Thermocapillary Flow Research Articles

Blockage of thermocapillary flows by surface-active impurities

Blockage of thermocapillary flows by surface-active impurities

On the influence of heat-induced evaporation over interfacial atomization driven by surface acoustic waves

In this work, the influence of evaporative flux over the atomization driven by surface acoustic waves (SAWs) of a Newtonian water drop is studied. The drop is placed on a heated substrate at constant temperature, higher than the saturation temperature at a given vapor pressure. In this manner, an interfacial temperature distribution arises along the drop free surface in terms evaporative mass flux and vapor recoil, which repercussion over aerosol size is studied by determining the asymptotic evolution equation governing the acoustically driven free surface. At such scenario, the connection between surface tension and temperature is also considered; thus, thermocapillary flow is incorporated into our drop model, described in terms of fundamental parameters, like the evaporation number, Marangoni number, and acoustic capillary number. Numerical solution of the evolution equation led us to obtain a simplified representation of the drop interfacial deformation mechanism, capable of predicting atomization and portraying the influence of evaporation over atomization. Subsequent analysis shows that the incorporation of evaporation at SAW atomization traduces in normal stresses counteracting the acoustic and thermocapillary effect, leading to the development of smaller drop aspect ratios with respect to the no-evaporative case. Being aware that the aerosol size is deeply related to the aspect ratio, we propose an analytical expression to estimate aerosol diameter under evaporative conditions. The results show that aspect ratio reduction leads to a decrement on aerosol size, up to two orders of magnitude, with respect to the no-evaporative case. Our study is a first approach providing insight about the importance of evaporation on aerosol regulation at SAW atomization.

Impact of heat flow from the cylinder sidewalls on thermocapillary droplet flow in a vibrating fluid: 3D study

Impact of heat flow from the cylinder sidewalls on thermocapillary droplet flow in a vibrating fluid: 3D study

Controlling Spatial Morphology of Microparticle Deposits via Thermocapillary Flows: Effect of Boundary Geometry.

The production of particle deposits with a desired distribution geometry has significant potential for materials science, printing, and coating technologies. Most methods for achieving well-defined assemblies rely on the spontaneous evaporation of colloidal solutions on substrates with predetermined properties, or on precise control of particle arrangement by external stimuli. Here, we present a combined method that enables the production of centimeter-scale microparticle deposits with a desired geometric shape. The method is based on controlling the massive transport of microparticles by thermocapillary flow in a layer of volatile liquid in a cell with borders of the desired geometry. Capillary forces cause the liquid to be distributed in the cell, forming corner wetting menisci and the flat layer in the central area. The formation of particle deposits occurs in two stages, determined by the flow regime. At the initial stage, the axisymmetric thermocapillary flow occurs in the flat part of the layer, resulting in the circular shape of the particle deposit. During the transition to the second stage of assembling thermocapillary flow is localized in the corner wetting menisci that results in reshaping the current particle deposit to match the geometry of the cell borders. Here, we demonstrated the creation of circular, square, and triangular shapes of the patterns of polystyrene microparticles using a point heater located at the geometric center of the cell. The proposed method is reliable, easy to implement, and potentially capable of producing a wide variety of deposit geometries, making it an attractive technique for patterning and modifying surface properties with particles of any type.

Numerical study on thermocapillary flows of power-law fluids with the lattice Boltzmann method

Numerical study on thermocapillary flows of power-law fluids with the lattice Boltzmann method

Thermocapillary convection in a cuboid pool with a sidewall of different temperature sections

Thermocapillary convection in a liquid cuboid pool with a sidewall of different temperature sections is investigated using 3D numerical simulation. One half section of the sidewall is heated but the other cooled. Numerical simulation is performed for Marangoni number (Ma) from 100 to 4.0 × 106 with Prandtl number (Pr) of 28.6 and aspect ratios (Ah and Aw) of 0.43. A temperature field with a weak thermocapillary convection flow is dominated by conduction for small Ma. As Ma increases, the flow becomes stronger and reveals a complex transition route to a chaotic state with a series of bifurcations. It has been demonstrated that the symmetry of the temperature field breaks between Ma = 102 and 103. A Hopf bifurcation occurs between Ma = 8.0 × 105 and 9.0 × 105 in which the flow becomes periodic. As Ma increases further, a sequence of period-doubling bifurcations occurs between Ma = 2.78 × 106 and 2.95 × 106. Once Ma exceeds 3.0 × 106, the flow becomes chaotic. Typical thermocapillary convection flows in the pool are characterized with topological analyses, spectral analyses, attractors, Lyapunov exponents, and fractal dimensions. Furthermore, the heat and mass transfer in the cuboid pool have been investigated.

