The Drop Coalescence Project (DROPCOAL) aims to explore how droplets coalesce and mix in the microgravity environment of the International Space Station (ISS). This research will provide valuable insights and lead to significant progress for biomedical and technological applications during long-term space missions.
The primary objective of the experiment is to investigate the interactions between two drops of different liquids of medical interest—such as water, ethanol, and Methylene Blue—while varying their diameters and contents and maintaining controlled relative approaching velocities.
The device that will allow us to perform these experiments is set to be launched on November 4, 2024, as part of SpaceX’s 31st resupply services mission. It will be carried by the Dragon spacecraft from Launch Complex 39A at NASA's Kennedy Space Center in Florida. The DROPCOAL payload will be installed in the ICE Cubes Facility within the European Physiology Module (EPM) Rack integrated into the Columbus module.
This project was conceived and proposed by a collaborative Science Team (ST) comprising the science coordinator unit that is the National Institute for Lasers, Plasma and Radiation Physics (INFLPR) in Romania, the Technical University Darmstadt (TUD) in Germany, and Carnegie Mellon University in the United States. This international partnership developed the Experiment Scientific Requirements (ESR), which serves as the framework for the experimental device titled “Setup for Investigation of Drops Coalescence in View of Medical Applications”.
Click here to find relevant information about how "droplets unite" on the ESA Science & Exploration website.
From an Experimental Concept to a Functional Payload
Starting from the Experiment Scientific Requirements developed by the Science Team, the European Space Agency (ESA) initiated a bidding process to select a contractor to build the device. Romanian In Space Engineering (RISE) in Măgurele, Bucharest, was selected to design and produce the device after winning the competitive bid. The payload, known as DROPCOAL, meets the requirements set by ESA and adheres to a series of established standards and specifications.
Click here to find more about the device development and testing.
Coalescence: The secret behind how droplets merge
Coalescence refers to the process of two or more droplets, bubbles, or particles merging to form a single, larger entity. A simple way to understand coalescence is to think about what happens when it rains. Rain formation involves a fascinating process called coalescence, where tiny water droplets in clouds collide and merge to create larger droplets. At first, these small droplets are too light to fall to the ground because warm air is lifting them. But as they collide with each other, they merge and grow bigger and heavier. Eventually, they become so large that they can no longer be supported by the warm air, and they fall to the ground as rain.
Why to study how droplets coalesce in microgravity?
Fluid dynamics in microgravity is especially intriguing because the low level of gravity makes buoyancy, thermal convection, and sedimentation insignificant, allowing for unique observations of liquid behavior.
Droplet coalescence and mixing are complex processes influenced by multiple factors. Evaporation and vapor flow around droplets can alter their interactions. Capillary and viscous forces shape the droplets and control their movement, while Marangoni stresses, driven by surface tension differences, create fluid flow. Additionally, flow instabilities within droplets affect the efficiency and type of their mixing.
During the experiments, different combinations of Methylene Blue water solution, pure water, and a water-ethanol solution will be used. This will allow us to explore the effects of surface tension gradients by two liquids with different surface tensions, water and ethanol. Additionally, Methylene Blue is used as a tracking dye to observe the mixing of liquids. The integrated humidity sensors will allow to analyze the evaporation rate of water. In microgravity, vapors tend to stay around the droplets, as the concepts of "heavy" or "light" become irrelevant, making surface tension one of the dominant forces.
To study the modifications in the shape of the droplets, the evolution of the liquid bridge, the oscillations dampening, and the mixing of liquids, it will be used a high-speed camera able to record up to 8000 frames per second (fps). The videos will be downloaded and post-processed to analyze and interpret the collected information. In addition to the images, we will analyze the data obtained from all the environmental sensors integrated into the device and the stamped data retrieved from the ISS sensors.
In each test, pairs of droplets between 2 and 5 mm in diameter will collide at controlled speeds ranging from 0.01 mm/s to 10 mm/s. One of the reasons for choosing these velocities is the time needed for the gas layer between the droplets to escape, the gas layer acting as a barrier for coalescence.
All of these will lead to over six months of experiments on ISS, and at least 560 runs will be conducted to study how droplets of coalesce in microgravity.
Despite active research in this area, droplet coalescence remains one of the biggest challenges in fluid mechanics. By performing these studies in microgravity, we hope to gain clearer insights into the fundamental behavior of fluid dynamics in space. The insights gained from this research will be useful for future human space exploration. Preparing medications and delivering doses as droplets to treat eye, nose, and skin or injections, and intravenous fluids are considered future applications.
