Numerical estimation of droplet motion on linear wettability gradient surface in microgravity environment

Self-propelled motion of sessile droplets on gradient surfaces is key to the advancement of microfluidic, nanofluidic, and surface fluidic technologies. Precise control over droplet dynamics, which often involves liquid–vapor phase transitions, is crucial for a variety of applications, including thermal management, self-cleaning surfaces, biochemical assays, and microreactors. Understanding how specific phase changes like condensation and evaporation affect droplet motion is essential for enhancing droplet manipulation and improving transport efficiency. We use the thermal Navier–Stokes–Korteweg equations to investigate the effects of condensation and evaporation on the motion and internal dynamics of droplets migrating across a surface with a linear surface energy profile. The study focuses on the early dynamics of self-propelled motion of a phase changing droplet at sub-micron scale before viscous forces are comparable with the gradient forces. Our results demonstrate that phase change significantly affects the self-propelled motion of droplets by reshaping interfacial mass flux distributions and internal flow dynamics. Condensation increases droplet volume and promotes extensive spreading toward regions of higher wettability, while evaporation reduces both volume and spreading. These changes in droplet shape and size directly affect the driving forces of motion, augmenting self-propulsion through condensation and suppressing it during evaporation. Additionally, each phase change type generates distinct internal flow patterns within the droplet, with condensation and evaporation exhibiting unique circulatory movements driven by localized phase changes near the contact lines.

This review reports the recent progress and future prospects of wettability gradient surfaces (WGSs), particularly focusing on the governing principles, fabrication methods, classification, characterization, and applications. While transforming the inherent wettability into artificial wettability via bioinspiration, topographic micro/nanostructures are produced with changed surface energy, resulting in new droplet wetting regimes and droplet dynamic regimes. WGSs have been mainly classified in dry and wet surfaces, depending on the apparent surface states. Wettability gradient has long been documented as a surface phenomenon inducing the droplet mobility in the direction of decreasing wettability. However, it is herein critically emphasized that the wettability gradient does not always result in droplet mobility. Indeed, the sticky and slippery dynamic regimes exist in WGSs, prohibiting or allowing the droplet mobility, respectively. Lastly, the stringent bottlenecks encountered by WGSs are highlighted along with solution-oriented recommendations, and furthermore, phase change materials are strongly anticipated as a new class in WGSs. In all, WGSs intend to open up new technological insights for applications, encompassing water harvesting, droplet and bubble manipulation, controllable microfluidic systems, and condensation heat transfer, among others.

The gradient wettability surface is processed on the surface of Si3N4 ceramics using a 1030 nm femtosecond laser. Both the microdimple morphology, surface contact angle, and surface chemical structure are determined using optical microscopy, optical profilometry, contact angle measuring instrument, and X‐ray photoelectron spectroscopy (XPS). The effects of average laser power, scanning passes, and microdimple distribution density on both surface morphology and contact angle are also studied. Accordingly, the wettability surfaces of different gradients are processed by changing the microdimple distribution density, and the flow state of the cutting fluid on the gradient wettability surface is observed using a high‐speed camera. The results show that both the diameter and depth of the microdimple increase with the increase in the average laser power and scanning passes. As the average laser power increases, the surface contact angle gradually reduces and then increases. Moreover, because of the increase in scanning passes, the surface contact angle gradually decreases. The microdimple morphology is an important factor affecting the wettability of the sample surface. Reason construction of the surface microdimple distribution density can obtain different gradients of wettability surface. Furthermore, with a larger gradient, the wetting rate of the cutting fluid will be faster.

Interaction of Different Types of Cells on Polymer Surfaces with Wettability Gradient

In view of the great importance of two geometrical parameters such as void fraction and interfacial area concentration to the accurate two-phase flow analysis at microgravity conditions, axial developments of flow parameters such as void fraction, interfacial area concentration, bubble Sauter mean diameter, and bubble number density were measured in bubbly flow at microgravity and low liquid Reynolds number conditions where the gravity effect on the flow parameters were pronounced. A total of seven data sets were acquired in the flow range of the void fraction from 1.01% to 3.36% and the liquid Reynolds number from 1,400 to 4,750. The measurements were also performed in the similar flow range at normal gravity conditions. The transport mechanisms of the flow parameters are discussed in detail based on the data measured at normal and microgravity conditions, and the drift-flux model developed at microgravity conditions are compared with the measured data.