SIMULACIONES NUMÉRICAS 3D DEL FLUJO DE UNA GOTA BAJO LOS EFECTOS DEUN GRADIENTE RADIAL DE TEMPERATURA

It has been shown experimentally that when a drop is deposited at the center of a substrate with an axial temperature gradient (hotter in the center), thermocapillarity effects makes an outward flow to appear so that the drop evolves towards a ring whose radius increases with time. Upon reaching a critical radius, the contact line becomes unstable, showing gentle undulations whose amplitudes grow with time. Using the lubrication approximation and adopting appropriate dimensionless variables, a parameter-free differential equation is obtained that governs this type of thermocapillary flow. Numerical solutions of this equation are presented to study the unstable stage. Experimental results are compared with those obtained from the numerical solutions.

Effect of Rotating Magnetic Field on the Thermocapillary Flow Instability in a Liquid Bridge

Effect of Rotating Magnetic Field on the Thermocapillary Flow Instability in a Liquid Bridge

Controlling a Free Surface With Thermocapillary Flows and Vibrations in Microgravity

Fluid manipulation and control is crucial for space exploration. Motivated by the “Thermocapillary-based control of a free surface in microgravity" (ThermoSlosh) experiment (Salgado Sánchez et al. in Acta Astronautica 205:57–67, 2023), we conduct here a detailed numerical analysis of interfacial dynamics in a two-dimensional cylindrical cell, half-filled with different silicone oils or a fluorinert, and subjected to thermal forcing and vibrations. The effect on the free surface dynamics of the applied temperature difference, vibrational amplitude, fluid viscosity, and contact angle is analyzed; both static and dynamic contact angle models are considered. Results strongly suggest that thermocapillary flows can be used to control the interface orientation within the cell, while supplemental vibrations can be added to increase the system responsiveness. This control can be further improved by using classical proportional-integral-derivative feedback to adjust the cell boundary temperatures in real-time. The proportional and derivative gains of the controller can be selected to optimize the stabilization time and/or energy cost, while the integral contribution is effective in reducing the steady-state error. Overall, the present analysis highlights the potential of using the thermocapillary effect for fluid management in reduced gravity, and evaluates different types of experimental tests that can be executed in the frame of the ThermoSlosh microgravity project.

Laser-induced thermocapillary flows on a flowing soap film

Laser-induced thermocapillary flows on a flowing soap film

Numerical Simulations on Thermocapillary Flow on Heated Sinusoidal Topography

Numerical Simulations on Thermocapillary Flow on Heated Sinusoidal Topography

Thermo-capillarity of immiscible binary fluids within serpentine micro-confinements

The thermocapillary flow of multiple liquid layers over contoured substrates finds applications in many industrial phenomena via micro-electromechanical systems/nano-electromechanical systems devices. This study theoretically explores the thermocapillary transport of binary-liquid layers in a wavy microchannel with serpentine wall profiles. Sinusoidal profiles are considered for the microchannel walls' temperature and surface topology. Patterned slip conditions at the walls are also accounted for in the analysis to make the problem more realistic. We semi-analytically solve the energy and momentum equations to understand the system's thermal and hydrodynamic characteristics for the limiting conditions of Ma, Re, and Ca. The leading- and higher-order solutions of the temperature and flow field are separately shown to demonstrate the advection terms' contribution to the transport equations. The circulatory flow obtained due to periodic thermal stimuli can be used to enhance the mixing efficiency of the system. We also present how the parameters like phase difference between the thermal stimulus, relative thermal conductivity, and relative film thickness ratio of the liquid layers can be altered to control the flow characteristics to obtain the most potent thermocapillary effect.

