Surface In Microgravity Research Articles

Controlling a Free Surface With Thermocapillary Flows and Vibrations in Microgravity

Fluid manipulation and control is crucial for space exploration. Motivated by the “Thermocapillary-based control of a free surface in microgravity" (ThermoSlosh) experiment (Salgado Sánchez et al. in Acta Astronautica 205:57–67, 2023), we conduct here a detailed numerical analysis of interfacial dynamics in a two-dimensional cylindrical cell, half-filled with different silicone oils or a fluorinert, and subjected to thermal forcing and vibrations. The effect on the free surface dynamics of the applied temperature difference, vibrational amplitude, fluid viscosity, and contact angle is analyzed; both static and dynamic contact angle models are considered. Results strongly suggest that thermocapillary flows can be used to control the interface orientation within the cell, while supplemental vibrations can be added to increase the system responsiveness. This control can be further improved by using classical proportional-integral-derivative feedback to adjust the cell boundary temperatures in real-time. The proportional and derivative gains of the controller can be selected to optimize the stabilization time and/or energy cost, while the integral contribution is effective in reducing the steady-state error. Overall, the present analysis highlights the potential of using the thermocapillary effect for fluid management in reduced gravity, and evaluates different types of experimental tests that can be executed in the frame of the ThermoSlosh microgravity project.

Dynamics of PCM melting driven by a constant heat flux at the free surface in microgravity

In this work, we analyze the thermocapillary-enhanced melting of n-octadecane driven by a constant heat flux, applied at the free surface, in microgravity. The material is enclosed in an open rectangular container of dimensions 2L×H, and its solid-to-liquid transition is described using an enthalpy-porosity formulation of the Navier–Stokes equations, assuming laminar and incompressible flow. We study the influence of key governing parameters, including the effect of the heated length l̂ϕ∈(0,1], the applied flux ϕ̂∈(0,8], and the container aspect ratio Γ∈[1.5,22.8]. Heat transport is analyzed by comparing the thermocapillary-enhanced process with that driven solely by conduction, and quantified by the enhancement ratio G, which simply compares melting times in each scenario. We find that G increases with ϕ̂ and Γ, and is maximum at an optimal heated length l̂ϕ≃0.5. Compared to previous works on the melting of n-octadecane in microgravity, the associated enhancement G is more moderate in this system, and oscillatory thermocapillary convection is not observed over the range of parameters explored.

The “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” experiment: Design and performance of the cuboidal cell

The experiment “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” aims to investigate the efficacy of thermal Marangoni convection in augmenting the heat transfer rate of passive phase change materials (PCMs) that incorporate a free surface in microgravity; the so-called thermocapillary-enhanced PCMs. Compared to thermal conduction, thermocapillary flows can increase heat transport by a factor of two or more. In addition to advancing scientific understanding, the experiment seeks to evaluate the practical feasibility of using thermocapillary-enhanced PCMs as passive thermal control devices for space missions and assess possible implementation challenges. By analyzing different PCM samples, the experiment will provide crucial insights into the heat and mass transport mechanisms of thermocapillary flows during melting and solidification. The present article primarily describes the design, thermal control and ground results of the cuboidal cell of the experiment. We illustrate here the comprehensive approach towards developing space experiments adhering to rigorous and demanding scientific and technical requirements. Along with the validation of an initial experiment prototype, the evaluation of ground tests is also addressed. The hardware developed demonstrates an adequate performance, while the obtained scientific results are coherent and compliant with the established requirements; the feasibility of the proposed design is thus demonstrated.

The “Thermocapillary-based control of a free surface in microgravity” experiment

Present and future challenges of space exploration require better and improved strategies for fluid control and management. The “Thermocapillary-based control of a free surface in microgravity” (ThermoSlosh) experiment aims to contribute directly to current knowledge and basic understanding of fluid phenomena in reduced gravity, in particular, to study the effectiveness of thermal forcing for fluid control in weightlessness and applications. The experiment proposes to analyze the dynamics of a free surface in a cylindrical cell, half filled with 5 cSt silicone oil, subjected to controlled temperatures and accelerations. Simulations suggest that the thermocapillary effect can be used in microgravity to control the orientation of the free surface within the cell. The response of the free surface to the applied thermal gradient is characterized using the rise time, the stabilization time, and the overshoot; these representative quantities further help evaluating the effectiveness of the strategy. The use of supplemental vibrations is shown to improve the overall performance of the thermal control. Finally, among various potential applications, the ability to control sloshing motion during the real microgravity scenario of an ISS reboosting maneuver is assessed. ThermoSlosh was recently presented to the International Space Science and Scientific Payload competition, and is part of the selected proposals for the competition final.

