Induced Marangoni Flow Research Articles

Step flow mechanism in dissolutive wetting Cu/Ni systems

The Cu-Ni system is a typical dissolutive system due to its mutual dissolution across a wide range of temperatures and compositions. We characterized the effects of Ni dissolution on the wetting behavior of liquid Cu by combining high-temperature wetting experiments, in-situ observation of spreading and solidification, microstructure analysis of the quenched droplets, and computational fluid dynamic (CFD) simulations. In the very early moment, at 1100 °C, when the Cu droplet is brought in contact with the Ni substrate, it oscillates due to capillarity and is dampened by inertial effects, while the significant Ni dissolution at 1150 °C largely reduced the initial oscillations. Later, a peculiar spreading behavior is observed and we propose to describe it through a 4-step mechanism: pinning of the contact line by a newly formed solid solution layer at the interface acting as a physical barrier, driving of liquid towards the solidified edge due to a Ni-concentration induced Marangoni flow, forming of a precursor film ahead of the solidified edge caused by the strong Cu-Ni interactions and Marangoni flow, and finally depinning due to overflow as a result of liquid accumulation at the solidified edge. The formation of a solid solution layer is confirmed by in-situ observation and quenching. The Ni-concentration induced Marangoni flow is characterized experimentally and further investigated by CFD simulations. The proposed step flow mechanism can be potentially relevant to other dissolutive wetting systems (e.g. Bi/Sn, Ag/Cu and Cu/Fe systems), which are crucial for high-temperature processing techniques.

One-step construction of light-responsive intelligent foam by the Hoffmeister effect and 2D black phosphorus: High stability and on-demand degradation

Stimulus-responsive liquid foams have gained much attention for use in various industrial applications. However, it remains challenging to construct such systems with integrated functionality of easy preparation, high stability, high foaming ability, and rapid on-demand degradation. Herein, by combining the Hofmeister effect and nanotechnology, a promising ultrastable and photoresponsive liquid foam was prepared that had a lifetime of several months and could be destroyed on demand in a few minutes. Specifically, the system was prepared by simply mixing a gelatine solution containing black phosphorus nanosheets (BPNs) and kosmotropic anions in the Hofmeister series with air in one step using only two syringes, and there were no chemical modifications or crosslinking agents required. The kosmotropic anions induced stronger hydrophobic interactions, bundling within molecular chains, and blockage of foam drainage channels, which significantly improved the foaming ability and the lifetime and mechanical properties of the foam. Moreover, rational structure design realized a promising on-demand degradation mechanism via a cascading “light trigger–heat generation–Marangoni flow generation” process occurring on the bubble surfaces. On this basis, the BPNs converted light into thermal energy, which induced Marangoni flow driven by surface tension gradients along the gas‒liquid interfaces, and the bubble film ruptured within seconds upon light illumination. The designed stimulus‒response systems combined stable, fast and repeatable processes without sacrificing the foaming abilities, thus providing a general way to control the stabilities of foams, bubbles and films.

Deterministic positioning of few aqueous colloidal quantum dots.

Emerging quantum technologies that critically require the integration of quantum emitters on photonic platforms are hindered by the control over their position, quantity, and scalability. Herein, we describe a facile strategy to deposit aqueous silica-coated quantum dots (QDs) in a template of polymethyl methacrylate (PMMA) nanoholes that leverages saturated ethanol vapor drop-casting and subsequent lift-off of the template. Ethanol vapor incorporation into water droplets during the drying process reduces the meniscus contact angle, which increases capillary forces and enhances particle confinement within the pinning contact region. Furthermore, induced Marangoni flow controls the particle transport dynamics inside the droplets, making large-scale deposition possible. Controlling the hole diameter of the template demonstrates changes in the number of QDs per hole, which is consistent with the Poissonian distribution with the best results of ∼40% single-particle yield from an ∼80% total site occupancy. This method employs a simple setup, eliminating the need for intricate optimization, yet offers the potential for deterministic patterning within complex photonic platforms.

