Marangoni Convection Research Articles

Experimental and Theoretical Study of Propulsion Mechanism with Neutrophil Rotation by using Concentration Marangoni Effect

In the immunological systems, neutrophil propulsion is considered to be driven by the concentration gradient of cytokines. However, the mechanism of propulsion has not been elucidated yet in previous research. In our investigations, assuming that neutrophil moves by the concentration Marangoni effect, the concentration gradient of cytokine on the neutrophil membrane was experimentally examined. The result showed that the cytokine concentration gradient on neutrophil membranes periodically changed. It indicates the possibility that neutrophils move while rotating under the cytokine concentration gradient. However, the neutrophil's rotation is difficult to be observed because experimental verification is limited due to the accuracy and sensitivity of the present measurement system. Therefore, no evidence has been found yet for a relationship between neutrophil velocity and rotation. In this study, assuming that the neutrophil is rotating, this relationship is investigated by theoretical and experimental approaches. As for the theoretical approach, the neutrophil velocity by the concentration Marangoni effect is calculated by using a special mathematical model. From this result, it was found that the velocity is high when the period is short. As for the experimental approach, to obtain the velocity and the concentration gradient on the membrane, neutrophil velocity and distribution of cytokine concentrations are observed with microscopy. In the experiment, the period of rotation was obtained from the time history of the concentration gradient by FFT (Fast Fourie Transform). From the results, the following three findings were obtained. At first, neutrophil velocity is not constant and vibrates. Second, when the rotation period is short, the velocity is fast. Third, when the distance between the neutrophil and the wall is close, the rotation period is longer and the velocity is lower. From these findings, it was concluded that neutrophil rotation is an important role in neutrophil’s propulsion mechanism.

Thermocapillary convection in a laser-heated Ni melt pool: A molecular dynamics study

Thermocapillary convection was investigated in a metallic system of pure Ni, at the nanoscale, by molecular dynamics. The system interface was irradiated by a heat flux, mimicking a focused laser source. The melt pool was submitted to a large temperature gradient that modified the surface tension along the interface. In liquid metal, because surface tension typically decreases with increasing temperature, the result is a gradient of surface tension along the free surface. The liquid metal, therefore, started to flow in the direction of high surface tension. Two counter-rotating convection cells developed, characteristic of those observed in welding and other material processing. A systematic estimation of relevant parameters in hydrodynamics allowed us to interpret the results in terms of Prandtl, Marangoni, and Péclet numbers. This study demonstrates the influence of laser power and system size on pool shape and flow characteristics.

Numerical study of ultrashort laser-induced microjet formation on the metal film based on the Navier–Stokes equation

A two-temperature model (TTM) for the electron-phonon thermal equilibrium is used to determine the heat distribution and laser fluence threshold for melting a thin metal film coated on a glass substrate and irradiated by an ultrashort laser pulse. This study proposes a novel model based on the Navier–Stokes equation to explain the formation of jet-shaped structures in the film's molten region. By solving this equation and obtaining the temporal evolution of the velocity distribution and displacement in the molten region, the Marangoni convection effect can be numerically demonstrated, and the circular motion of the fluid can describe the formation of a jet-shaped structure in the central region of the radiation. The results are compared to those obtained by numerically solving the thermo-elastoplastic equations, and also, to the previously reported experimental results to ensure the accuracy of the microjet height calculated by the Navier–Stokes equation. Good agreement is observed, particularly when the temperature of the irradiated area is significantly over the film's melting temperature. In addition, several calculations are performed for various pulse fluences. In both models, increasing the pulse fluences leads to an increase in the height of microjets.