The Study of an Integrated Sensor for Partial Nucleate Pool Boiling at Low Heat Flux

To study local convention and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well-characterized flat surface in microgravity, a microelectromechanical system (MEMS) sensor has been fabricated to pulse single bubble boiling. In this article, we present the design and fabrication process of an MEMS microchip, which can excite a bubble nucleus with accurate spatial and temporal positioning on the topside of the microchip, and meanwhile, the temperature on the heating surface around and underneath the growing bubble can be measured. The key points and difficulties of chip fabrication have been discussed, including the characteristics of the ultrathin Ti/Pt (10/35 nm) film, such as the effect of annealing on resistivity and its temperature coefficient of resistance (TCR). Then, the thermal cycling experiment has verified its stability. The results of the preliminary ground tests of the flight module SOBER-SJ10 show that the microchip works well as expected and laid a good foundation for meeting the requirements of future space experiments.

Thermocapillary-driven dynamics of a free surface in microgravity: Control of sloshing

Numerical simulations are used to analyze the dynamics of a free surface excited by thermal modulations at the lateral boundaries that generate a time-dependent thermocapillary flow. Fluid parameters are selected to be representative of 5 cSt silicone oil. Following the work of Gligor et al. [“Thermocapillary-driven dynamics of a free surface in microgravity: Response to steady and oscillatory thermal excitation,” Phys. Fluids 34, 042116 (2022)], the response of the free surface to oscillatory thermal excitation is characterized by the displacement of the contact points, and a frequency sweep is used to obtain a Bode-type diagram that reveals a resonance peak in the vicinity of the first sloshing mode. The ability of the thermocapillary flow to excite this sloshing mode suggests a control strategy that uses thermal modulations to dampen sloshing motion. After the response of the isothermal surface to a generic pulse-like inertial perturbation is measured, a classical proportional integral derivative control is implemented and the effect of its gains is considered separately. The efficacy of the controller is characterized by the decay time of the contact point oscillations and by a cost function. The effect of possible delays in the control loop is accounted for. Finally, a controller with a derivative gain is selected and used to dampen the motion induced by a reboosting maneuver of the International Space Station.

Thermocapillary-driven dynamics of a free surface in microgravity: Response to steady and oscillatory thermal excitation

A numerical analysis of the thermocapillary-driven dynamics of a free surface in microgravity is presented for an open container of liquid subjected to steady or oscillatory thermal excitation. The response to this forcing is analyzed for parameters representative of common silicone oils. In contrast to previous investigations, we permit large-scale unconstrained motion of the contact points and deformation of the free surface, which allows us to study the interaction between free surface dynamics and thermocapillary flow. First, the response of the free surface to steady thermal excitation is considered and characterized by the asymmetry of the contact points. Linear dependence of this asymmetry on the applied Marangoni number is found, which is amplified by the vibroequilibria effect when supplemental (high-frequency) vibrations are introduced. In low-viscosity liquids, the transient dynamics of the free surface includes sloshing modes, suggesting that thermal modulation may be used to excite them. The free surface response to oscillatory thermal excitation is then studied for a wide range of parameters, including variations in contact angle β, viscosity ν, container length L, and fluid height H. We perform a frequency analysis and obtain Bode-type diagrams for the contact point oscillations, characterizing the low-frequency response by its amplitude and phase with respect to the thermal forcing, and demonstrate a resonance peak corresponding to the principal sloshing mode. Overall, these results indicate the potential of oscillatory thermal excitation for fluid control in microgravity.

Surface boundary-dendrite interactions in thin metallic Al-alloy samples

Crystallographic orientations and growth directions of dendrites in thin Al-15 wt.%Cu (200 µm) and Al-29 wt.%Ge (500 µm) alloys were investigated employing in situ X-radiography and post-mortem X-ray tomography and electron backscatter diffraction. The Al-Cu alloy was solidified under microgravity conditions, while the Al-Ge sample was processed horizontally on ground. It was found that (i) dendrites nucleate on both sides of the surface in microgravity, but accumulate under normal gravity near the top surface due to buoyancy, (ii) dendrites grow along preferred in-plane crystallographic directions, which are 〈100〉 for Al-15 wt.%Cu and 〈100〉, 〈110〉 and 〈210〉 for Al-29 wt.%Ge, indicating that the solid-liquid interfacial anisotropy of this Al-Ge composition is similar for these 〈xy0〉 directions, (iii) slightly inclined dendrite arms in Al-Cu grow along the sample boundary after contact, whereas tips of inclined dendrite arms in Al-Ge never touch the boundary, but develop new primary tips originating from secondary branches.

