Marangoni flow in micro-channels

A novel electrochemical method for driving fluids in micro-channels is presented. The principle is based upon the onset of Marangoni flow along the interface between an aqueous solution (mobile phase) and an organic electrolyte polymer gel coated on the inner walls of the micro-channel. The gradient of surface tension responsible for the fluid motion arises from local changes in the surface charge. The excess charge is determined by the ionisation of surfactant species at the gel coating|aqueous electrolyte interface which is effectively dependent on the Galvani potential difference. Potential differences of less than a volt between two closely spaced silver band electrodes along the micro-channel can generate zones of high and low surface tension, promoting the motion of the aqueous electrolyte.     

Superspreading driven by Marangoni flow

The spontaneous spreading (called superspreading) of aqueous trisiloxane ethoxylate surfactant solutions on hydrophobic solid surfaces is a fascinating phenomenon with several practical applications. For example, the ability of trisiloxane ethoxylate surfactants to enhance the spreading of spray solutions on waxy weed leaf surfaces, such as velvetleaf (Abutilion theophrasti), makes them excellent wetting agents for herbicide applications. The superspreading ability of silicone surfactants has been known for decades, but its mechanism is still not well understood. In this paper, we suggest that the spreading of trisiloxane ethoxylates is controlled by a surface tension gradient, which forms when a drop of surfactant solution is placed on a solid surface. The proposed model suggests that, as the spreading front stretches, the surface tension increases (the surfactant concentration becomes lower) at the front relative to the top of the droplet, thereby establishing a dynamic surface tension gradient. The driving force for spreading is due to the Marangoni effect, and our experiments showed that the higher the gradient, the faster the spreading. A simple model describing the phenomenon of superspreading is presented. We also suggest that the superspreading behavior of trisiloxane ethoxylates is a consequence of the molecular configuration at the air/water surface (i.e. small and compact hydrophobic part), as shown by molecular dynamics modeling. We also found that the aggregates and vesicles formed in trisiloxane solutions do not initiate the spreading process and therefore these structures are not a requirement for the superspreading process.     

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.     

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.     

Marangoni flow alters wetting: Coffee ring and superspreading

Colloidal nano altered wetting is an everyday phenomenon with various applications. It occurs when an aqueous colloidal droplet is placed on a wet solid and evaporates with a pinned contact line (to form a shape similar to a coffee ring) and when a ‘superspreader’ is placed on a partially wet solid surface with an unpinned contact line. This article reviews recent advances on the mechanisms behind these two phenomena. For the coffee ring phenomenon, besides the evaporation flow, the thermal and solutal Marangoni flow are critical to the radial flow as they carry solutes from the droplet's interior to the contact line. The particle self-assembly into a layered structure at the droplet edge involves particle structuring under confinement and the stagnation flow. While superspreading is not fully understood, it is proposed that the Marangoni flow plays an essential role in superspreading.     

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.     

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.     

Double-stratified Marangoni boundary layer flow of Casson nanoliquid: probable error application

Journal of Thermal Analysis and Calorimetry - The present research is manifested to observe the impact of double stratification in the Marangoni convective flow of the Casson nanoliquid model...     

Interfacial forces and Marangoni flow on a nematic drop retracting in an isotropic fluid

Nematic-isotropic interfaces exhibit novel dynamics due to anchoring of the liquid crystal molecules on the interface. The objective of this study is to demonstrate the consequences of such dynamics in the flow field created by an elongated nematic drop retracting in an isotropic matrix. This is accomplished by two-dimensional flow simulations using a diffuse-interface model. By exploring the coupling among bulk liquid crystal orientation, surface anchoring and the flow field, we show that the anchoring energy plays a fundamental role in the interfacial dynamics of nematic liquids. In particular, it gives rise to a dynamic interfacial tension that depends on the bulk orientation. Tangential gradient of the interfacial tension drives a Marangoni flow near the nematic-isotropic interface. Besides, the anchoring energy produces an additional normal force on the interface that, together with the interfacial tension, determines the movement of the interface. Consequently, a nematic drop with planar anchoring retracts more slowly than a Newtonian drop, while one with homeotropic anchoring retracts faster than a Newtonian drop. The numerical results are consistent with prior theories for interfacial rheology and experimental observations.     

Marangoni效应与液膜振荡*

本文系统地总结了Marangoni效应的形成、发展及其与液膜振荡现象的关系,用Marangoni效应定量分析讨论了3种不同的液膜振荡体系,给出了有关数学公式,并与液膜振荡的化学本质学说作了对比。     

Droplet behavior on flat and textured surfaces: co-occurrence of Deegan outward flow with Marangoni solute instability

It is demonstrated experimentally that the solution droplet behavior is governed by the co-occurrence of outward hydrodynamic and surface tension (solute Marangoni) induced flows. Potassium ferrocyanide allows the effective visualization of the processes taking place in the droplet. Wetting properties of the substrate govern the shape evolution of the evaporated droplet. Quantitative estimation of the wetting properties of textured polymer surface is presented.     