Influence of External Magnetic Field on 3D Thermocapillary Convective Flow in Various Thin Annular Pools Filled with Silicon Melt

Thermocapillary convection flows can have an impact on the homogeneity of floating zone semiconductor crystals. An external magnetic field can also help to reduce this non-homogeneity. The goal of this research is to minimize thermocapillary convection in various thin annular pools filled with silicon melt. A three-dimensional (3D) numerical technique is proposed that employs an implicit finite volume formulation. The steady-state thermocapillary flow in six thin annular pools (R=0.3, 0.4, 0.5, 0.6, 0.7, and 0.8) subjected to an externally induced magnetic field was observed. Under magnetic field influence, the effects of increasing annular gap, R on the hydrothermal wave number and azimuthal pattern are obtained. The results reveal that hydrothermal waves m=14, m=11, m=8, m=6, m=4, and m=3 are observed in steady flow for R=0.3; 0.4; 0.5; 0.6; 0.7, and R=0.8, respectively. The maximum temperature occurs in the intermediate zone between the inner and outer walls when there is no magnetic field. Under a strong enough magnetic field, isothermal lines change form and become concentric circles. As the amplitude of the magnetic field (Ha) grows, the azimuthal velocity and temperature at the free surface reduce, and the asymmetric 3D flow becomes axisymmetric steady when Ha surpasses a threshold value.

Analytical simulation of magneto-marangoni convective flow of Walter-B fluid with activation energy and Soret–Dufour effects

Marangoni convection is discovered by varying gradients of surface tension. Marangoni induced flow plays a vital role in melting of coating flow technology, drying wafers, crystals, soap film stabilization, wielding and microfluidics, in which the flow creates unwanted impacts under gravity on micro-level in the same manner as buoyancy-induced natural convection. The Magneto-Marangoni convective flow of Walter-B fluid over a vertical permeable surface is addressed in the current research. The Dufour–Soret effects are taken into account along with activation energy and radiation. Flow through a porous media is modeled via Darcy and Forchheimer theory. The surface tension gradient becomes stronger by increasing the Marangoni convection parameter, which results in stronger induced flows and more efficient heat and mass movement inside the liquid. The result is a more uniform distribution of these qualities throughout the liquid as the temperature and concentration profiles drop. With higher viscoelastic parameter levels, the fluid accelerates and the velocity profile increases due to decreased viscosity. Due to an augmentation in the Dufour and Soret number, the thermal and concentration of the Walter-B fluid boost up respectively.

A multiphase model for exploring electrochemical Marangoni flow

A multiphase numerical model based on the volume of fluid (VOF) method is proposed to simulate the transient, electrochemically-generated Marangoni flow in a system comprising a NaOH electrolyte and a eutectic gallium–indium (EGaIn) metal droplet. The model incorporates appropriate equations to accurately represent the transport phenomena, including flow, electric potential, and electric current density, within the entire system. The model includes the transient variation in the interfacial tension as a function of electric current density at the interface, leading to the generation of Marangoni flow and enabling the tracking of droplet shape evolution. Notably, the model successfully captures the elongation of the droplet towards the cathode, which is validated through comparison with available experimental data.

Simulation analysis of cathodic ablation under the arcing action of vacuum rotary contacts

During the breaking process of the switchgear, a rotational movement of 1-10 degrees per millisecond is added to the contacts. Allowing for more even distribution of arc energy affects the cathodic ablation process. The cathode ablation process under the action of a vacuum rotary contact arc and the effect of rotary motion on the cathode surface are studied during arc ignition in a vacuum metal vapour arc in the paper. A three-dimensional (3D) cathodic ablation transient model with contact rotation characteristics is developed by considering the effects of arc pressure and the Marangoni effect, combined with the hydrodynamic equations and heat transfer equations. Based on a 3D transient model, the action of the arc on the cathode surface is compared and analyzed for different modes of contact movement. The results show that there is a lower cathode surface temperature, a smaller melt zone and a smaller ablation volume by the contact rotation method.