Free surface reconstruction of opaque liquids in microgravity, Part 1: Design and on-ground testing

This paper describes an inexpensive, non-invasive and highly adaptable surface reconstruction device for opaque liquids. The instrument was developed to study the lateral sloshing of ferrofluids in microgravity as part of the UNOOSA DropTES 2019 StELIUM project. Its design is driven by the geometrical and mechanical constraints imposed by ZARM’s drop tower, where the experiment was launched in November 2019. The launch catapult and deceleration systems impose strong axial g-loads to a system that is confined in the reduced capsule environment. Redundant procedures are implemented to measure the first two lateral sloshing frequencies and damping ratios of the magnetic liquid, as well as its equilibrium surface in microgravity. Ideal vertical resolutions between 0.25 and 0.4 mm/px can be achieved with the configuration here proposed. The final performance depends, among other factors, on the correct application of the robust calibration procedure that is documented in this work.

DEM Analysis and Evaluation of Hopping Motion on a Sandy Surface in Microgravity

DEM Analysis and Evaluation of Hopping Motion on a Sandy Surface in Microgravity

Partial Nucleate Pool Boiling at Low Heat Flux: Preliminary Ground Test for SOBER-SJ10

Focusing on partial nucleate pool boiling at low heat flux, SOBER-SJ10, one of 27 experiments of the program SJ-10, has been proposed to study local convection and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well characterized flat surface in microgravity. An integrated micro heater has been developed. By using a local pulse overheating method in the experimental mode of single bubble boiling, a bubble nucleus can be excited with accurate spatial and temporal positioning on the top-side of a quartz glass substrate with a thickness of 2 mm and an effective heating area of 4.5 mm in diameter, and then grows under an approximate constant heat input provided by the main heater on the back-side of the substrate. Ten thin film micro-RTDs are used for local temperature measurements on the heating surface underneath the growing bubble. Normal pool boiling experiments can also be carried out with step-by-step increase of heating voltage. A series of ground test of the flight module of SOBER-SJ10 have been conducted. Good agreement of the measured data of single phase natural convection with the common-used empirical correlation warrants reasonable confidence in the data. It is found that the values of the incipience superheat of pool boiling at different subcooling are consistent with each others, verifying that the influence of subcooling on boiling incipience can be neglected. Pool boiling curves are also obtained, which shows great influence of subcooling on heat transfer of partial nucleate pool boiling, particularly in lower heat flux.

Bubble dynamics in nucleate pool boiling on micro-pin-finned surfaces in microgravity

Bubble dynamics is an important phenomenon, which basically affects the nucleate boiling heat transfer coefficient. Nucleate boiling heat transfer of gas-saturated FC-72 on micro-pin-finned surface was experimentally investigated in microgravity environment by utilizing the drop tower facility in Beijing. The dimensions of the silicon chips were 10 mm × 10 mm × 0.5 mm (length × width × thickness) on which two kinds of micro-pin-fins with the dimensions of 30 × 30 × 60 μm3, 50 × 50 × 120 μm3 (width × thickness × height, named PF30-60, PF50-120) were fabricated by the dry etching technique. The experimental data were presented for the bubble departure radius on micro-pin-finned surface. Experimental results showed that the bubble detachment radius increases with increasing heat flux, but the traditional force balance model failed to predict the bubble detachment radius on micro-pin-finned surfaces especially at high heat fluxes. Therefore, a new modified model for predicting bubble departure radius on micro-pin-finned surface in microgravity was developed. In this model, both bubble force balance and bubble coalescence are considered as two main factors influencing the size of bubble departure radius, and the predictions agree much better with the experimental data at moderate and high heat fluxes than the force balance model.

Gravity‐dependent differentiation and root coils in Arabidopsis thaliana wild type and phospholipase‐A‐I knockdown mutant grown on the International Space Station

Arabidopsis roots on 45° tilted agar in 1-g grow in wave-like figures. In addition to waves, formation of root coils is observed in several mutants compromised in gravitropism and/or auxin transport. The knockdown mutant ppla-I-1 of patatin-related phospholipase-A-I is delayed in root gravitropism and forms increased numbers of root coils. Three known factors contribute to waving: circumnutation, gravisensing and negative thigmotropism. In microgravity, deprivation of wild type (WT) and mutant roots of gravisensing and thigmotropism and circumnutation (known to slow down in microgravity, and could potentially lead to fewer waves or increased coiling in both WT and mutant). To resolve this, mutant ppla-I-1 and WT were grown in the BIOLAB facility in the International Space Station. In 1-g, roots of both types only showed waving. In the first experiment in microgravity, the mutant after 9days formed far more coils than in 1-g but the WT also formed several coils. After 24days in microgravity, in both types the coils were numerous with slightly more in the mutant. In the second experiment, after 9days in microgravity only the mutant formed coils and the WT grew arcuated roots. Cell file rotation (CFR) on the mutant root surface in microgravity decreased in comparison to WT, and thus was not important for coiling. Several additional developmental responses (hypocotyl elongation, lateral root formation, cotyledon expansion) were found to be gravity-influenced. We tentatively discuss these in the context of disturbances in auxin transport, which are known to decrease through lack of gravity.

