In the phase analysis, accuracy can degrade when the intensity of bright and dark lines in interference fringes varies significantly spatially, or when the phase change Delta phi is temporally discontinuous. In this study, we improved the interference fringe analysis method by focusing on a local region in phase analysis and classifying phase inter-frame differences delta phi' in phase unwrapping. The objective of this study is to clarify how to set the threshold for classifying delta phi' and the accuracy of the phase phi and phase change Delta phi obtained by the proposed method. Both experimentally obtained in a microgravity and artificially generated interference fringes were analyzed using the proposed method. As a result, the obtained spatial distribution of Delta phi had a few outliers. In addition, it was revealed that a reliable threshold for classifying delta phi' can be automatically determined by using the total amount of tolerable noise and estimated delta phi'. The phase error and the phase change error were as small as 0.025 pi and 0.034 pi rad, respectively, which is less than one-fifth of the conventional method.
A numerical simulation study was carried out to investigate the relative contributions of thermal and solutal Marangoni convection (thermo-solutal Marangoni convection) on transport structures in a liquid bridge under zero gravity. The liquid bridge in the model represents a three dimensional half-zone configuration of the Floating Zone (FZ) growth system. Three dimensional field equations of the liquid zone, i.e. continuity, momentum, energy, and diffusion equations, were solved by the PISO algorithm. Computations were performed using the open source software OpenFOAM. The numerical simulation results show that the parameter plane spanned by the two Marangoni numbers with an aspect ratio 0.4 can be classified into three main zones: axisymmetric steady flow, steady flow with wave number, and oscillatory flow and the azimuthal wave number depends on the thermal Marangoni number at oscillatory flow zone.
This paper proposes a methodology to determine and validate the local crystal growth rate for vertices of the solid-liquid (S-L) interface at each frame. The crystal growth in levitated Ti and Ti-6Al-4V melts was successfully observed for the first time using a high-speed camera (HSC) installed in the electrostatic levitation furnace in the International Space Station. The displacement of the S-L interface vertices on the sample surface per unit time is defined as the local crystal growth rate. The mathematical equations to geometrically calculate the displacement and error sources were derived, and their parameters were acquired via image analyses. Quantification error, lens distortion, and crystal movement were identified as error sources. Subsequently, the equations for evaluating the error sources were derived. According to the proposed method, the quantification error is dominant under the experimental setup used in this study. For all samples, consequent plots without overlapping error bars exist. Therefore, the variations in the local crystal growth rate are significant. The proposed methodology revealed that the local crystal growth rate decreased at first and then increased during both former and latter halves of the solidification process on the sample surface. These changes in the local crystal growth rate can be attributed to the preferred orientation of crystal growth.
Space agriculture has attracted attention as a solution to the logistical and psychological challenges of long-term missions, such as those planned under the Artemis program. We investigated the effect of soil pressure on the early-stage enlargement of sweet potato root tubers, a promising crop for space agriculture. Growth experiments were conducted under three soil pressure treatments-low, high, and control-in planters filled with different media. Sweet potatoes were grown in a controlled environment, and growth parameters, including the fresh weights of shoots, roots, and root tubers, were measured 40 days after transplanting. Root lengths were analyzed using WinRHIZO software. The results showed a negative correlation between soil pressure and the fresh weight of shoots and roots, whereas root tubers indicated a positive correlation. Total root lengths decreased under the high pressure, while the mass of fine-diameter roots remained relatively unchanged. These findings suggest that moderate soil pressure may be beneficial for storage root development, although it may suppress the growth of other organs. Our study highlights the importance of maintaining appropriate soil pressure in microgravity environments to promote successful sweet potato growth.
