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Journal ArticleDOI

Thermocapillary migration and interaction dynamics of droplets in a constricted domain

28 Feb 2019-Physics of Fluids (AIP Publishing LLCAIP Publishing)-Vol. 31, Iss: 2, pp 022106
TL;DR: In this article, an in-house solver with isosurface based interface reconstruction developed in OpenFOAM has been employed to carry out numerical simulations of the thermocapillary migration of a single droplet in a constricted domain with constriction comparable to the droplet size.
Abstract: Migration of confined droplets in a stationary fluid medium due to thermocapillary forces is considered. An in-house solver with isosurface based interface reconstruction developed in OpenFOAM has been employed to carry out numerical simulations. Thermocapillary migration of a single droplet in a constricted domain with constriction comparable to the droplet size shows that the migration velocity has non-monotonic dependence on the droplet radius. In the case of two droplets migrating in a constricted domain, the relative slowdown of a larger droplet when a smaller droplet is trailing behind reveals the possibility of interaction which is not observed in larger domains. The effects of the constricted domain size, the initial distance of separation, the radius of the trailing droplet, and the Marangoni number are analysed for this configuration. It is observed that the constriction size and Marangoni number have more influence on the interaction and dictate whether the droplets may coalesce or move with a constant separation distance. The final steady state separation distance between the droplets does not depend on the initial separation distance, but it varies with the radius of the trailing droplet. The results from the present study reveal the physical mechanisms influencing the thermocapillary migration of droplets in constricted domains and interactions between the migrating droplets.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors present the front-tracking-based simulation results of the thermocapillary effects on compound droplets in a certain limited domain, and the effects of these parameters on the compound droplet eccentricity are also considered.
Abstract: Compound and simple droplets have been studied and appeared in many life applications, e.g., drug processing and microfluidic systems. Many studies have been conducted on the thermocapillary effects on simple droplets, but similar studies on compound droplets are quite rare. Filling this missing gap, this paper presents the front-tracking-based simulation results of the thermocapillary effects on compound droplets in a certain limited domain. The compound droplet consists of a single inner core that is initially concentric with the outer one. Various dimensionless parameters including Reynolds number from 1 to 50, Marangoni number from 1 to 100, droplet radius ratio from 0.3 to 0.8, and viscosity ratios from 0.1 to 6.4 are varied to reveal their influences on the migration of a compound droplet from cold to hot regions. Initially, the inner droplet moves faster than the outer one, and when the leading surface of the inner droplet touches the outer one, the inner and outer droplets migrate at the same speed. The effects of these parameters on the compound droplet eccentricity are also considered.

8 citations

Journal ArticleDOI
TL;DR: In this article , a three-dimensional color-gradient lattice Boltzmann model is used to investigate the droplet migration behavior on a series of wettability-confined tracks subject to a uniform temperature gradient.
Abstract: Thermocapillary migration describes the phenomenon whereby liquid droplets move from warm to cold regions on a nonuniformly heated hydrophilic surface. Surface modifications can be applied to manipulate this migration process. In the present study, a three-dimensional color-gradient lattice Boltzmann model is used to investigate the droplet migration behavior on a series of wettability-confined tracks subject to a uniform temperature gradient. The model is validated by simulating the thermocapillary-driven flow with two superimposed planar fluids in a heated microchannel and the capillary penetration of a wetting fluid in a capillary tube. An in-depth study of the wettability-confined tracks confirms the capacity to manipulate the droplet migration process, that is, the wettability-confined tracks can accelerate thermocapillary migration compared with a smooth surface. The effects of changes in the viscosity ratio and interfacial tension are investigated, and it is found that a lower viscosity ratio and larger interfacial tension cause the droplet to migrate faster. Moreover, a systematic study of the track vertex angle is conducted, and the mechanism through which this parameter influences the droplet migration is analyzed. Then the effect of the track wettability on droplet migration is explored and analyzed. Finally, a serial wettability-confined track is designed to realize long-distance droplet migration, and the narrow side width of the connection region is found to play a key role in determining whether the droplets can migrate over long distances. The results provide some guidance for designing tracks that enable precise droplet migration control.

7 citations

Journal ArticleDOI
TL;DR: In this paper , a non-dimensionlized thermal radiation number is proposed to quantitatively depict the intensity ratio of the thermal radiation flux to the uniform temperature gradient, and the steady migration velocity decreases with the increasing of Reynolds and Marangoni numbers and increases with the increase of thermal radiation numbers.
Abstract: Thermocapillary migration of a droplet in a vertical temperature gradient controlled by uniform and non-uniform thermal radiations is theoretically analyzed and numerically investigated. A non-dimensionlized thermal radiation number is proposed to quantitatively depict the intensity ratio of the thermal radiation flux to the uniform temperature gradient. From the momentum and energy equations at zero limits of Reynolds and Marangoni numbers, analytical results for the uniform and non-uniform thermal radiations are determined. The steady migration velocity raises with the increasing of the thermal radiation number. By using the front-tracking method, it is observed that thermocapillary droplet migration under the uniform thermal radiation at moderate Marangoni and moderate thermal radiation numbers reaches a steady process. The steady migration velocity decreases with the increasing of Marangoni number and increases with the increasing of thermal radiation number. Moreover, the intensity of thermal energy transferred from the interface to both fluids depends on the volume heat capacity ratio. For the larger/smaller volume heat capacity ratio, more heat is transferred into the continuous phase fluid/the droplet. Furthermore, when the uniform thermal radiation is replaced by the non-uniform ones, the time evolutions, the structures of temperature fields, and parameter dependencies of thermocapillary droplet migration at moderate Marangoni and moderate thermal radiation numbers remain qualitatively unchanged. This study provides a profound understanding of thermocapillary droplet migration in a vertical temperature gradient controlled by thermal radiations, which is of great significance for practical applications in microgravity and microfluidic fields.

5 citations

References
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Journal ArticleDOI
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TL;DR: In this article, it has been demonstrated that small bubbles in pure liquids can be held stationary or driven downwards by means of a sufficiently strong negative temperature gradient in the vertical direction, due to the stresses resulting from the thermal variation of surface tension at the bubble surface.
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894 citations

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308 citations

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TL;DR: In this paper, the effects of gravitation on the behavior of fluids and the shapes of materials for the years 1977-1982 are reviewed, with a focus on ground-based research which complements and gives direction to experimentation in the reduced-gravity environment of space.
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216 citations

Journal ArticleDOI
TL;DR: In this article, the attachment probability of inclusions on a bubble surface is investigated based on fundamental fluid flow simulations, incorporating the turbulent inclusion trajectory and sliding time of each individual inclusion along the bubble surface as a function of particle and bubble size.
Abstract: Fundamentally based computational models are developed and applied to quantify the removal of inclusions by bubbles during the continuous casting of steel. First, the attachment probability of inclusions on a bubble surface is investigated based on fundamental fluid flow simulations, incorporating the turbulent inclusion trajectory and sliding time of each individual inclusion along the bubble surface as a function of particle and bubble size. Then, the turbulent fluid flow in a typical continuous casting mold, trajectories of bubbles, and their path length in the mold are calculated. The change in inclusion distribution due to removal by bubble transport in the mold is calculated based on the computed attachment probability of inclusions on each bubble and the computed path length of the bubbles. In addition to quantifying inclusion removal for many different cases, the results are important to evaluate the significance of different inclusion-removal mechanisms. The modeling approach presented here is a powerful tool for investigating multiscale phenomena in steelmaking and casting operations to learn how to optimize conditions to lower defects.

156 citations