Numerical Investigation of Water Droplet Impact on Solid Surface in Microgravity

A numerical study based on VOF model has been carried out to investigate the dynamics of water droplet impact on solid surface in microgravity in comparison with that in normal gravity to discuss the differences of the extinguishing mechanism of water mist in different gravity level. Water droplets with different initial diameters and impact velocities were considered. The simulated results show that the deformation process in microgravity lags behind that in normal gravity. And it was also found that Dmaxand spread velocities are smaller in microgravity as the potential energy decreases and the time taken for a liquid droplet to reach its maximum spread has no obvious regularity. Hence, the effect of cooling the fuel surface and diluting fuel vapour with water mist in microgravity may be not as good as that in normal gravity.The critical impact Weber number for water droplet breaking up in microgravity is lower than that in normal gravity as the reduction of the value of Bond number, which may result in diluting fuel vapour with water mist in microgravity being more effective than that in normal gravity in some case.

Bubble Behavior and Heat Transfer of Nucleate Pool Boiling on Micro-Pin-Finned Surface in Microgravity

Nucleate pool boiling on micro-pin-finned surface structure is proposed for efficiently cooling electronic components with high heat flux in microgravity, and was verified by experiments performed utilizing the drop tower Beijing. Micro-pin-fins with the dimensions of 50 × 60 μm2 (thickness × height) and the space of 50 μm were fabricated on the chip surface by the dry etching technique. FC-72 was used as the working fluid. Nucleate pool boiling of FC-72 on a smooth surface was also tested for comparison. Unlike much obvious deterioration of heat transfer of nucleate pool boiling on the smooth surface in microgravity, constant heater surface temperature of nucleate pool boiling for the micro-pin-finned surface was observed, even though a large coalesced bubble completely covered the surface under microgravity condition. The performance of high efficient heat transfer on micro-pin-finned surface is independent of the gravity, which stems from the sufficient supply of fresh liquid to the heater surface due to the capillary forces.

DEPRESSURIZATION EFFECTS ON THE THERMAL FIELDS AND HEAT TRANSFER DURING HEMI-SPHERICAL BUBBLE GROWTH ON A HEATED SURFACE

A numerical simulation has been carried out which predicts the growth and wall heat transfer characteristics of a bubble atop a heated flat surface in an otherwise quiescent pool of liquid. In accordance with the experimental conditions and observations of Merte et al [29] the simulations are carried out on a constant heat flux surface in microgravity and the bubble maintains a hemispherical shape with no contribution of a microlayer. The model, computational technique and interface tracking methodology provide very high spatial and temporal resolution. This is true for the micron sized nucleus in metastable equilibrium with its surrounding liquid at the end of the measured waiting time, through the surface tension, transition and heat transfer controlled growth domains where the bubble expands to macrosized. The simulations indicate that the rapid depressurisation of the vapour bubble as it expands occurs in conjunction with a like drop in the vapour temperature. This establishes a substantial temperature gradient and subsequent evaporative cooling effect of the heater surface near the moving triple interface. The influence of bulk liquid advection and transient conduction are discussed in relation to the bubble dynamics.

Quasi-homogeneous boiling nucleation on a small spherical heater in microgravity

Pool boiling experiments were conducted in the European Space Agency (ESA) multi-user facility, the bubble, drop, particle unit (BDPU) in the microgravity environment of space. A part of the study involved the heating of a small sphere immersed in R-123 to the onset of nucleate boiling. An analysis of the nucleation process is presented, based on a prior work for so-called quasi-homogeneous nucleation with a flat heater surface in microgravity. Reasonably good qualitative agreement exists between the analysis and measurements.

Bubble growth in a uniform and spatially distributed temperature field

A theory has been developed which has been shown to predict experimental bubble growth data for both spherical growth in an unbounded liquid and hemispherical growth at a heated plane surface in microgravity. The theory is able to accommodate both spatial and temporal variations in the temperature and velocity fields in the liquid surrounding the bubble as it grows. Utilising the present theory, the complicated thermal and hydrodynamic interactions between the vapour, liquid and solid have been manifested for a single isolated bubble growing on a heated plane surface from inception.

Terrestrial and microgravity boiling heat transfer in a dielectrophoretic force field

In order to maintain steady nucleate boiling in microgravity another force must be imposed on the boiling process to replace the buoyancy force. The objective of this study is to investigate the effectiveness of a static electric field for maintaining nucleate boiling in microgravity. Semi-transparent gold-film heaters are used to measure the instantaneous average heater surface temperature and to provide a bottom view of the boiling process. Three electrode geometries are designed with this heater: a diverging plate, a flat plate, and a pin electrode. Depending on the heat flux level and voltage strength, it was found that each of these electrodes is able to effectively move the boiling bubbles away from the vicinity of the heater surface in microgravity. Both flow visualization and measured heat transfer data are obtained to verify these results.