Two-color thermal imaging of the melt pool in powder-blown laser-directed energy deposition

This work demonstrates an experimental technique to image melt pool temperature fields with a commercial color camera for laser-directed energy deposition (L-DED) processes. The technique relies on two-color thermography to construct spatially-resolved temperature fields and is demonstrated on 316L stainless steel (SS) melt pools from solidus to near-boiling temperatures. This two-color thermal imaging system negates the need for a priori knowledge of melt pool emissivity or the camera’s view factor and was validated with a calibrated tungsten filament lamp between temperatures of 1220 K and 2850 K. In-situ temperature measurements are combined with ex-situ cross-sectional geometry to determine the material’s effective absorptivity and coefficient of temperature-dependent surface tension, which is responsible for Marangoni convection, in a multi-physics computational fluid dynamics (CFD) model. Measured peak melt pool temperatures are important for understanding the molten convection within the melt pool, and measured cooling rates can be related to the resulting part microstructure. Peak temperatures from 1750 K to 3000 K show that below the boiling point, temperature increases with increasing laser power density and decreases with increasing scanning velocity. Thermal images are used to estimate cooling rates in the melt pool tail, which increase with the ratio of scanning velocity to power. Our thermal imaging strategy will advance measurement science in L-DED processes by validating multi-physics CFD models, quantifying cooling rates, providing real-time feedback control, and allowing process mapping of critical melt pool behaviors.

Laser Injection Technique for Liquid Crystal Droplets with Arbitrary Shape

We propose a simple and precise method for processing three-dimensional droplet structures of liquid crystal with arbitrary shapes. By irradiating liquid crystal emulsions with a laser beam, a photothermally induced Marangoni effect is introduced. This leads to the injection of water solution into the liquid crystal droplets. With the help of the elasticity of liquid crystal, the injected water solution spontaneously forms into monodispersed guest droplets which latterly self-assemble into branched droplet chains. This method enjoys great flexibility due to its non-contact and in-situ processing manner, allows the simultaneous and multiple processing of all liquid crystal droplets under the irradiation region, and offers excellent compatibility with technologies such as microfluidics. The proposed method is applicable in material science, photonics, and sensors.

The Role of Te, As, Bi, and Sb in the Noble Metals (Pt, Pd, Au, Ag) and Microphases during Crystallization of a Cu-Fe-S Melt

Quasi-equilibrium directional crystallization was performed on a melt composition (at. %): 18.50 Cu, 32.50 Fe, 48.73 S, 0.03 Pt, Pd, Ag, Au, Te, As, Bi, Sb, and Sn, which closely resembles the Cu-rich massive ores found in the platinum-copper-nickel deposits of Norilsk. Base metal sulfides (BMS) such as pyrrhotite solid solution (Fe,Cu)S1±δ (Poss), non-stoichiometric cubanite Cu1.1Fe1.9S3 (Cbn*), and intermediate solid solution Cu1.0Fe1.2S2.0 (Iss) are progressively precipitated from the melt during the crystallization process. The content of noble metals and semimetals in the structure of BMS is below the detection limit of SEM-EDS analysis. Only tin exhibits significant solubility in Cbn* and Iss, meanwhile Pt, Pd, Au, Ag, As, Bi, Sb, and Te are present as discrete composite inclusions, comprising up to 11 individual phases, within their matrices. These microphases correspond to native Au, native Bi, hessite Ag2Te, sperrylite Pt(As,S)2, hedleyite Bi2Te, michenerite PdTeBi, froodite PdBi2, a solid solution of sudburite-sobolevskite-kotulskite Pd(Sb, Bi)xTe1−x, geversite PtSb2, and a multicomponent solid solution based on geversite Me(TABS)2, where Me = Σ(Pt, Pd, Fe, Cu) and TABS = Σ(Te, As, Bi, Sb, Sn). Most of the inclusions occur as thin layers between BMS grain boundaries or appear drop-shaped and subhedral to isometric grains within the sulfide matrix. Only a small fraction of the trace elements form mineral inclusions of sizes ≤ 0.5 μm in Poss, most likely including PtAs2 and (Pt,Pd)S. It is likely that the simultaneous presence of noble metals (Pt, Pd, Au, Ag) and semimetals (As, Te, Bi, Sb) in the sulfide melt leads to the appearance of liquid droplets in the parent sulfide melt after pyrrhotite crystallization. The solidification of droplets during the early stages of Cbn* crystallization may occur simultaneously with the cooling of later fractions of the sulfide melt, resulting in the formation of Iss. In addition, abundant gas voids containing micro-inclusions were observed in Cbn* and Iss. These inclusions showed similar chemical and mineral compositions to those in BMS matrices, i.e., the presence of gas bubbles did not affect the main features of noble metal fractionation and evolution. Therefore, it is reasonable to assume that ore particles suspended in the melt are either trapped by defects at the crystallization front or transported towards gas bubbles via the Marangoni effect.