This study experimentally evaluated the effect of internal bubbles (void fraction) on the combustion behavior of polypropylene (PP) spheres under microgravity. In previous research, it has been demonstrated that bubble generation during the burning of polymethyl methacrylate (PMMA) spheres clearly affected the burning rate constant. However, because PMMA exhibits unique pyrolysis characteristics -being composed predominantly of monomers- the observed void effect may not necessarily apply to other polymers. In this study, PP spheres with various initial internal void fractions were used as test specimens and burned under microgravity conditions generated using a drop tower test facility. A total of thirteen productive runs were successfully conducted, showing that the burning rate constant of PP spheres exhibited a positive correlation with the initial void fraction in the range of 0-40%, while remaining nearly constant above 40%. This trend indicates that the material-dependent physical properties play a dominant role in the process from bubble growth to bursting. Extrapolation to zero initial void fraction yielded an ideal (with the least initial void effect) burning rate constant of 0.60-0.72 mm2/s (indicating the potential uncertainty range), with the corresponding B-number ranging from 0.42-0.53. These findings suggest that bubble dynamics still significantly influence the burning behavior of PP and confirm the validity of void-fraction-based evaluation methods for identifying the intrinsic burning properties of polymers.
We established a Free Drift (FD) method to remotely measure the force generated on an isolated object simulated using an Electrostatic Levitation Furnace (ELF). The FD method can be applied in high power laser ablation experiments, where the electric charge of the sample changes drastically during the irradiation. The analyses to deduce the thrust are described in detail. Using this method in the ELF on the measured the thrust generated by laser beam irradiation to an Al droplet for the first time. The momentum coupling coefficient (Cm ) when irradiated by a laser beam with a laser power density of 103 -104 W/cm2 was around 10-9 N/W.
Self-assembled structures of submicron-sized colloidal particles are anticipated to serve as innovative optical materials, including photonic crystals. Colloidal particles with high refractive indices, which are beneficial for optical applications, often have a high specific gravity. During the selfassembly process on the ground, they experience gravitational sedimentation. Consequently, a microgravity environment is ideal for studying their structural formation. Our group has participated in colloidal crystallization and cluster formation experiments aboard the International Space Station. In the index similar or equal to 2.4, specific gravity similar or equal to 3) with a diameter of approximately one micrometer. Here, we report the construction of two-dimensional (2D) colloidal crystals of the titania particles on the Earth, by applying the knowledge of particle fabrication obtained during the preparation of the space experiment. Furthermore, we present a preliminary study on stacking the 2D crystal layers of the titania particles. These results will be valuable for constructing photonic crystals on the Earth and for future space experiments on colloidal self-assembly.
This study reports on the phase separation and solidification behavior of Fe-Cu alloys under microgravity, utilizing an electrostatic levitation furnace (ELF) aboard the International Space Station (ISS). Spherical Fe-Cu samples of varying compositions were first prepared on Earth using an aerodynamic levitator in an argon atmosphere, then selected samples were melted and resolidified in space without a container. Detailed thermal histories were obtained, revealing distinct cooling plateaus and recalescence events, indicative of complex solidification dynamics. Microstructural analysis by electron probe microanalysis showed the formation of a core-shell structure, with Fe-rich phases forming the inner regions and Cu-rich phases surrounding them, as well as evidence of iron oxide formation likely due to residual oxygen. The cooling rates and resulting morphologies were found to agree well with phase field simulations, suggesting that slower cooling in microgravity promotes the development of well-defined core-shell structures through enhanced droplet movement and mixing. These findings provide new insights into phase separation and solidification mechanisms in Fe-Cu alloys under microgravity and will be compared with results from other compositions in future work.