Effects of surfactant size and concentration on the internal flow fields of moving slug and Disk-like droplets via μ-PIV

• • A laser-based phase-locking system is developed and integrated with a Micro Particle Image Velocimetry system enabling quantitative measurement of the internal velocity of moving droplets. • • Effects of surfactant size on the flow field of moving droplets are investigated by choosing two surfactants, Sodium Dodecyl Sulphate (SDS) and Tween 20, which have a large difference in adsorption timesale. • • Effects of surfactant concentration (zero to 10X) on the internal flow field of moving droplets are evaluated and discussed. • • Effects of droplet shape (slug and spherical) and droplet generation regimes on the internal flow field of moving droplets with surfactant are investigated and discussed. • • It is revealed that achievement of complete remobilisation in the internal flow depends on surfactant type, concentration, and droplet operating regime. Understanding the effects of surfactant on the internal flow field of moving droplets is of fundamental and practical importance. A laser-based phase-locking system was developed and coupled with a micro particle image velocimetry system enabling quantitative measurement of the internal droplet flow filed. Five concentrations (zero to 10X critical micelle concentration) of each of the two surfactants, Sodium Dodecyl Sulphate (SDS) and Tween 20, which present a large difference in adsorption timescale, were evaluated. Slug- and disk-shaped droplets were considered because of their different contact area with channel walls and thus the resulted friction. Squeezing and transition droplet formation regimes which normally yield monodispersed droplets were evaluated using SDS. Flow retardation was observed at low surfactant concentrations for slug droplets, which was primarily attributed to the induced Marangoni flow opposing the internal flow. Achievement of complete remobilisation in the internal flow depends on surfactant type, concentration, and droplet operating regime.

Buoyancy-Marangoni Fingering of a Miscible Spreading Drop

We experimentally investigate the interfacial instability that emerges when a water droplet is deposited on a bath of glycerol-water solution. Despite the absence of surface tension to stabilize short-wavelength undulations, we observe finite-size fingers that grow and pinch off from the drop. We show that the fingering patterns formed in the experiments resultes from a balance between the outward buoyancy effect and inward Marangoni flow. This induced Marangoni flow inhibits small perturbations and acts as an effective surface tension on the miscible interface of the spreading drop. To characterize the final size and shape of the drop, we perform systematic experiments by varying the drop volume and the glycerol-water volume fraction. In addition, we have developed scaling arguments for the drop’s final radius using key physical forces, and show that the final wavelength is inversely proportional to the Bond number.

Disk-Ring Deposition in Drying a Sessile Nanofluid Droplet with Enhanced Marangoni Effect and Particle Surface Adsorption.

The present study is to explore the central particle deposition from drying a sessile nanofluid droplet experimentally and theoretically. Normally, a pinned colloidal droplet dries into a coffee-ring pattern as a result of moving the particles to a three-phase line by the radial direction capillary flow. However, the strong evaporation can generate the nonuniform temperature at the evaporating droplet interface and the droplet periphery temperature is higher than that close to the droplet centerline. The induced Marangoni flow would reversibly transport the particles at the periphery toward the centerline. We have thus designed the experiments to increase the droplet evaporation rate in vacuum conditions and accordingly to enhance the Marangoni effect. We have observed distinguishable disk deposition inside the outer coffee ring. A three-dimensional diffusion-limited cluster-cluster aggregation Monte Carlo model has been developed to simulate the deposition process. With modeling the Marangoni effect, particle adsorption at the liquid-air interface and particle aggregation behaviors, the formation of the disk pattern inside a coffee ring has been simulated. The qualitative agreement has been found in the comparison of local deposition distribution between the related experiment and simulation.

Speciation of thioarsenicals through application of coffee ring effect on gold nanofilm and surface-enhanced Raman spectroscopy

Thioarsenicals, such as dimethylmonothioarsinic acid (DMMTAV) and dimethyldithioarsinic acid (DMDTAV), have been increasingly discovered as important arsenic metabolites, yet analysis of these unstable arsenic species remains a challenging task. A method based on surface-enhanced Raman spectroscopy (SERS) detection in combination with the coffee ringeffect for separation is expected to be particularly useful for analysis of thioarsenicals, thanks to minimal sample pretreatment and unique fingerprint Raman identification. Such a method would offer an alternative approach that overcomes limitations of conventional arsenic speciation techniques based on high performance liquid chromatography separation and mass spectrometry detection. A novel analytical method based on combination of the coffee ringeffect and SERS was developed for the speciation of thiolated arsenicals. A gold nanofilm (AuNF) was employed not only as a SERS substrate, but also as a platform for the separation of thioarsenicals. Once a drop of the thioarsenicals solution was placed onto the AuNF and evaporation of the solvent and the ring stamp formation onto AuNF began, the SERS signal intensity substantially increased from center to edge regions of the evaporated droplet due to the presence of the coffee ring effect. Through calculating the pKa’s of DMMTAV and DMDTAV and accordingly manipulating the chemical environment, separation of these thioarsenicals was realized as they travelled different distances during the development of the coffee ring. The migration distances of individual species were influenced by a radial outward flow of a solute, the thioarsenicals-AuNF interactions and a thermally induced Marangoni flow. The separation of DMMTAV (center) and DMDTAV (edge) on the coffee ring, in combination with fingerprint SERS spectra, enables the identification of these thioarsenicals by this AuNF-based coffee ring effect-SERS method.