Pose Control of Millimeter-Scale Objects in a Laser-Powered Thermocapillary Manipulation Platform

The miniaturization of technological components requires tools capable of manipulating them. Noncontact micromanipulation has recently gained popularity since it minimizes component damage and adhesion problems. However, the simultaneous position and orientation (pose) control using noncontact tools remains an open challenge. In this work, we propose the pose control of millimeter-scale structures at the air–water interface using laser-powered thermocapillary flows. We achieve autonomous pose control by steering two laser spots around the structure in a closed-loop manner. The controller consists of a linear pose controller and a model inversion. The pose controller defines the required force and torque to reach the target. The model inversion defines the required laser positions to obtain said force and torque. This inversion is performed with an optimization algorithm, without any prescribed strategy. The pose stabilization was validated experimentally, obtaining a position and orientation precision of 0.15 body lengths and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2^{\circ }$</tex-math></inline-formula> , respectively. Additionally, we show the experimental path-following with independent orientation control, obtaining a similar position precision.

The “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” experiment: Design and performance of the cuboidal cell

The experiment “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” aims to investigate the efficacy of thermal Marangoni convection in augmenting the heat transfer rate of passive phase change materials (PCMs) that incorporate a free surface in microgravity; the so-called thermocapillary-enhanced PCMs. Compared to thermal conduction, thermocapillary flows can increase heat transport by a factor of two or more. In addition to advancing scientific understanding, the experiment seeks to evaluate the practical feasibility of using thermocapillary-enhanced PCMs as passive thermal control devices for space missions and assess possible implementation challenges. By analyzing different PCM samples, the experiment will provide crucial insights into the heat and mass transport mechanisms of thermocapillary flows during melting and solidification. The present article primarily describes the design, thermal control and ground results of the cuboidal cell of the experiment. We illustrate here the comprehensive approach towards developing space experiments adhering to rigorous and demanding scientific and technical requirements. Along with the validation of an initial experiment prototype, the evaluation of ground tests is also addressed. The hardware developed demonstrates an adequate performance, while the obtained scientific results are coherent and compliant with the established requirements; the feasibility of the proposed design is thus demonstrated.

Linear stability analysis on the thermocapillary flow of molten silicon in a liquid bridge between unequal disks under a rotating magnetic field

This paper explores the stability of thermocapillary flow in a molten silicon liquid bridge situated between two unequal disks and subjected to a rotating magnetic field. Two different heating strategies are examined, and the static deformation of the free surface is taken into account by solving the Young–Laplace equation. Using linear stability analysis based on the Legendre spectral element method, the critical conditions are determined, and the underlying instability mechanism is uncovered through energy analysis. When heated from the bottom disk, three transitions between axisymmetric steady flow and three-dimensional oscillatory flow are observed within a narrow range of 7000 ≤ Ta ≤ 9000. In contrast, when heated from the top disk, the rotating magnetic field significantly enhances the flow stability. Furthermore, based on the energy analysis, we identify two types of instability caused by purely hydrodynamic mechanisms.

EHD flow and heat transfer of hybrid nanofluid in a free surface cavity fitted with an internal hot obstacle

The objective of this study is to numerically investigate the effects of electrohydrodynamics (EHD) on coupled free convective thermocapillary flow. The study focuses on a free-surface cavity filled with a hybrid nanofluid (MWCTN- Fe3O4-thermal oil) and containing a hot rigid obstacle. An electric field is applied between the obstacle and the lower cavity wall to induce additional instabilities in this region. This technique aims to eliminate the dead zone typically present in traditional natural convection and is expected to greatly improve heat transfer efficiency. To achieve this objective, the equations governing the EHD phenomenon, as well as the equations defining the physicochemical properties of the hybrid nanofluid, are solved using the finite volume method (FVM) coupled with a blocked-off region procedure. A parametric study is conducted, involving the Marangoni number (surface tension), the thermal Rayleigh number (buoyancy force), the electrical Rayleigh number (electrical force), the nanoparticle concentration, the size and the position of the obstacle. It has been demonstrated that applying of an electric field to the convective-thermocapillary flow of the nanofluid (combining two enhancement techniques) significantly enhances heat transfer, resulting in a more than 230 % increase in the Nusselt number.

Wavy Marangoni effect and strategic slip-pattern-driven species transport in micro-confined sandwiched liquid films