The periodic oscillation of diffusion flame, known as (bulk) flame flickering, is a well-known dynamic behavior connected with flow-associated instabilities triggered by gravity-induced buoyant flow. In this study, the flickering behavior of a wick assisted flame was experimentally examined under partial gravity fields ranging from 1 G to 0.075 G, where G is the normal gravity acceleration (9.81 m/s(2)). The subjected gravity was controlled using the "Slope-Sliding-Method environment 2," which allows repeatable and finely adjustable gravitational conditions in laboratory experiments. Temporal variations in flame height were recorded and analyzed using fast Fourier transform to extract characteristic frequencies. The specific objectives of this work are (1) to determine the gravity dependence on flickering frequency and (2) to observe the "critical" gravity, where flickering is suppressed under a substantially reduced gravity regime. The results indicate that the well-known Strouhal-Froude number correlation is preserved down to similar to 0.17 G, while distinctively different flickering behavior occurs below this point owing to a different mode of periodic oscillation. Theoretical analysis indicates that the characteristic delay time by the conductive heat transfer in the present wick-assisted flame system plays a role under such highly reduced gravity conditions. Although special care must be taken to investigate the "critical" gravity, this work is the first to reveal that weak flickering is preserved, even under a Lunar or Martian gravity environment.
An upgraded partial-gravity generator based on the slope-sliding method proposed in our previous work (Nakamura and Sekimoto, 2014) was developed. This generator consists only of a sliding rack and variable-angle base with a low-friction surface, and the experimental rack is launched from the bottom of the slope. During the sliding operation, the rack experiences partial gravity in the normal direction of the sliding surface. The time for which partial gravity is achieved depends on the length of the slope: in the present test, it was at most 1.0 s, which is sufficient to capture the dynamic response of a flame to the gravity level. To check the performance, the g-sensor signals and behavior of a flame were analyzed. The precision achieved by this method was within 0.05G, which is quite satisfactory. Flame-image analyses revealed the expected features of partial gravity, suggesting that the present methodology is an effective and convenient means of realizing a partial-gravity environment.
A microgravity experiment to elucidate the cool flame dynamics in a multi-droplet system was conducted using the TEXUS-60 sounding rocket. The cool flame position was successfully observed through formaldehyde chemiluminescence. This allowed the study of cool flame propagation speed along n-decane droplet arrays, with droplet spacings of 8 mm (nine droplets) and 16 mm (five droplets). The spread speed was analyzed at an ambient temperature of 570 K and a pressure of 0.1 MPa, using air as the ambient gas. For both droplet configurations, the cool flame occurred from nearly the same position 4.7 s after the droplet was inserted and settled at the combustion position. The cool flame spread speeds decreased from around 250 mm/s to approximately 100 mm/s after the spontaneous ignition, and then increased. The maximum speed reached around 600 mm/s for the 16 mm droplet spacing case. Under these experimental conditions, the results suggest that in the first stage the cool flame spread follows the premixed propagation mode, while in the latter stage the acceleration may be driven by the sequential spontaneous ignition.
The surface tension of molten Zircaloy (Zry-2 and Zry-4), a Zr-1 at% Sn model alloy, and pure zirconium with varying hafnium content was measured using the oscillating droplet method with electromagnetic levitation in atmospheres with controlled oxygen partial pressure (P-O2). The surface tension of molten zirconium was not significantly affected by the presence of hafnium impurities in the 0.05-2.3 at% range. When the P-O2 was buffered at low levels (<= 10(- 4) Pa) using Ar-He-H-2-CO2 gas, oxygen gradually dissolved into the molten Zr-1 at% Sn alloy, resulting in a time-dependent decrease in surface tension. In contrast, under Ar- He gas (P-O2 approximate to 10(- 2) Pa), both oxygen content and surface tension remained nearly constant over time due to kinetic limitations in gas-phase transport. The surface tension of molten Zry-2 and Zry-4 decreased linearly with increasing temperature and was nearly identical to that of the molten Zr- 1 at% Sn alloy under the Ar-He atmosphere. These values were consistently lower than those of pure molten zirconium. These experimental results, along with Butler model analysis, indicate that tin plays a dominant role in reducing surface tension through surface segregation. The maximum expanded uncertainty was within +/- 1%, confirming the reliability of the measurements.