Gas-Phase Induced Marangoni Flow Causes Unstable Drop Merging.

The merging of drops plays a key role in many processes from simple rain to complex coating applications. In technical applications, often liquids with different surface tensions merge on a substrate like inkjet printing. For a suitable set of surface tensions, one drop can form a stable wetting film that is covering the other drop. Here, we explore the dynamics of driven wetting films and show a route toward their instability when these wetting films are driven by an external source of energy, which is Marangoni stress in our case. The wetting becomes unstable via a fingering instability and can be observed in various liquid combinations. The vapor of the liquid with the lower surface tension induces a Marangoni driven flow inside the other drop that pulls the wetting film. The concentration of the driving vapor can be controlled through the spreading velocity of the corresponding drop. We use this dependence to map out the characteristics of the instability. For very high or very low spreading velocities, no instability is observed. This is summarized in a stability diagram, which has three different regimes. A detailed analysis reveals a strong coupling of the characteristics of the fingering instability to the spreading velocity. The use of the spreading velocity as a control parameter is justified by a simplified 1D model that motivates how the spreading velocity controls the concentration profile of the second liquid vapor before and at contact. The strength of the Marangoni flow that drives the instability depends on the exposure time of the sitting drop to the vapor concentration profile around the spreading drop.

Electrically induced spreading of EGaIn on Cu substrate in an alkali solution under wetting and non-wetting conditions

Electrically actuated movement of liquid metals is quite promising in mechanical engineering and applied physics. However, little work has so far been performed on the effect of current polarity on the actuation of the eutectic gallium–indium (EGaIn) alloy moving on metal substrates. In this work, the spreading of EGaIn on copper (Cu) substrate under different direct-current (DC) polarities in the 0.5 mol/L NaOH solution was investigated, and the underlying mechanisms were addressed. When the Cu substrate was connected to cathode, the wetting was essentially attributed to the reduction in the oxidized surface of the Cu substrate; whereas, under the opposite polarity, the spreading was related to the tension gradient along the EGaIn–solution interface, which induced Marangoni flow, despite the fact that the EGaIn–Cu system was non-wetting in this case.

Controlling self-assembly and buckling in nano fluid droplets through vapour mediated interaction of adjacent droplets

HypothesisSessile droplets of contrasting volatilities can communicate via long range (∼O (1) mm) vapour-mediated interactions which allow the remote control of the flow driven self-assembly of nanoparticles in the drop of lower volatility. This allows morphological control of the buckling instability observed in evaporating nanofluid droplets. ExperimentsA nanofluid droplet is dispensed adjacent to an ethanol droplet. Asymmetrical adsorption induced Marangoni flow (∼O (1) mm/s) internally segregates the particle population. Particle aggregation occurs preferentially on one side of the droplet leaving the other side to develop a relatively weaker shell which buckles under the effect of evaporation driven capillary pressure. FindingsThe inter-droplet distance is varied to demonstrate the effect on the precipitate shape (flatter to dome shaped) and the location of the buckling (top to side). In addition to being a simple template for hierarchical self-assembly, the presented exposition also promises to enhance mixing rates (∼1000 times) in droplet-based bioassays with minimal contamination.

Coalescence of small bubbles with surfactants

Bubble coalescence is central to many important technological processes, such as separations, cleaning of oil spills, microfluidics, emulsification and foaming. It is well known that surfactants, which are frequently present as additives or contaminants, delay coalescence by slowing the drainage of the liquid film separating the approaching bubbles before they make contact. However, the coalescence and surfactant transport mechanisms developed after surfactant-laden bubbles make initial contact remain poorly understood. Here, we characterize these mechanisms using high-fidelity numerical simulations to predict the evolution of bubble interfaces, surfactant spreading, and induced Marangoni flows. Our results show that the surfactant initially accumulates on the tiny meniscus bridge formed between the coalescing bubbles due to the rapid and highly localized contraction of meniscus area. At the same time, a Marangoni-driven convective flow is generated at the interface, which drags the accumulated surfactant away from the joining meniscus and toward the back of the bubbles. Together, these transport mechanisms affect the rate bubble coalescence by dynamically modifying the local pull of surface tension on the bubble interfaces.

