layer for positive and negative Marangoni numbers. Copyright 2001 Academic Press.     

Marangoni stresses and surface compression rheology of surfactant solutions. Achievements and problems

In the presence of soluble surfactants, the motion of liquid surfaces involves Marangoni effects. As a consequence, the surfaces exhibit elastic responses, even frequently behaving as rigid surfaces, especially at low surfactant concentration. The Marangoni effects can be conveniently quantified introducing surface viscoelastic compression parameters that characterize the mechanical response of the surface near equilibrium. Many experimental techniques allow measuring the viscoelastic parameters. However, many difficulties are encountered during the interpretation of the surface response in the various types of hydrodynamic velocity fields involved in the different techniques. The role of adsorption and desorption energy barriers appears crucial, despite the fact that little is known yet about their values. In this short review, we will present examples illustrating the different problems.     

Formation of parallel strips in thin films of polystyrene/poly(vinyl pyrrolidone) blends via spin coating on unpatterned substrates

Patterns of parallel strips, consisting of alternating polystyrene (PS) and poly(vinyl pyrrolidone) (PVP) regions, were observed in thin films spin cast from a PS/PVP/chloroform solution on unpatterned substrates. The formation of anisotropic patterns, manifested not only in thickness variation but also in composition variation, was found to be driven by Marangoni instability, with the PS and PVP streams flowing toward the preferred regions as the phase separation induced by solvent evaporation proceeded. The initial viscosity of the polymer solution and the thickness of the spin-cast films were lumped into one single parameter to study the phase morphology development at various initial polymer solution concentrations. Interestingly, the ratio of the square of the film thickness to the viscosity, a parameter loosely related to the Marangoni number, was found to reach a maximum value at the concentration where the strip patterns were most well-developed.     

Formation of nanowire striations driven by Marangoni instability in spin-cast polymer thin films

Parallel striations made of silver nanowires were formed through the Marangoni instability induced during spin casting of poly(2-vinyl pyridine)/silver nanowire/chloroform solutions. The striation patterns of the silver nanowires resembled those obtained from spin casting of the corresponding neat polymer solutions, indicating essentially the same driving mechanism (i.e., the Marangoni instability). The silver nanowires were found to concentrate in the valleys of the striation pattern to balance the nonuniform surface tension distribution in the polymer thin film. The resulting nanowire striation patterns were found to depend on polymer concentration, rotational speed, and nanowire loading. Interestingly, this nanowire striation phenomenon was found to be independent of the substrate characteristics, hydrophobic or hydrophilic.     

Experimental studies on the geometrical characteristics determining the system behavior of surface tension autooscillations

Autooscillation of the surface tension is a phenomenon related to Marangoni instability periodically arising and fading by dissolution of a surfactant droplet under a water-air interface. A detailed experimental investigation was performed to clear up the influence of the system geometry on development and characteristics of autooscillations. It was found that the aspect ratio is an additional dimensionless parameter that determines the system behavior equally to the Marangoni number. The influence of the cell diameter, capillary immersion depth, and droplet radius on the autooscillation period and amplitude was studied as well.     

Effect of interfacial mobility on rupture of thin stagnant films on a solid surface due to random mechanical perturbations

Previous analysis of Narsimhan [G. Narsimhan, J. Colloid Interface Sci. 287 (2005) 624-633] for the evaluation of rupture of a nondraining thin film on a solid support due to imposed random mechanical perturbations modeled as a Gaussian white noise has been extended for partially mobile gas-liquid interfaces. The average rupture time of film is evaluated by first passage time analysis (as the mean time for the amplitude of perturbation to become equal to film thickness). The interfacial mobility is accounted for through surface viscosity as well as Marangoni effect. The mean rupture time for partially mobile gas-liquid interface, as characterized by two dimensionless groups, dimensionless surface viscosity and Marangoni number, lies between the two extreme limits for fully mobile and immobile films. The critical wavenumber for minimum rupture time is shown to be insensitive to interfacial mobility. However, the critical dimensionless surface viscosity and critical Marangoni number at which the behavior of thin film deviates from that of fully mobile film and the behavior approaches that of fully immobile film are smaller for higher Hamaker constants, smaller film thickness and smaller surface potentials.     

Spontaneous oscillations due to solutal Marangoni instability: air/water interface

Systems far from equilibrium are able to self-organize and often demonstrate the formation of a large variety of dissipative structures. In systems with free liquid interfaces, self-organization is frequently associated with Marangoni instability. The development of solutal Marangoni instability can have specific features depending on the properties of adsorbed surfactant monolayer. Here we discuss a general approach to describe solutal Marangoni instability and review in details the recent experimental and theoretical results for a system where the specific properties of adsorbed layers are crucial for the observed dynamic regimes. In this system, Marangoni instability is a result of surfactant transfer from a small droplet located in the bulk of water to air/water interface. Various dynamic regimes, such as quasi-steady convection with a monotonous decrease of surface tension, spontaneous oscillations of surface tension, or their combination, are predicted by numerical simulations and observed experimentally. The particular dynamic regime and oscillation characteristics depend on the surfactant properties and the system aspect ratio.      