In microfluidic systems, analyte/reagent mixing is essential to achieve rapid chemical/biochemical reactions. The low Reynolds number (Re) flow, however, makes passive mixing difficult and necessitates the use of some external stimulus to cause disturbance in the system. Here, we report how a periodic thermocapillary effect may interact with strategically patterned wall wettability and boost the mixing dynamics in a micro-confined ternary-liquid film system. Approximate, yet without compromise on the physics involved, analytical solutions to the energy and Navier–Stokes equations under creeping flow conditions are obtained to comprehend the thermal and hydrodynamic characteristics of the thermo-capillarity. In the binary fluid limit, our model is validated with the findings of Pendse &amp; Esmaeeli (Intl J. Therm. Sci., vol. 49, 2010, pp. 1147–1155), and good agreement is found. The flow characteristics in the ternary-liquid system due to discrete wall temperatures is also demonstrated via finite-element-based numerical simulations, and agreement with the semi-analytical solution is noted. The qualitative study of the flow pattern reveals that enhanced mixing is obtained as a result of vortical motion created by the interplay of the periodic thermo-capillarity-driven interfacial flow and wall slip. The observation is further consolidated from the numerical simulation of the species transport equation. The species distribution so obtained is compared with fully developed laminar flow for the same inlet concentration. Our study further investigates the effects of the fluids’ relative thermal conductivity, wall slip length, relative film thickness and thermal (or slip) phase differences on the mixing efficiency of the proposed arrangement. While wall slip, relative film thickness and relative thermal conductivity regulate mixing in the top and bottom layers, phase difference determines how well the middle fluid layer mixes. Slip at the walls creates vortex distortions to get a strong churning effect. The lower thermal conductivity of the top and bottom fluids weakens the thermocapillary flow; however, the dominance of the patterned slip improves mixing in such cases.

Selective Patterned Growth of ZnO Nanoneedle Arrays

Abstract Nanostructured coatings are widely used to improve the sensitivity of various types of sensors by increasing the active surface area compared to smooth films. However, for certain applications (in some cases), it may be necessary to achieve selectivity in the coating process to ensure that nanostructures only form in specific areas leaving interelectrode spaces free of nanostructures. This article discusses several methods for creating intricate ZnO nanostructured patterns, including area selective application of Zn acetate seeds followed by hydrothermal growth, selective thermal decomposition of zinc acetate via laser irradiation followed by hydrothermal growth, and the electrochemical deposition method. These methods enable ZnO nanostructures to grow onto designated surface areas with customised, patterned shapes, and they are rapid, cost-effective, and environmentally benign. The article examines the process of producing a nanostructured coating with a complex shape and discusses several factors that can impact the quality of the final product. These include the influence of the thermocapillary flows and the “coffee stain” effect on the deposition of a seed layer of zinc oxide from an ethanol solution of zinc acetate. Additionally, the study found that using a protective screen during the growth of nanostructures can reduce the occurrence of unintended parasitic structures in areas lacking a seed layer. Overall, the article presents various techniques and strategies to improve the quality of nanostructured coatings. We have proven that the use of laser radiation to create a seed layer does not impact the final morphology of the resulting nanostructures. However, when combined with computer-controlled technology, this approach allows for the creation of intricate patterns made up of micrometre-sized lines which cannot be achieved by using other methods. The article also demonstrates an electrochemical technique for obtaining zinc oxide nano-structures that can selectively coat metal electrodes without requiring a seed layer.

Linear stability of thermocapillary flow in a droplet attached to a hot or cold substrate

Linear stability of thermocapillary flow in a droplet attached to a hot or cold substrate

Phase-field-based lattice Boltzmann model for simulating thermocapillary flows.

This paper proposes a simple and accurate lattice Boltzmann model for simulating thermocapillary flows, which can deal with the contrast between thermodynamic parameters. In this model, two lattice Boltzmann equationsare utilized to solve the conservative Allen-Cahn equationand the incompressible Navier-Stokes equations, while another lattice Boltzmann equationis used for solving the temperature field, where the collision term is delicately designed such that the influence of the contrast between thermodynamic parameters is incorporated. In contrast to the previous lattice Boltzmann models for thermocapillary flows, the most distinct feature of the current model is that the forcing term used in the present thermal lattice Boltzmann equationis not needed to calculate space derivatives of the heat capacitance or the order parameter, making the scheme much more straightforward and able to retain the main merits of the lattice Boltzmann method. The developed model is first validated by considering the thermocapillary flows in a heated microchannel with two superimposed planar fluids. It is then used to simulate the thermocapillary migration of a two-dimensional deformable droplet, and its accuracy is consistent with the theoretical prediction when the Marangoni number approaches zero. Finally, we numerically study the motion of two recalcitrant bubbles in a two-dimensional channel where the relationship between surface tension and temperature is assumed to be a parabolic function. It is observed that due to the competition between the inertia and thermal effects, the bubbles can move against the liquid's bulk motion and towards areas with low surface tension.