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Research Containing: Marangoni

Oscillatory cellular patterns in three-dimensional directional solidification

by on 22 August 2016in Biology & Biotechnology, Physical Sciences No comment

We present a phase-field study of oscillatory breathing modes observed during the solidification of three-dimensional cellular arrays in microgravity. Directional solidification experiments conducted onboard the International Space Station have allowed us to observe spatially extended homogeneous arrays of cells and dendrites while minimizing the amount of gravity-induced convection in the liquid. In situ observations of transparent alloys have revealed the existence, over a narrow range of control parameters, of oscillations in cellular arrays with a period ranging from about 25 to 125 min. Cellular patterns are spatially disordered, and the oscillations of individual cells are spatiotemporally uncorrelated at long distance. However, in regions displaying short-range spatial ordering, groups of cells can synchronize into oscillatory breathing modes. Quantitative phase-field simulations show that the oscillatory behavior of cells in this regime is linked to a stability limit of the spacing in hexagonal cellular array structures. For relatively high cellular front undercooling (i.e., low growth velocity or high thermal gradient), a gap appears in the otherwise continuous range of stable array spacings. Close to this gap, a sustained oscillatory regime appears with a period that compares quantitatively well with experiment. For control parameters where this gap exists, oscillations typically occur for spacings at the edge of the gap. However, after a change of growth conditions, oscillations can also occur for nearby values of control parameters where this gap just closes and a continuous range of spacings exists. In addition, sustained oscillations at to the opening of this stable gap exhibit a slow periodic modulation of the phase-shift among cells with a slower period of several hours. While long-range coherence of breathing modes can be achieved in simulations for a perfect spatial arrangement of cells as initial condition, global disorder is observed in both three-dimensional experiments and simulations from realistic noisy initial conditions. In the latter case, erratic tip-splitting events promoted by large-amplitude oscillations contribute to maintaining the long-range array disorder, unlike in thin-sample experiments where long-range coherence of oscillations is experimentally observable.

Related URLs:
http://www.ncbi.nlm.nih.gov/pubmed/26565251

Transition to chaotic thermocapillary convection in a half zone liquid bridge

by on 22 August 2016in Biology & Biotechnology, Physical Sciences

A series of fluid physics microgravity experiments with an enough long run time were performed in the ‘‘KIBO,’’ the Japanese Experiment Module aboard the International Space Station, to examine the transition to chaos of the thermocapillary convection in a half zone liquid bridge of silicone oil with a Prandtl number of 112. The temperature difference between the coaxial disks induced the thermocapillary-driven flow, and we experimentally demonstrated that the flow fields underwent a tran- sition from steady flow to oscillatory flow, and finally to chaotic flow with increasing temperature differ- ence. We obtained the surface temperature time series at the middle of the liquid bridge to quantitatively evaluate the transition process of the flow fields. By Fourier analysis, we further confirmed that the flow fields changed from a periodic, to a quasi-periodic, and finally to a chaotic state. The increasing nonlin- earity with the development of the flow fields was confirmed by time-series chaos analysis. The deter- mined Lyapunov exponent and the translation error indicated that the flow fields made transition to the chaotic field with the increasing temperature difference.

Related URLs:
http://www.sciencedirect.com/science/article/pii/S0017931015005323

Growth of InxGa1−xSb alloy semiconductor at the International Space Station (ISS) and comparison with terrestrial experiments

by on 22 August 2016in Physical Sciences No comment

BACKGROUND: InxGa1 − xSb is an important material that has tunable properties in the infrared (IR) region and is suitable for IR-device applications. Since the quality of crystals relies on growth conditions, the growth process of alloy semiconductors can be examined better under microgravity (μG) conditions where convection is suppressed.;AIMS: To investigate the dissolution and growth process of InxGa1 − xSb alloy semiconductors via a sandwiched structure of GaSb (seed)/InSb/GaSb(feed) under normal and μG conditions.;METHODS: InxGa1 − xSb crystals were grown at the International Space Station (ISS) under μG conditions, and a similar experiment was conducted under terrestrial conditions (1G) using the vertical gradient freezing (VGF) method. The grown crystals were cut along the growth direction and its growth properties were studied. The indium composition and growth rate of grown crystals were calculated.;RESULTS: The shape of the growth interface was nearly flat under μG, whereas under 1G, it was highly concave with the initial seed interface being nearly flat and having facets at the peripheries. The quality of the μG crystals was better than that of the 1G samples, as the etch pit density was low in the μG sample. The growth rate was higher under μG compared with 1G. Moreover, the growth started at the peripheries under 1G, whereas it started throughout the seed interface under μG.;CONCLUSIONS: Kinetics played a dominant role under 1G. The suppressed convection under μG affected the dissolution and growth process of the InxGa1 − xSb alloy semiconductor.