Surfactant adsorption and Marangoni flow in liquid jets. I. Experiments

The adsorption of surfactants at an expanding liquid surface has been studied in a gravity-driven laminar water jet with Reynolds numbers in the range from 1000 to 2000. Surface concentrations of hexadecyltrimethylammonium bromide (C(16)TAB) were deduced from ellipsometric measurements, using a calibration made previously with neutron reflection. Simultaneous measurements of the velocity profile within the jet were made with laser Doppler velocimetry. These two noninvasive techniques were able to measure conditions to within 1 mm of the nozzle, where rates of surface expansion were as high as 300 s(-1). For the laminar jet without surfactant, the measurements are in excellent agreement with CFD calculations and with the theoretical result that the surface velocity varies as z(1/3), where z is the distance from the nozzle. Close to the nozzle the high rate of surface expansion drives both rapid diffusional transport to the surface, and rapid convection on the surface, resulting in a low concentration of surfactant. Higher concentrations of surfactant downstream cause a Marangoni stress which decelerates the surface-an effect clearly shown by the velocity data. In the presence of 0.2 M salt, which significantly depresses the cmc, the adsorption of C(16)TAB is greatly reduced, probably because it forms cylindrical micelles, which diffuse much more slowly than free monomers. The apparatus is shown to be a very suitable platform for investigating surfactant adsorption and Marangoni flows under carefully controlled hydrodynamic conditions.     

Investigation of electro-oxidation of methanol and benzyl alcohol at boron-doped diamond electrode: evidence for the mechanism for fouling film formation

Boron-doped diamond (BDD) and glassy carbon (GC) electrodes are compared for electrochemical oxidation of methanol and benzyl alcohol. Cyclic voltammograms reveal that BDD electrode produces good oxidation signals for both methanol and benzyl alcohol, while GC produces no significant oxidation signal for either methanol or benzyl alcohol. Amperometric measurement of oxidation of methanol and benzyl alcohol on BDD shows development of a fouling film for benzyl alcohol but not for methanol. Prolonged (24 h) polarization of the BDD electrode at +2.0 V in benzyl alcohol generated enough fouling film for investigation by AFM, SEM, Raman, and FTIR techniques. AFM and SEM microscopy images confirm a fouling film confined to the low-lying regions of the polycrystallite BDD surface, indicating that the active sites of benzyl alcohol oxidation are located within these low-lying regions. The fouling material generated in the process of benzyl alcohol oxidation was identified from Raman and FTIR spectroscopy as polyester. Experiments confirm the fouling film can be removed and the electrode surface reactivated by brief polarization at +3.0 V. Amperometric results of concentration dependence confirm the BDD electrode is well suited for quantitative analysis applications of methanol and benzyl alcohol, with recognizable oxidation currents at micromolar concentration levels.     

Gas–liquid mass transfer in the presence of ionic surfactant: effect of counter‐ions and interfacial turbulence

This work investigated the effect of counter‐ions and interfacial turbulence on oxygen transfer from gas to liquid phase containing ionic surfactant, and experiments were performed in a mechanically stirred reactor with flat gas–liquid interface. Counter‐ions in terms of hydration ability and polarizability influence the interfacial coverage of ionic surfactants (i.e. cetytrimethylammonium bromide (CTAB) and cetytrimethylammonium chloride) with the same hydrocarbon chain length, producing hindrance but in different extent on oxygen transfer. The addition of electrolyte (NH₄Br) substantially reduced the interfacial tension and surface charge of micelles (zeta potential) in CTAB system, and this salt effect greatly compressed interfacial double layer leading to gas transfer inhibition. The surface charge, aggregation number as well as stability of micelles formed above the critical micelle concentration could also alter interfacial configuration of surfactant layer reflected by gas absorption rate. Liquid turbulence was analyzed to decide the role of surfactant present in water on gas–liquid mass transfer, since Marangoni instability effect playing positive role should be taken into consideration under moderate liquid flow, while in turbulent system, contribution of Marangoni effect became overshadowed and consequently surfactant pose ‘barrier’ effect on gas transfer due to its surface active nature. Copyright © 2013 John Wiley & Sons, Ltd.     

表面波技术研究TBP对C12E8表面流变性质的影响

The influence of antifoaming agent TBP on the surface adsorption and on the surface rheological properties of foaming agent C12E8 is investigated by the damped longitudinal wave method, and the antifoaming mechanism of the antifoaming agent TBP on C12E8 is explored in this paper. The experimental results show that while the concentration of foaming agent C12E8 is smaller than cmc and at ω=6.28 rad•s-1, the decreases of surface dilational modulus, surface dilational elasticity, surface dilational viscosity and the increases of the phase angle when the concentration of TBP increase for the C12E8+TBP aqueous solution (C12E8=0.074 mmol•L-1) have been related to the diffusional exchange between surface and bulk and to the Marangoni effect, which depends on the restored mechanism of surface tension gradient due to the stretched surface. This has been related to the mechanism of antifoaming action.     

Regular self-motion of a liquid droplet powered by the chemical marangoni effect

We describe here our recent work on spontaneous regular motion of liquid droplet powered by the chemical Marangoni effect under spatially symmetric conditions. It is shown that a spontaneously crawling oil droplet on a glass substrate with a nonequilibrium chemical condition of cationic surfactant exhibits regular rhythmic motion in a quasi-one-dimensional vessel, whereas irregular motion is induced in a two-dimensionally isotropic environment. Such behavior of a droplet demonstrates that spontaneous regular motion can be generated under fluctuating conditions by imposing an appropriate geometry. As another system, we introduce alcohol droplet moving spontaneously on water surface. The droplet spontaneously forms a specific morphology depending on its volume, causing specific mode of translational motion. An alcohol droplet with a smaller volume floating on water surface moves irregularly. On the other hand, a droplet with a larger volume undergoes vectorial motion accompanied by deformation into an asymmetric shape. This result suggests a scenario on the emergence of regular motion coupled with geometrical pattern formation under far-from-equilibrium conditions.