质量、热量传递过程中的Marangoni效应*

由质量、热量传递引发,表面张力梯度驱动的Marangoni效应不但对化学工程、材料工程和热能工程等领域里的一系列过程具有重要的影响,而且具有非线性耗散系统理论研究的一个具有实际意义的课题。对Marangoni效应的实验及理论研究有助于增进对微观传热、传质机理的理解,它的合理利用也可以提高某些过程的效率。迄今为止,对Marangoni效应的认识还不能满足理论研究和工程应用的要求。按期在各相关领域内对Marangoni的研究十分活跃,本文回顾了这些研究成果。     

Faraday Ripples, Parametric Resonance, and the Marangoni Effect

In general, the combined actions of two destabilizing mechanisms do not simply add to each other. Here we show that there is a subtle interplay between parametric excitation and thermal gradients leading to interfacial instability, overstability, and generation of surface waves. The case studied refers to the stability of a liquid layer with an open free surface subjected to a transverse temperature gradient (with the Marangoni effect) and also subjected to the simultaneous action of periodic vibrations normal to the layer. Stability is examined in the weak viscosity approximation by applying a multiscale method. To a first approximation, whatever the imposed thermal gradient, vibrations with fairly large amplitude are responsible for excitation of ripples with half the imposed vibration frequency, but their amplitude depends on the Marangoni number. However, as the Marangoni number increases, the critical amplitude decreases from the excitation threshold of Faraday ripples, and after passing through a minimum it monotonically increases with increasing thermal gradient. Another salient finding is that the threshold of the Marangoni overstability is found to be independent of the imposed vibration frequency and amplitude. Copyright 2001 Academic Press.     

Marangoni Flow Driven Instabilities and Marginal Regeneration

Fingering instabilities in films moving along wetted surfaces, dimpling in horizontal liquid films, and the drainage of vertical soap films by marginal regeneration are caused by surface tension gradients along the perimeter of the thin film. These gradients lead to a mechanical instability which involves Marangoni type liquid flow. It is possible to describe the conditions for the onset of marginal regeneration with a critical number of the ratio between the driving force for the Marangoni flow and the friction of film elements that move relative to their surroundings. This ratio is called the Mysels number. A linear stability analysis leads to a scaling relation lambda approximately h(Ca)(-1/3) between the wavelength lambda of the instability and the capillary number Ca (Ca=/etaV(s)/gamma. In experiments with several Marangoni-driven instabilities this scaling relation has been found; it illustrates the general applicability in the understanding of flow phenomena of this type. Copyright 2001 Academic Press.     

Marangoni effect reverses coffee-ring depositions

We show here both experimentally and theoretically that the formation of "coffee-ring" deposits observed at the edge of drying water droplets requires not only a pinned contact line but also suppression of Marangoni flow. For simple organic fluids, deposition actually occurs preferentially at the center of the droplet, due to a recirculatory flow driven by surface-tension gradients produced by the latent heat of evaporation. The manipulation of this Marangoni flow in a drying droplet should allow one in principle to control and redirect evaporation-driven deposition and assembly of colloids and other materials.     

On the theory of steady-state current flow through electrolytes

A method of solving electrodiffusion equations, which generalizes the method of joining asymptotic solutions, is developed. Approximate solutions for prelimit and beyond-limit currents are obtained. The limiting values of field strength at which a boundary layer is formed are found. The coordinates and field strength values at which the Boltzmann distribution of the concentration of species is observed are found.     

Extending the Ree-Eyring generalized flow theory to non-equilibrium conditions

Summary It is stated that theRee-Eyring treatment of the entanglement-disentanglement transition does not satisfactorily describe the non-equilibrium states. By introducing theKubát theory of relaxation theRee-Eyring flow theory can be extended to those states, in agreement with experimental observations.

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Zusammenfassung Es wird gezeigt, da? dieRee-Eyringsche Behandlung der Verflechtungs-Entfleohtungs-Umlagerung die Nicht-Gleichgewichtszust?nde nicht befriedigend beschreibt. Durch Einführung derKubátschen Theorie der Relaxation wird dieRee-Eyringsche Flie?theorie in übereinstimmung mit experimentellen Beobachtungen an solchen Zust?nden erweitert.

     

Interfacial Wave Motions Due to Marangoni Instability

Collisions and reflections of solitary waves and (periodic) wave trains driven by surface tension gradients (Marangoni stresses) exhibit a wealth of astonishing features. Depending on the angle between the incoming wave crests, the outgoing waves show in their trajectories after collision negative phase shift for small enough angles, no phase shift at about pi/2 and hence no appreciable change in their trajectories, or positive phase shift, accompanied by the appearance of a phase-locked third wave or Mach-Russell stem at wider crossing angles. Synchronous wave collisions exhibit regular but complex dynamic network patterns whose formation and dependence on the size and the shape of the container are discussed. Although wave reflections share some of these features, corresponding apparently to the outcome of the virtual collision of a wave with its mirror image, there are significant differences that are described here. Copyright 2001 Academic Press.     

Evaporation Induced Spontaneous Micro-Vortexes through Engineering of the Marangoni Flow

Vortex flow fields are widely used to manipulate objects at the microscale in microfluidics. Previous approaches to produce the vortex flow field mainly focused on inertia flows. It remains a challenge to create vortexes in Stokes flow regime. Here we reported an evaporation induced spontaneous vortex flow system in Stokes flow regime by engineering Marangoni flow in a micro-structured microfluidic chip. The Marangoni flow is created by nonuniform evaporation of surfactant solution. Various vortexes are constructed by folding the air–water interface via microstructures. Patterns of vortexes are programmable by designing the geometry of the microstructures and are predictable using numerical simulations. Moreover, rotation of micro-objects and enrichment of micro-particles using vortex flow is demonstrated. This approach to create vortexes will provide a promising platform for various microfluidic applications such as biological analysis, chemical synthesis, and nanomaterial assembly.     

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

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.