Colloidal Manipulation through Plasmonic and Non‐plasmonic Laser‐Assisted Heating

AbstractIn spite of the long‐term awareness of the conversion of light to heat even in materials with low absorption coefficient via the photothermal effect and consequent usage of the effect to evaluate thermo‐optic properties of the materials, only recently has the thermal field created via photon‐to‐phonon conversion been exploited for manipulation of colloidal objects as well as living cells. As compared to conventional direct photon‐assisted manipulation via optical tweezers, the optothermal manipulation technique employs much lower optical source power and can manipulate particles over a long range. In this review, the working mechanisms, concepts, and applications of a series of recently established optothermal techniques are discussed for the manipulation of diverse species including micro/nanoparticles, biological cells, molecules, and micelles in various fluidic environments. The physical mechanism of the optical manipulation that relies on the coordinated action of thermal convection, Marangoni convection, thermophoresis, thermoelectricity, depletion attraction, and thermo‐osmotic flow is discussed in detail. With their low‐power operation, diverse functionalities, and simple optics employed, optothermal manipulation techniques are increasingly finding a wide range of applications in colloidal science, life sciences, materials science, and nanoscience, as well as in the developments of colloidal functional devices and nanomedicine.

Conditions of enhanced evaporation for nanofluids droplet and inhibition of coffee-ring effect under buoyancy and Marangoni convection

Evaporation of nanofluid droplets whose size is larger than capillary length is common in industrial processes, but there is currently a lack of research on the evaporation kinetics and more complex deposition patterns under the combined effects of buoyancy and Marangoni convection of large evaporating droplet. Experimental investigations on evaporation of Al2O3H2O nanofluid sessile droplet with initial contact line diameter of 4 mm on heating PTFE coating substrate are reported. The variation in contact angle, contact radius, and droplet volume over time under different conditions were obtained through experiments. The average evaporation rate is calculated. Internal flow characteristics are inferred through the surface temperature distribution detected by the top view infrared camera. The constant contact radius mode takes place at the beginning of evaporation. Then mixed evaporation mode and stick slip mode may occur for different substrate temperature, nanoparticle mass concentration and nanoparticle size. Increasing nanoparticle size causes a decrease in average evaporation rate. Whether the use of nanofluids can improve evaporation rate and heat transfer or not depends on multiple factors. Convection cells exist, and three kinds of deposition patterns are formed due to the underlying coupled transport phenomena. Under the combined effect of buoyancy and Marangoni convection in this article, the most common coffee-ring pattern does not appear. The deposition patterns are directly affected by the pinning stability, which decrease at higher temperature.

Direct joining of quartz glass and copper by nanosecond laser

In present study, quartz glass and copper, which are with a large difference of coefficient of thermal expansion by 38 times, were successfully welded without any interlayer by nanosecond (ns) laser under non-optical contact for the first time. By reasonably tuning the welding parameters, well-formed quartz glass/copper joints without any defects such as microcracks or voids can be achieved. The welded seam includes laser-unirradiated smooth region and laser-irradiated spindle melted region and both the region are composed of “Cu2O″-SiO2 system (Cu–O–Si system) and CuO. The formation of such a particular microstructure feature is attributed to the recoil pressure from ns laser ablation and the Marangoni convection at the interface. The parameters of ns laser welding including laser power, scanning velocity and the number of scans influence the laser energy input and thermal accumulation greatly during welding, thus affecting the interfacial feature and the mechanical property of the resultant joints. The shear strength of the joint can reach 33.4 MPa, which is about 80% of quartz glass shear strength. The metallurgical bonding from interfacial reaction and mechanical interlocking from physical spindle structures are the main reasons for achieving a high shear strength of the joint. We expect this study could provide a new idea for joining materials with large differences in thermophysical properties.