Alkali cyanides in the iron blast-furnace

T-Consciousness Fields (TCFs) are introduced as non-physical entities, with their effects initiated through the human mind. It is hypothesized that information transmitted via TCFs may influence the behavior of studied subjects. The present experiment aimed to investigate the effects of TCFs on cell cycle progression and ATP production in the HEK-293 cell line under both microgravity (MG) and Earth gravity (1G) conditions. To achieve this, cultured cells were exposed to TCFs for 24 hours following an initial 24-hour incubation period, under either microgravity (MG) conditions, simulated by clinostat rotation, or a standard Earth gravity (1G) environment. Untreated cells served as the control group. Flow cytometry was used to assess the distribution of cells across different phases of the cell cycle, and ATP concentration was measured by evaluating luciferase enzyme activity. The results showed a significant increase in the sub-G1 phase under microgravity (MG) conditions (p < 0.05), indicating elevated apoptosis. In contrast, TCFs-treated samples maintained sub-G1 levels comparable to those under Earth gravity (1G). Additionally, TCFs treatment significantly increased the S phase under MG (p < 0.05) and the G2 phase under 1G (p < 0.05). ATP concentrations were markedly reduced under MG in both treated and untreated samples compared to 1G (p < 0.05). However, in the clinostat environment, ATP levels in TCFs-treated samples were approximately twice as high as those in untreated controls (p < 0.05), whereas no significant difference was observed under 1G conditions. These findings suggest that the reduced sub-G1 phase in TCFs-treated samples under MG reflects apoptosis inhibition, while increased ATP levels and prolonged S phase indicate enhanced cellular efficiency under stress. The distinct outcomes under MG and 1G imply that the information transmitted by TCFs may vary depending on environmental conditions. Further studies are needed to elucidate the underlying mechanisms of TCF action.

Experimental investigation of liquid film thickness and heat transfer during condensation in microgravity

Ontogeny of plants under various gravity condition.

XRMON-SOL: Isothermal equiaxed solidification of a grain refined Al–20 wt%Cu alloy

Experimental testing of mini heat pipes under microgravity conditions aboard a suborbital rocket

Tuning the spontaneous motion of pure nanodroplets on programmed surfaces has progressed notably. However, impurities exist in droplets more often than not. We present molecular dynamics simulations of the dynamic behaviors of a mixture of water droplets and particles at the nanoscale, as they are placed on a wettability gradient substrate. We uncover three different styles of motion, namely, sweeping, separating, and pinning, as interaction strengths among the substances vary. By investigating the potential energies between the droplet and the nanoparticle/substrate, we explain the competition of the driving force and the impedance due to an imbalance of van der Waals force, which is responsible for differentiating the styles. We also find in some specific cases the phenomenon of tailing, where one side of the droplet is pinned by the nanoparticle as the other side moves forward to generate an extremely large difference of the contact angles. The results provide a fundamental understanding of the dynamic behaviors at the nanoscale and may inspire potential applications in a broad range, such as drug delivery, dust cleaning of nanomachines, liquid purification, etc.