Interfacial mechanisms for stability of surfactant-laden films.

Thin liquid films are central to everyday life. They are ubiquitous in modern technology (pharmaceuticals, coatings), consumer products (foams, emulsions) and also serve vital biological functions (tear film of the eye, pulmonary surfactants in the lung). A common feature in all these examples is the presence of surface-active molecules at the air-liquid interface. Though they form only molecular-thin layers, these surfactants produce complex surface stresses on the free surface, which have important consequences for the dynamics and stability of the underlying thin liquid film. Here we conduct simple thinning experiments to explore the fundamental mechanisms that allow the surfactant molecules to slow the gravity-driven drainage of the underlying film. We present a simple model that works for both soluble and insoluble surfactant systems in the limit of negligible adsorption-desorption dynamics. We show that surfactants with finite surface rheology influence bulk flow through viscoelastic interfacial stresses, while surfactants with inviscid surfaces achieve stability through opposing surface-tension induced Marangoni flows.

Effect of directed energy deposition processing parameters on laser deposited Inconel® 718: Microstructure, fusion zone morphology, and hardness

Single-bead, laser-deposited Inconel® 718 tracks atop substrates of the same composition were studied to ascertain the influence of laser power, processing speed, working distance, and substrate preheat on the fusion zone geometry, microstructure, and hardness. Modifying working distance encompassed both a change in powder flow distribution and beam diameter. Laser power and processing speed linearly affected fusion zone width and area, though laser power was found to have the most significant effect of all processing parameters. Preheating the substrates increased the width of the fusion zone by an average of 16% and led to a more uniform hardness throughout. The fusion zone cross-section was found to morph from semicircular to double-parabolic (wavy) with increasing laser power. This was attributed to surface tension induced Marangoni flow and the influence of surface-activated species on surface tension. The applicability of coupled parameters, including linear heat input and normalized enthalpy were investigated. Given the limited data available on the influence of processing parameters, particularly working distance and substrate temperature, on fusion zone geometry and hardness, results reported here may aid experimentalists and modelers working on cladding and additive manufacturing processes.

Numerical study on solutal Marangoni instability in finite systems with a miscibility gap

Solutal Marangoni instability (SMI) is investigated both in 2D and 3D using a combined Cahn-Hilliard and Navier-Stokes model in a finite system. Fe-Sn is chosen as a representative alloy system since the phase diagram reveals a region with a miscibility gap, where two liquid phases, namely, the Fe-rich phase L1 and the Sn-rich phase L2, are in chemical equilibrium. In 3D, considering a perturbed liquid cylinder (L2 phase) with a length of λ and a radius of R0 embedded in the middle of a simulation box of λ × H × H (length × width × height) surrounded by the phase L1, we find that the perturbation induced Marangoni flow is either clockwise or anti-clockwise depending on the mean curvature difference between the convex and concave regions which is affected by the ratio of λ/R0. The critical ratio of λ/R0 for SMI is shown to be invariant for different Marangoni numbers as well as independent of the geometrical properties of the L1 phase. In 2D, a perturbed liquid pipe with a length of λ and a radius of R0 embedded in the middle of a simulation box of λ × H (length × height) is taken into account. Due to different curvature constitution, the critical ratio of λ/R0 for SMI depends on the height of the L1 phase.

Large Convective Heat Transfer Enhancement in Microchannels With a Train of Coflowing Immiscible or Colloidal Droplets

We show that heat transfer in microchannels can be considerably augmented by introducing droplets or slugs of an immiscible liquid into the main fluid flow. We numerically investigate the influence of differently shaped colloidal or simply pure immiscible droplets to the main liquid flow on the thermal transport in microchannels. Results of parametric studies on the influence of all major factors connected to microchannel heat transfer are presented. The effect of induced Marangoni flow at the liquid interfaces is also taken into account and quantified. The calculation of the multiphase, multispecies flow problem is performed, applying a front tracking method, extended to account for nanoparticle transport in the suspended phase when relevant. This study reveals that the use of a second suspended liquid (with or without nanoparticles) is an efficient way to significantly increase the thermal performance without unacceptably large pressure losses. In the case of slug-train coflow, the Nusselt number can be increased by as much as 400% compared with single liquid flow.