Experimental study on workability and permeability of sandy soils conditioned with thickened foam

Water spewing and muck plugging often occur during earth pressure balance (EPB) shield machines tunnelling in water-rich sandy strata, even though the conventional foam has been employed to condition sandy soils. In this study, a novel thickened foaming agent suitable for EPB shield tunnelling in water-rich sandy strata is developed. In contrast to conventional foam-conditioned sands, the thickened foam-conditioned sand has a low permeability due to the consistent filling of soil pores with the thickened foam, and the initial permeability coefficient decreases by approximately two orders of magnitude. It also exhibits a suitable workability, which is attributed to the enhanced capability of the thickened foam to condition sandy soils. In addition, the effect of concentration on the stability of the foam is explained by the Gibbs-Marangoni effect, and conditioning mechanisms for the thickened foam on sands are discussed from the evolution of foam bubbles.

Thin liquid films on a slippery vertical cylinder in presence of chemical reaction

In industrial settings, chemical reactions often affect thin liquid films that flow down vertical cylinders. However, the effect of non-isothermal reactions on the dynamics of these films is not fully comprehended. To address this issue, this study investigates the behavior and stability of thin film flows on uniformly heated vertical cylinders that exhibit wall slippage when subjected to a pseudo-zero-order exothermic or endothermic chemical reaction. A reduced model is developed for thin liquid films flowing down vertical cylinders in the presence of exothermic or endothermic chemical reactions, assuming the film thickness is much smaller than the cylinder radius. The heat from the reaction and/or wall heating initiates a thermocapillary Marangoni effect, which is further enhanced by wall slippage, directly affecting the dynamics of the free surface. Linear and weakly nonlinear stability analyses show that wall slip amplifies the instability, while exothermic reactions stabilize the system and endothermic reactions destabilize it. It is also found that chemical reactions significantly influence the supercritical stable, subcritical unstable, unconditional stable, and explosive zones of the thin film. A direct numerical simulation confirms the results of linear and weakly nonlinear analyses.

Oscillation and evolution characteristics of nanofluid thermocapillary convection in rectangular cavity

To analyze the oscillation and evolution characteristics of nanofluid thermocapillary convection instability, a two-phase mixture model was utilized to simulate nanofluid thermocapillary convection in a rectangular cavity and investigate the impact of nanoparticle volume fraction on the oscillatory flow. The results indicate that the two-phase mixture model has a lower critical temperature difference compared to the single-phase model, and the oscillation mode exhibits distinct changes for small nanoparticle volume fractions. At a high Marangoni number, the thermocapillary convection displays periodic oscillatory behavior, with the flow field composed of several dynamic convection vortices. An increase in nanoparticle volume fraction results in a decrease in the critical temperature difference, as well as the amplitude and variation range of temperature oscillation. However, the oscillation period shows an approximate linear increase. Furthermore, there is a non-uniform distribution of nanoparticles at the vicinity of solid wall, especially a significant concentration gradient near the side wall.

Dynamic mode decomposition of thermocapillary convection in GaAs melt liquid bridge between unequal ends