The total number of inhomogeneously distributed grains in solidified metallic samples is essential data for understanding solidification mechanisms, but estimating the total number of grains is challenging when the number density of grains is spatially inhomogeneous. This study presents a method for estimating the total number of grains from a single central cross section, by inferring local variations in grain density through multiple measurement lines. Based on this concept, an estimation formula for spherical samples processed by electrostatic levitation (ESL) was developed using Voronoi tessellation (VT). The formula consists of three principal components. First, a quartic relationship between the number of grains per unit length, NL, and the square of the number of grains per unit area, NA2, is statistically derived. This relationship enables the direct estimation of the number of grains per unit volume, NV, in local domains from NL measurements. Second, the sample is partitioned into spherical segments. The volume of each segment, Vi, is calculated based on the geometric properties of the sphere. Regional number of grains is then obtained by integrating the number of grains per unit volume, NV, over each volume of segment. Finally, a correction factor, DC, is introduced to prevent overcounting when a grain is intersected by more than one measurement line. Validation against VT models demonstrates that the proposed method reliably estimates the true grain number, with results falling within acceptable uncertainty ranges. In addition, the proposed formula estimated that an ESL sample contained approximately 989 +/- 77 grains under the measurement condition that the interval between measurement lines, triangle d, is less than the average grain size, N-1 perpendicular to.
A series of microgravity experiments using 50 mdroptower for burning polymer sphere have been performed to elucidate the role of the "abrupt" dynamic behaviors, such as soot/bubble bursting, on determination of its burning rate constant. With 23 successive runs at the same condition, we could learn precisely the source of the variation of burning behaviors and the strategy to define the burning character of polymers in a precise manner. Specimen is a layered PMMA (poly-methyl methacrylate) coated over a small ceramic ball supported by 14 mu m SiC fiber. Tiny amount of ignitor-gel is employed to attain quasi-steady burning within the limited microgravity time (< 2.5 s). It was found that there are two distinctive stages during the event; square of specimen diameter (d(2)) nearly stays at constant in early stage (Stage I), whereas it almost linearly decreases in time (Stage II) enabling us to define the burning rate constant (K, mm(2)/s) for given conditions. Notably, soot bursting is found to be mainly occurred in Stage I, while the bubble bursting is pronounced in Stage II. With 23 runs of microgravity experiment, burning rate constant, K, is found to be varied randomly, revealing that the randomness of bubble generation in molten specimen is the key and its control is crucial to obtain the characteristic K value. Present work is firstly achieved with sufficient number of microgravity tests of polymeric material for the specific condition and pointed out the importance to take it account the inherent less-repeatable burning character of polymers; such that bubble-generating/growing and "abrupt" bursting processes are essentially unavoidable. It is suggested that certain attempt to suppress such dynamic behaviors is strongly demanded in order to extract "ideal" value of the burning character.
Samples of stainless steel (SS)- boron carbide (B4C) alloys were levitated in the Electrostatic Levitation Furnace onboard the International Space Station (ISS-ELF) to measure the thermophysical properties of their melts. Melting of samples of two different compositions (SS-12.3, and 28 mass % B4C) were attempted in the furnace. Even though only one sample (SS-12.3 mass% B4C) could be melted, its density was successfully obtained. The sample was weighed upon return to the ground, its density (rho) at its liquidus phase was determined to be rho(T) =5829-0.52(T-Tl) (kg/m3) , where Tl is its liquidus temperature (2100 K).
