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Experimental research on thermocapillary migration of drops by using digital holographic interferometry

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Abstract

The thermocapillary migration of drops in a rectangular cell, with a heated top wall and a cooled bottom wall, was investigated experimentally on the ground. The rectangular test cell was 70 mm high, with a horizontal cross section of 40 mm × 40 mm. In the present experiment, 30 cSt silicon oil was used as the continuous phase, and a water–ethanol mixture was used as the drop phase, respectively. The drops ranged in size from 1.87 to 6.94 mm in diameter and were injected into the continuous phase, where the temperature gradients ranged from 0.193 to 0.484 °C mm−1. In order to measure the temperature distribution of the liquid, a digital holographic interferometry was used, which was non-contact, full-field, and in-situ. The holograms were recorded, and then the corresponding wrapped phase distributions images were numerically reconstructed. The temperature distribution of the continuous phase liquid in the cell had been obtained following the unwrapping. Also, through an algebra layer analysis, the temperature distribution around the drop during the thermocapillary migration was obtained. As a result, the drop was colder than the continuous phase liquid, and a thermal wake existed behind the drop. The influence of convective transport on the drop migration was also investigated for the Marangoni number in the range of 7–174. With the increasing of the Marangoni number, the dimensionless interface temperature difference decreased, which was caused by the convective transport enhanced results in the drop thermocapillary migration velocity becoming decreased. The data were compared with previous space experiments to explain the phenomena of the drop migration. Finally, with the increasing Marangoni numbers, the length of the thermal wake region increased, and the thermal wake region became extended.

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Abbreviations

δ :

Optical path difference (m)

n :

Refractive index

μ :

Dynamic viscosity of the continuous phase (Pa s)

ρ :

Density of the continuous phase (kg m−3)

Λ :

Thermal conductivity of the continuous phase (W m−1 °C−1)

σ T :

Temperature coefficient of the interfacial tension (N m−1 °C−1)

ν :

Kinetic viscosity (m2 s−1)

λ :

Laser wavelength (m)

g :

Gravitational acceleration (m s−2)

n 0 :

The initial state of refractive index

R :

Drop radius (m)

μ′:

Dynamic viscosity of the drop phase (Pa s)

ρ′:

Density of the drop phase (kg m−3)

Λ′:

Thermal conductivity of the drop phase (W m−1 °C−1)

κ :

Thermal diffusivity (m2 s−1)

Γ :

Temperature gradient (°C m−1)

Δϕ :

Phase difference (rad)

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Acknowledgments

This research study was funded by the National Natural Science Foundation of China (Grant No. 11372328), by the Strategic Priority Research Program on Space Science, the Chinese Academy of Sciences: SJ-10 Recoverable Scientific Experiment Satellite (Grant Nos. XDA04020405 and XDA04020202-05), and by China Manned Space Engineering program. The authors wish to thank the team led by Professor Jianlin Zhao of Northwestern Polytechnical University for providing assistance in using the digital holographic interferometer.

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Authors and Affiliations

  1. National Microgravity Laboratory, Institute of Mechanics, Chinese Academy of Sciences, Beijing, 100190, China

    Shuoting Zhang, Li Duan & Qi Kang

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  1. Shuoting Zhang
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  2. Li Duan
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  3. Qi Kang
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Correspondence to Qi Kang.

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Zhang, S., Duan, L. & Kang, Q. Experimental research on thermocapillary migration of drops by using digital holographic interferometry. Exp Fluids 57, 113 (2016). https://doi.org/10.1007/s00348-016-2193-x

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  • DOI: https://doi.org/10.1007/s00348-016-2193-x

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