The effects of the radius ratio on the stability of thermocapillary convection in the GaAs melt (Pr=0.068) liquid bridge, situated between unequal ends, are studied by three-dimensional direct numerical simulation. The simulated results reveal that, for the first time, the GaAs melt flow transforms from a stable axisymmetric flow into a three-dimensional steady flow through the first bifurcation at 0.5≤Γr≤0.9. The bifurcation phenomenon was previously common in the range of small Prandtl numbers(0.001≤Pr≤0.057). Additionally, we observe that the stability of the thermocapillary liquid bridge is initially weakened and then slowly improved as the radius ratio Γr decreases. The second bifurcation occurs with the further increase of Ma, and the flow becomes oscillatory. Remarkably, two different oscillation forms are identified, depending on the difference in the degree of symmetry of the flow field structure during its oscillation process. Once the flow reaches a stable periodic oscillation, the Dynamic Mode Decomposition (DMD) technique is used to study the inherent spatio-temporal evolution of the temperature field. By combining the spectrum obtained from the Fast Fourier Transform (FFT), we provide evidence for the existence of mixed forms with different azimuthal wavenumbers from a dynamic perspective, intuitively displaying the different components of the oscillation and examining the contribution of different modes to the oscillation. The dynamic model analysis demonstrates that oscillation form I is a mixed form with m = 1 and m = 2, while oscillation form II belongs to the form where m = 2 is absolutely dominant.

Volume of fluid based modeling of thermocapillary flow applied to a free surface lattice Boltzmann method

Thermocapillary flow, also termed as thermal Marangoni convection, is an important material transport mechanism in many technical processes. Thus, it has to be considered in the corresponding simulations. In this work, a novel numerical model for the implementation of thermocapillary flow into a volume of fluid based free surface lattice Boltzmann method is presented. It is based on the idea of discretizing the surface normal vector in accordance to the discretization scheme of the velocity space. This approach is used to interpolate the temperature at the surface and to calculate the local surface gradients.The result is a simple and efficient numerical model, which is able to calculate the Marangoni stress along a liquid surface with sufficient accuracy and without the need to explicitly reconstruct the interface. We show that the Marangoni stress can readily be incorporated into a free surface lattice Boltzmann model by adjusting the boundary condition at the surface. In doing so, no additional forcing scheme has to be applied.In order to verify the presented model, it is tested using several simulation setups. As the lattice Boltzmann method is particularly suitable for complex geometries, a special attention is paid to the behavior at curved surfaces. Additionally, a simple test case for thermocapillary flow that can be regarded as a modified version of the Couette flow problem, is presented.

Motion of magnetic motors across liquid–liquid interface

HypothesisIn a number of applications related to chemical engineering and drug delivery, magnetic nanoparticles should move through a liquid–liquid interface in the presence of surfactant molecules. However, due to the action of capillary forces, this is not always possible. The mechanism of particle motion through the interface essentially depends on the intensity of the Marangoni flow, which is induced on the interface during its deformation. ExperimentsIn this paper we study the motion of nanoparticles Fe3O4 through the water-tridecane interface under the action of a nonuniform magnetic field when using different surfactants. FindingsIf the linear size of the magnetic motor turns out to be less than a certain critical value, then it is not able to move between phases due to the action of capillary forces on the interface. Depending on the type and concentration of the surfactant used, various mechanisms for the motor motion through the liquid–liquid interface can be carried out. In one of them, a liquid phase is transferred through the interface along with a movable motor, while in the other, it is not.

Evaporation-induced hydrodynamics control plasmid transfer during surface-associated microbial growth

Droplet evaporation is a general process in unsaturated environments that results in micro-scale hydrodynamic flows which in turn determine the spatial distributions of microbial cells across surfaces. These spatial distributions can have significant effects on the development and functioning of surface-associated microbial communities, with consequences for important processes such as the spread of plasmids. Here, we experimentally quantified how evaporation-induced hydrodynamic processes modulate the initial deposition patterns of microbial cells (via the coffee ring effect and Marangoni convection) and how these patterns control the spread of an antibiotic resistance-encoding plasmid during surface-associated growth. We found that plasmid spread is a function of the initial density of cells deposited along the droplet periphery, which is a manifestation of the coffee ring effect. Using an individual-based model, we systematically linked how the different initial cell deposition patterns caused by the relative strengths of the coffee ring effect and Marangoni convection determine the extent of plasmid transfer during surface-associated growth. Our study demonstrates that evaporation-induced hydrodynamic processes that are common in nature can alter crucial ecological properties of surface-associated microbial communities and control the proliferation of plasmids, with consequences on the spread of antibiotic resistance and other plasmid-encoded traits.