Related URLs:
http://www.nature.com/articles/npjmgrav201511

Space experiment on the instability of Marangoni convection in large liquid bridge – MEIS-4: effect of Prandtl number

by on 9 June 2015in Physical Sciences No comment

Microgravity experiments on the thermocapillary convection in liquid bridge, called Marangoni Experiment in Space (MEIS), are carried out in "KIBO" of ISS. Three series of experiments, MEIS-1, 2, and 4, have been conducted so far. This paper reports the results obtained from MEIS-4, in which 20cSt silicone oil ( Pr = 207) is used to generate large liquid bridges. They are suspended between coaxial disks that are 50mm in diameter, with their maximum length equal to 62.5mm. MEIS-4 aims at (1) determining the critical temperature difference for the onset of oscillatory flow; (2) realizing high Marangoni number conditions for high Pr fluid; (3) clarifying the effects of volume ratio, heating rate, hysteresis, and cooled disk temperature; and (4) observing whether the hydrothermal wave with azimuthal mode number m = 0 appears or not. The main results are presented and compared with those obtained in MEIS-1 and 2, which utilized liquid bridges of 5cSt silicone oil ( Pr = 67).

Related URLs:
http://stacks.iop.org/1742-6596/327/i=1/a=012029

3-D PTV measurement of Marangoni convection in liquid bridge in space experiment

by on 9 June 2015in Physical Sciences No comment

Microgravity experiments have been conducted on the International Space Station in order to clarify the transition processes of the Marangoni convection in liquid bridges of high Prandtl number fluid. The use of microgravity allows us to generate large liquid bridges, 30 mm in diameter and up to 60 mm in length. Three-dimensional particle tracking velocimetry (3-D PTV) is used to reveal complex flow patterns that appear after the transition of the flow field to oscillatory states. It is found that a standing-wave oscillation having an azimuthal mode number equal to one appears in the long liquid bridges. For the liquid bridge 45 mm in length, the oscillation of the flow field is observed in a meridional plane of the liquid bridge, and the flow field exhibits the presence of multiple vortical structures traveling from the heated disk toward the cooled disk. Such flow behaviors are shown to be associated with the propagation of surface temperature fluctuations visualized with an IR camera. These results indicate that the oscillation of the flow and temperature field is due to the propagation of the hydrothermal waves. Their characteristics are discussed in comparison with some previous results with long liquid bridges. It is shown that the axial wavelength of the hydrothermal wave observed presently is comparable to the length of the liquid bridge and that this result disagrees with the previous linear stability analysis for an infinitely long liquid bridge.

Related URLs:
http://dx.doi.org/10.1007/s00348-011-1136-9

Instability and associated roll structure of Marangoni convection in high Prandtl number liquid bridge with large aspect ratio

by on 9 June 2015in Physical Sciences No comment

This paper reports the experimental results on the instability and associated roll structures (RSs) of Marangoni convection in liquid bridges formed under the microgravity environment on the International Space Station. The geometry of interest is high aspect ratio (AR = height/diameter ≥ 1.0) liquid bridges of high Prandtl number fluids (Pr = 67 and 207) suspended between coaxial disks heated differentially. The unsteady flow field and associated RSs were revealed with the three-dimensional particle tracking velocimetry. It is found that the flow field after the onset of instability exhibits oscillations with azimuthal mode number m = 1 and associated RSs traveling in the axial direction. The RSs travel in the same direction as the surface flow (co-flow direction) for 1.00 ≤ AR ≤ 1.25 while they travel in the opposite direction (counter-flow direction) for AR ≥ 1.50, thus showing the change of traveling directions with AR. This traveling direction for AR ≥ 1.50 is reversed to the co-flow direction when the temperature difference between the disks is increased to the condition far beyond the critical one. This change of traveling directions is accompanied by the increase of the oscillation frequency. The characteristics of the RSs for AR ≥ 1.50, such as the azimuthal mode of oscillation, the dimensionless oscillation frequency, and the traveling direction, are in reasonable agreement with those of the previous sounding rocket experiment for AR = 2.50 and those of the linear stability analysis of an infinite liquid bridge.

Related URLs:
http://scitation.aip.org/content/aip/journal/pof2/27/2/10.1063/1.4908042

3-D Flow Measurement of Oscillatory Thermocapillary Convection in Liquid Bridge in MEIS

by on 9 June 2015in Physical Sciences No comment

Marangoni Experiment in Space (MEIS) has been conducted in the International Space Station (ISS) in order to clarify the transition processes of thermocapillary convection in liquid bridges. The use of microgravity allows us to generate long liquid bridges, 30mm in diameter and up to 60mm in length. Several flow visualization techniques have been applied to those large liquid bridges. 3-D PTV is used to reveal highly three-dimensional flow patterns that appear after the transition. Three CCD cameras are used to observe the motions of the tracer particles from different view angles through the transparent heated disk made of sapphire. Particle images are recorded in the HDD recording system in ISSand they are downloaded to the ground for data analysis. A conventional 3-D PTV technique and a newly-developed multi-frame particle tracking method are combined to obtain the results that can help better understanding of oscillatory 3-D flow fields in the liquid bridges. It is shown that the flow pattern changes from a 2-D axisymmetric steady flow to an oscillatory 3-D non-axisymmetric flow under the supercritical conditions.