Visualization of low Reynolds boundary-driven cavity flows in thin liquid shells

Classic examples of low-Reynolds recirculating cavity flows are typically generated from lid-driven boundary motion at a solid–fluid interface, or alternatively may result from shear flow over cavity openings. Here, we are interested in an original family of boundary-driven cavity flows occurring, in contrast to classic setups, at fluid–fluid interfaces. Particle image velocimetry (PIV) is used to investigate the structure of internal convective flows observed in thin liquid shells. Under the specific configuration investigated, the soap bubble’s liquid shell is in fact in motion and exhibits sporadic local “bursts”. These bursts induce transient flow motion within the cavity of order Re ∼ O(1). The combination of PIV and proper orthogonal decomposition (POD) is used to extract dominant flow structures present within bubble cavities. Next, we show that thermally induced Marangoni flows in the liquid shell can lead to forced, (quasi) steady-state, internal recirculating flows. The present findings illustrate a novel example of low-Reynolds boundary-driven cavity flows.

Criterion for Reversal of Thermal Marangoni Flow in Drying Drops

The thermal Marangoni flow induced by nonuniform surface temperature has been widely invoked to interpret the deposition pattern from drying drops. The surface temperature distribution of a drying droplet, although being crucial to the Marangoni flow, is still controversial. In this paper, the surface temperature in the drop central region is analyzed theoretically based on an asymptotic analysis on the heat transfer in such region, and a quantitative criterion is established for the direction of the surface temperature gradient and the direction of the induced Marangoni flow of drying drops. The asymptotic analysis indicates that these two directions will reverse at a critical contact angle, which depends not only on the relative thermal conductivities of the substrate and liquid, but also on the ratio of the substrate thickness to the contact-line radius of the droplet. The theory is corroborated experimentally and numerically, and may provide a potential means to control deposition patterns from drying droplets.

Stationary regimes of axisymmetric thermal wake interaction of two buoyant drops at low Reynolds and high Peclet number

Axisymmetric motion of a leading fluid drop and a trailing gas bubble (or thermally nonconducting drop) in a viscous fluid under the combined effect of gravity and thermocapillarity is considered under the assumption of negligible inertia effects and of nondeformable interfaces. The ambient fluid far from the inclusions is isothermal and the temperature of the leading particle differs from that of the continuous medium. At large Peclet number, thermal boundary layers are present along the fluid-liquid and the gas-liquid interfaces, and thermal wakes are formed downstream from the particles. The interaction of the thermal wake, shed from the leading inclusion, with the thermal boundary layer on the surface of the trailing one causes a nonuniform temperature distribution on the surface of the latter. The induced Marangoni flow results in the change of the flow pattern, the velocity of both particles, and the equilibrium separation distance. In the present paper, the influence of the Marangoni effect on the drag force and the rate of heat transfer from the drops translating at given velocity is studied as well as on the equilibrium velocities and separation distance of the drops freely migrating under the action of gravity. The analysis considers particles at arbitrary separation distance and takes full account of thermal and hydrodynamic interactions.

Dynamics of Surfactant-Driven Fracture of Particle Rafts

We investigate the dynamic fracture of a close-packed monolayer of particles, or particle raft, floating at a liquid-gas interface induced by the localized addition of surfactant. Unusually for a two-dimensional solid, our experiments show that the speed of crack propagation here is not affected by the elastic properties of the raft. Instead it is controlled by the rate at which surfactant is advected to the crack tip by means of the induced Marangoni flows. Further, the velocity of propagation is not constant in time and the length of the crack scales as t(3/4). More broadly, this surfactant-induced rupture of interfacial rafts suggests ways to manipulate them for applications.

On the flow-induced Marangoni instability due to the presence of surfactant

The flow-induced Marangoni instability due to the presence of surfactant is examined for long-wavelength perturbations. A unified view of the underlying mechanisms is provided through revisiting both falling film and two-fluid Couette flow systems. The analysis is performed by inspecting the corresponding coupled set of evolution equations for the interface and surfactant concentration perturbations. While both systems appear to have very similar sets of equations consisting of base flows and Marangoni effects, the origins of stability/instability are identified and illustrated from a viewpoint of vorticity. The base flow rearranges the surfactant distribution and the induced Marangoni flow tends to stimulate the interface's growth. But this destabilizing effect is reduced by effects combining the interface travelling motions and the Marangoni recoil. The competition between these opposing effects determines the system stability, and is elucidated using equations in concert with observations from initial value problems. Moreover, a criterion for the onset of instability can be established in line with the same rationale. The present work not only furnishes a lucid way to clarify the instability mechanisms, but also complements previous studies. Extension to the weakly nonlinear regime is also discussed.