Ammonia oxidation is an important electrochemical reaction needed in urine purification processes for water reclamation in the International Space Station (ISS). The objective of the microgravity experiment was to compare the ammonia oxidation capabilities of different catalysts at a half cell and in an alkaline fuel cell. Different Pt catalysts were studied, from nanoparticles on carbon supports to platinum nanocubes with preferential crystalline (100) sites. Under microgravity conditions the performance of a fuel cell was diminished by the absence of buoyancy since nitrogen gas was produced, blocking the catalytic sites. The following catalysts were studied: platinum nanocubes (ca. 10nm in diameter), platinum nanocubes on carbon Vulcan and platinum on carbon nanoonion support prepared by the RoDSE method [1]. This research has shown the difference between the performance of the catalysts in an ammonia fuel cell under microgravity and ground experimental conditions, in terms of normalized current densities [1]. In the case of a fuel cell with platinum black at the anode there is a reduction in the produced mass-normalized current densities of around 7.5% between the hypergravity and microgravity experimental episodes. This decrease was shown to be reversed when gravity effects were once again applied to the fuel cell, confirming the direct influence from microgravity on the electrochemical oxidation of ammonia. For the half cell, a decrease in performance of 20-65% over ground-based controls for the electrochemical oxidation of ammonia was found [2,3]. This decrease in current is theorized to be due to the lack of buoyancy driven mixing that occurs in microgravity. In microgravity only diffusional mixing occurs and, therefore, molecules may be unable to reach the nanocatalyst interface at the same rate as on the ground. During the first oxidation peak at ca. 0.7 V vs. Ag QRE, the formation of N2 molecule, during microgravity, conditions creates a stagnant gas-surface interaction that avoids ammonia molecules to reach the Pt surface. This situation leads to a significant peak current decrease compared to the ground experiments. On the other hand, during the second oxidation peak an oxynitride layer is formed at the interface of the electrode without the formation of N2 molecule. This latter peak does not have the effect of microgravity due to the surface chemistry reaction and the oxide adsorbates are no readily desorbed. Therefore, no major differences between the microgravity and ground conditions are observed in such an oxynitride layer region. These findings indicate that significant reductions in electrode performance for ammonia electrooxidation may occur if catalyst design measures are not taken in consideration to mitigate these microgravity effects. This conclusion is applicable specifically to electrochemical processes, but also may be relevant to all process constrained by nanoscale mixing of diffusion limited structures such as thermal catalysts, separation membranes, microbial growth and human cellular functions. For these microgravity experiments, a tailored electrochemical lab was designed and constructed that would handle 2g and 0g gravitational conditions (see Figure 1) [2,3]. References Poventud-Estrada, C. M.; Acevedo, R.; Morales, C.; Betancourt, L.; Diaz, D. C.; Rodriguez III, M. A.; Larios, E.; José-Yacaman, M.; Nicolau, E.; Flynn, M.; and Cabrera, C. R., “Microgravity Effect on Chronoamperometric Ammonia Oxidation at Platinum Nanoparticles Modified Mesoporous Carbon Supports”, Microgravity Science and Technology 2017, 29(5), 381-389. https://doi.org/10.1007/s12217-017-9558-5Acevedo, R.; Poventud-Estrada, C.M; Morales-Navas, C.; Martinez-Rodriguez, R.A.; Ortiz-Quiles, E.; Vidal-Iglesias, F.; Sollá-Gullón, J.; Nicolau, E.; Feliu, J.M.; Echegoyen, L.; Cabrera, C.R., “Chronoamperometric Study of Ammonia Oxidation in a Direct Ammonia Alkaline Fuel Cell under the Influence of Microgravity”, Microgravity Science and Technology 2017, 29(4),253–261. http://dx.doi.org/10.1007/s12217-017-9543-zNicolau, E., Poventud-Estrada, C.M., Arroyo, L., Fonseca, J., Flynn, M., Cabrera, C.R.: Microgravity effects on the electrochemical oxidation of ammonia: A parabolic flight experiment. Electrochim. Acta 2012, 75, 88-93. doi:10.1016/j.electacta.2012.04.079 Figure 1

This paper presents a novel technique for droplet transportation using a wettability gradient surface that is generated by EWOD (Electrowetting-on-dielectric). This technique can be achieved by applying a voltage to only two interdigitated and wedge-shaped electrodes: one coated with and the other without a dielectric SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film. Several experimental results showed that a decrease in SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thickness reduces the minimum required voltage, and that an application of square-wave voltage permits lower voltage transportation of a droplet than that of DC voltage. Also, droplet velocity depended on frequency of an applied square-wave voltage, and the velocity of a 15 mul deionized (DI) water droplet reached the maximum at 60 Hz.

Characteristics of developing vertical bubbly flow under normal and microgravity conditions