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Instability of thermocapillary convection in long liquid bridges of high Prandtl number fluids in microgravity

by on 9 June 2015in Physical Sciences No comment

This paper reports experimental results on the instability of thermocapillary convection in long half-zone liquid bridges of high Prandtl number fluids (Pr=67, 112 and 207 for 5, 10 and 20 cSt silicone oils, respectively). The experiments were carried out in microgravity on the International Space Station, which allowed sufficiently long waiting period for the development of instability. Critical temperature differences were measured for liquid bridges of 30 and 50 mm diameters and up to 62.5 mm length. The resultant critical Marangoni numbers (Mac) were obtained for a wide range of aspect ratio (=height/diameter), AR, up to AR=2.0. Linear stability analyses for Pr=67 were also carried out to obtain numerical data for comparison. The present experimental results for Pr=67 indicate 5.0×103<Mac<2.0×104 for large AR (AR>1.25) and they are in good agreement with the present linear stability analysis result. In contrast, the present results are considerably smaller than the previous data (Pr=74) taken in the Space Shuttle experiments. It is shown that this difference is due to the effect of heating rate of the liquid bridge. The data for oscillation frequency and azimuthal mode number are also presented. The non-dimensional oscillation frequencies as well as Mac for Pr=67 have shown a sudden decrease at around AR=1.25, suggesting the bifurcation of neutral stability curves.

Related URLs:
http://www.sciencedirect.com/science/article/pii/S0022024815002353

Effect of capillary and marangoni forces on transport phenomena in microgravity

by on 9 June 2015in Physical Sciences No comment

The Constrained Vapor Bubble (CVB) experiment concerns a transparent, simple, "wickless" heat pipe operated in the microgravity environment of the International Space Station (ISS). In a microgravity environment, the relative effect of Marangoni flow is amplified because of highly reduced buoyancy driven flows as demonstrated herein. In this work, experimental results obtained using a transparent 30 mm long CVB module, 3 mm x 3 mm in square cross-section, with power inputs of up to 3.125 W are presented and discussed. Due to the extremely low Bond number and the dielectric materials of construction, the CVB system was ideally suited to determining if dry-out as a result of Marangoni forces might contribute to limiting heat pipe performance and exactly how that limitation occurs. Using a combination of visual observations and thermal measurements, we find a more complicated phenomenon in which opposing Marangoni and capillary forces lead to flooding of the device. A simple one-dimensional, thermal-fluid flow model describes the essence of the relative importance of the two stresses. Moreover, even though the heater end of the device is flooded and the liquid is highly superheated, boiling does not occur due to high evaporation rates.

Related URLs:
http://www.ncbi.nlm.nih.gov/pubmed/25874586
http://pubs.acs.org/doi/abs/10.1021/acs.langmuir.5b00428

The effect of support fibers on micro-convection in droplet combustion experiments

by on 9 June 2015in Physical Sciences No comment

This study reports experimental evidence of gas phase micro-convection induced by support fibers used in droplet combustion experimentation. Soot aggregates formed during combustion of n-octane and n-decane droplets (initial diameters ranging from 0.5 mm to 5 mm) provide natural seeds to reveal the thermal and flow asymmetries involved. The experiments are carried out in an environment that reduces the influence of forced and buoyant convection for both free-floating (unsupported) and fiber-supported droplets. Under these conditions, the soot trapping patterns (due to a balance of thermophoretic and flow-induced drag) would be spherical. However, this situation is only observed for unsupported droplets, or for fiber-supported droplets when the fiber is small relative to the droplet diameter. For Do < 1 mm a ground based drop tower employed two 14 μm diameter SiC fibers to fix the droplet’s position during burning; unsupported droplets were also examined. For Do > 1 mm the International Space Station provided capabilities for anchoring test droplets onto a single 80 μm SiC fiber, and for deploying unsupported droplets. Results clearly indicate that a non-symmetric gas flow field exists in some cases (i.e., for 1 mm < Do < 3 mm, with an 80 μm fiber) near to where the fiber enters the droplet. This gas motion originates from the presence of the fiber that introduces asymmetries in the temperature and flow fields resulting in localized force imbalances on the soot particles, which cause vortical flow patterns near the fiber. This may in part be explained by flow asymmetries induced by droplet shape distortions coupled with heat exchanges between the fiber and surrounding gas and conduction into the droplet, resulting in a Marangoni flow near the droplet surface. For very small fibers (or for unsupported droplets) spherical soot shells are found suggesting that no thermal and flow asymmetries exist.

Related URLs:
http://www.sciencedirect.com/science/article/pii/S1540748914003320