To clarify the effects of oxygen on cool-flame, hot- flame, and two-stage ignition, n-decane droplet ignition experiments were conducted at various ambient oxygen volume fractions. The ambient oxygen volume fraction was set to 15%, 21%, or 35%, with nitrogen added as the balance gas. Ambient pressure and temperature were varied from 0.1 to 0.5 MPa and 530 to 890 K, respectively. The results showed that the lower ambient temperature limit for hot-flame ignition decreased as the oxygen volume fraction increased. In contrast, the effects on the upper ambient temperature limit for cool- flame ignition were minimal, likely due to the limited impact on the equilibrium constant of low- temperature oxidation reactions. Consequently, the no-ignition region narrowed with the increase of ambient oxygen volume fraction. One of the most significant findings was the observation of two-stage ignition at 0.2 MPa when the oxygen volume fraction was 35%. This suggests that the two-stage ignition behavior of hydrocarbon fuel droplets can be evaluated at 0.2 MPa, a condition under which combustion experiments can be conducted on the Kibo module in accordance with safety regulations.
Since the discovery of a dust crystal by Hayashi et al. (Jpn. J. Appl. Phys., 33 (1994) L804), various interesting phenomena have been observed and actively studied in complex plasmas (also called dusty plasmas) containing mu m- sized charged microparticles, or dust particles. Although microparticles are macrosopic compared to electrons and ions, the dusty plasmas are treated as fluids when studying their collective behavior. Here we revisit and redescribe one of our representative experimental results on such a complex plasma: the dynamic circulation of microparticles in three dimensions. The experiments were performed under gravity using a glass-tube apparatus whose axis was vertical. Argon gas plasma was generated using radio-frequency (rf) discharge. A neodymium magnet was placed outside the bottom of the glass tube to apply a magnetic field to the complex plasma. Monodisperse acrylic resin spheres of mu m order were used as microparticles. The microparticles were visualized with the naked eye by irradiating a thin fan-shaped green laser light. Their motions were recorded as still or moving images. A conical cloud of microparticles was formed, and individual particles in the cloud circulated and behaved similarly to tea leaves in a teacup.
The density, viscosity, and surface tension of Hastelloy (Ni-Cr-Mo alloy) in the molten state were measured under vacuum conditions. Due to the lack of data in the open literature, the values of these physical properties were compared with those of other compositions of similar nickel-based alloys (CMSX, RENE, Inconel). There was not much difference in viscosity and surface tension when compared with other alloys. The high viscosity of Hastelloy C-276 is considered to be the result of surface oxidation, and the viscosity increases as the surface tension decreases.
This study aims to establish a simple predicting method for impurity diffusion coefficients of elements, including the effect of atomic weight in liquid Sn based on a hard-sphere model. The impurity diffusion coefficients of Al, Au, and Cu in liquid Sn at 573 K were measured using the shear cell technique and stable density layering. Furthermore, we proposed different prediction methods for impurity diffusion coefficients of elements using a multiple regression analysis based on (6Sn/6i)cpis, where 6i and cpis are atomic diameter and thermodynamic factor of solute element i in solvent element s, respectively. Impurity diffusion coefficients of elements, the value (6Sn/6i)cpis of which is approximately 1.2 or small in liquid Sn near the melting point, were simply proportional to the product of the following three factors with the self-diffusion coefficient of Sn as the slope: (i) the atomic diameter ratio of Sn to the solute element, (ii) 0.182 power of the reduced mass ratio of Sn to the solute element, and (iii) thermodynamic factor. The uncertainty of the impurity diffusion coefficients of reference values and Al was within 6%. However, the impurity diffusion coefficients of elements, the value (6Sn/6i)cpis of which is approximately 1.2 or large, were approximately equal to the self-diffusion coefficient of Sn. This was owing to the inhibited atomic diffusion of Au and Cu in liquid Sn.
We have developed passive devices for conducting two-step enzyme reactions with a time lag. They are "n-paraffin separator device" and "osmotic timer device." We evaluated the feasibility of these devices using two reactions, cellulose synthesis and cellulose decomposition, catalyzed by different enzymes. We have verified that the n-paraffin separator device works well for the cellulose synthesis in space and the osmotic pressure timer device releases the cellulase with a time lag. This paper further reports that the devices functioned autonomously and carried out the two-step enzyme reactions in the International Space Station.
