Wetting film dynamics during evaporation under weightlessness in a near-critical fluid

Hegseth, John; Oprisan, Ana; Garrabos, Yves; Nikolayev, Vadim; Lecoutre-Chabot, Carole; Beysens, Daniel

doi:10.1103/physreve.72.031602

Cited by 23 publications

(19 citation statements)

References 12 publications

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“…It is also observed that, in one-component liquid-gas systems, the contact angle increases with the heat flux (see Fig. 8), a phenomenon that has been theoretically predicted and has been experimentally confirmed [70,80,81].…”

Section: B Flow Geometry and Initial Statementioning

confidence: 62%

Droplet motion in one-component fluids on solid substrates with wettability gradients

Qian

2012

Phys. Rev. E

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CitationDroplet motion in one-component fluids on solid substrates with wettability gradients 2012, 85 (5) Physical Review E Droplet motion on solid substrates has been widely studied not only because of its importance in fundamental research but also because of its promising potentials in droplet-based devices developed for various applications in chemistry, biology, and industry. In this paper, we investigate the motion of an evaporating droplet in onecomponent fluids on a solid substrate with a wettability gradient. As is well known, there are two major difficulties in the continuum description of fluid flows and heat fluxes near the contact line of droplets on solid substrates, namely, the hydrodynamic (stress) singularity and thermal singularity. To model the droplet motion, we use the dynamic van der Waals theory [Phys. Rev. E 75, 036304 (2007)] for the hydrodynamic equations in the bulk region, supplemented with the boundary conditions at the fluid-solid interface. In this continuum hydrodynamic model, various physical processes involved in the droplet motion can be taken into account simultaneously, e.g., phase transitions (evaporation or condensation), capillary flows, fluid velocity slip, and substrate cooling or heating. Due to the use of the phase field method (diffuse interface method), the hydrodynamic and thermal singularities are resolved automatically. Furthermore, in the dynamic van der Waals theory, the evaporation or condensation rate at the liquid-gas interface is an outcome of the calculation rather than a prerequisite as in most of the other models proposed for evaporating droplets. Numerical results show that the droplet migrates in the direction of increasing wettability on the solid substrates. The migration velocity of the droplet is found to be proportional to the wettability gradients as predicted by Brochard [Langmuir 5, 432 (1989)]. The proportionality coefficient is found to be linearly dependent on the ratio of slip length to initial droplet radius. These results indicate that the steady migration of the droplets results from the balance between the (conservative) driving force due to the wettability gradient and the (dissipative) viscous drag force. In addition, we study the motion of droplets on cooled or heated solid substrates with wettability gradients. The fast temperature variations from the solid to the fluid can be accurately described in the present approach. It is observed that accompanying the droplet migration, the contact lines move through phase transition and boundary velocity slip with their relative contributions mostly determined by the slip length. The results presented in this paper may lead to a more complete understanding of the droplet motion driven by wettability gradients with a detailed picture of the fluid flows and phase transitions in the vicinity of the moving contact line.

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Section: B Flow Geometry and Initial Statementioning

confidence: 62%

Droplet motion in one-component fluids on solid substrates with wettability gradients

Qian

2012

Phys. Rev. E

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“…A non-intrusive, optical grid deflection technique [30][31][32] has been used to capture the film shape in situ, during the PHP functioning. A wire grid (formed by parallel wires) is placed between the diffuse light source and the cell, cf.…”

Section: Film Shape Reconstructionmentioning

confidence: 99%

In situ investigation of liquid films in pulsating heat pipe

Fourgeaud

Nikolayev

Ercolani

et al. 2017

Applied Thermal Engineering

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International audienceTo understand functioning of the pulsating (or oscillating) heat pipe (PHP), a liquid film deposited by an oscillating meniscus is studied experimentally inside the simplest, single branch PHP, which is a straight capillary sealed from one end. The PHP capillary is of rectangular section of high aspect ratio. The evaporator is transparent so that the films can be studied by two complementary optical methods: grid deflection method and interferometry. We were able to measure the dynamic film profile during the self induced meniscus oscillations. It has been shown that the PHP films have the same origin as those of the Taylor bubbles; their thickness right after deposition is well described by the classical formulas. The film shape in PHPs differs from the classical wedge-shaped film observed in capillary heat pipes because both of the larger thickness and of the receding triple liquid-vapor-solid contact line. The film slope is very weak, with a growing in time ridge adjacent to the contact line. It is shown that this ridge is the dewetting ridge. Its dynamics is defined mainly by the capillary effects. Such results can be generalized to the conventional multi-branch PHP

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“…The laser stability after 1 h was estimated to be better than 0.3%. An optical microscope of 3 μm resolution was used to record a small region (less than 1 mm 2 ) of the SCU with a sampling rate of 25 Hz [35]. Here we analyze about 200 images (8 s of recording) above T c , respectively, about 575 images (23 s of recording) below T c .…”

Section: B Opticsmentioning

confidence: 99%

Dynamic structure factor of density fluctuations from direct imaging very near (both above and below) the critical point of SF6

Oprisan

Bayley

et al. 2012

Phys. Rev. E

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Large density fluctuations were observed by illuminating a cylindrical cell filled with sulfur hexafluoride (SF(6)), very near its liquid-gas critical point (|T-T(c)|< 300 μK) and recorded using a microscope with 3 μm spatial resolution. Using a dynamic structure factor algorithm, we determined from the recorded images the structure factor (SF), which measures the spatial distribution of fluctuations at different moments, and the correlation time of fluctuations. This method authorizes local measurements in contrast to the classical scattering techniques that average fluctuations over the illuminating beam. We found that during the very early stages of phase separation the SF scales with the wave vector q according to the Lorentzian q(-2), which shows that the liquid and vapor domains are just emerging. The critical wave number, which is related to the characteristic length of fluctuations, steadily decreases over time, supporting a sustained increase in the spatial scale of the fluctuating domains. The scaled evolution of the critical wave number obeys the universal evolution for the interconnected domains at high volume fraction with an apparent power law exponent of -0.35 ± 0.02. We also determined the correlation time of the fluctuations and inferred values for thermal diffusivity coefficient very near the critical point, above and below. The values were used to pinpoint the crossing of T(c) within 13 μK.

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Wetting film dynamics during evaporation under weightlessness in a near-critical fluid

Cited by 23 publications

References 12 publications

Droplet motion in one-component fluids on solid substrates with wettability gradients

Droplet motion in one-component fluids on solid substrates with wettability gradients

In situ investigation of liquid films in pulsating heat pipe

Dynamic structure factor of density fluctuations from direct imaging very near (both above and below) the critical point of SF6

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