The Origins of the DROPCOAL Project
The initial proposal for the DROPCOAL project was submitted to the “Orbit Your Thesis” program in 2019 by the INFLPR team. The DROPCOAL project officially started in October 2019 and builds on the flight experience and results gained by the Romanian team in two previous projects:
The HyperMed project was part of the European Space Agency’s “Spin Your Thesis” program, conducted at ESTEC in Noordwijk, Netherlands, in September 2015. This project focused on studying tiny droplets (microliter-sized) of water and phenothiazine solutions (medicines). The research team analyzed how these droplets behaved when they interacted with different surfaces, hydrophilic or hydrophobic. The experiments were conducted in conditions simulating hypergravity, where the force of gravity was increased to levels between 2 and 20g. Click here for more information about the HyperMed project on the ESA website.
The DropTES 2018 project was part of the United Nations Office for Outer Space Affairs (UNOOSA) Drop Tower Experiment Series, conducted at the ZARM Drop Tower in Bremen, Germany, in November 2018. In this project, researchers studied how droplets of a medicine called chlorpromazine, which were treated with a laser, interacted with aluminum surfaces in short-term microgravity conditions. For more information about this project, click on ZARM or UNOOSA .
Meet the DROPCOAL Science Team
The DROPCOAL project is primarily led by a Science Team composed of young scientists, including PhD and master's students. The project was initially coordinated by Prof. Mihail Lucian Pascu, who oversaw its development from the proposal stage through its approval by the European Space Agency (ESA) in the first years. In 2021, Dr. Mihai Boni became the Science Team coordinator.
The DROPCOAL Science team today includes leading researchers from:
- INFLPR - Laser Department - Laser Spectroscopy and Optics Group (SOL):
- Prof. Mihail Lucian Pascu
- Dr. Mihai Boni (coordinator)
- Dr. Ionut-Relu Andrei
- Dr. Agota Simon (deputy coordinator)
- Ing. Ionut-Petrisor Ungureanu
- Faculty of Physics, University of Bucharest:
- Prof. Mircea Bulinski
- TUD - Institute for Fluid Mechanics and Aerodynamics:
- Prof. Ilia Roisman
- Dr. Ing. Jan Breitenbach
- Dr. Ing. Benedikt Schmidt
- Dr. Ing. Simon Burgis
- TUD - Mathematical Modeling and Analysis Department
- Prof. Dieter Bothe
- Carnegie Mellon University:
- Prof. Stephen Garoff
The Science Team benefited from the permanent contribution and support by Dr. Sebastien Vincent-Bonnieu, from the Directorate of Human and Robotic Exploration, ESA.
The previous experience of colleagues from TU Darmstadt in the framework of the Collaborative Research Center SFB TRR 75, brings valuable expertise in supporting the theoretical modeling of the gas gap, and computations of drop coalescence. This experience will contribute significantly to the CFD computations and overall research goals of the project.
The research component of the DROPCOAL project is further strengthened by the contributions of many additional researchers from our institutes and collaboration with industry specialists.
Over the past five years, we have dedicated ourselves to the DROPCOAL project, conducting extensive experiments both in Earth’s gravity (1 g) and in microgravity environments to prepare for the upcoming mission on the International Space Station. This thorough approach has provided us with results under various conditions, ensuring a rich database for our research.
Parabolic Flight Campaign - Experiments in Microgravity
To examine different needle geometries and outer coatings related to the formation of the most stable drops, a microgravity experiment was conducted during the DropCoal project by the Science Team. This series of exciting experiments took place during the 79th ESA Parabolic Flight Campaign (PFC) in October 2022 at Novespace, a subsidiary of the French Space Agency (CNES, Centre National d’Études Spatiales) in Bordeaux-Mérignac, France. The campaign was funded by the European Space Agency (ESA) and the German Aerospace Center (DLR), with experiments carried out onboard the Airbus A310 ZERO-G aircraft.
The Romanian team was sponsored by Hamilton, throught Hamilton Syringe Grant (more details here) and financially supported by INFLPR.
The ESA Parabolic Flight Campaign provided several advantages, including the ability to conduct multiple microgravity phases on each flight day, with several flights throughout the two-week duration. Researchers were directly involved in their experiments during breaks between the parabolic flights.
Ultimately, these experiments aimed to select and identify the important characteristics of the needles for future use in DROPCOAL experiments. Click here for more information about our experience in the 79th ESA Parabolic Flight Campaign.
Check out the large, spherical droplets captured in our microgravity experiment, now featured on the ESA website. Click here!
Support Experiments Conducted in Earth-Based Laboratories
Different experiments and support activities were performed by the Science Team. At the INFLPR laboratories, for instance, over 8 months, we carried out Chemical Compatibility Tests on various components of the fluidic circuit using ethanol and water. As part of this process, we performed bi-monthly surface tension measurements on different liquid samples received from the payload developer. We closely monitored whether the surface tension of ethanol increased due to material permeability, indicating a decrease in the Ethanol concentration, or if the surface tension of water decreased due to contamination from polymers or other impurities.
Additional Chemical Compatibility Tests were conducted using optical characterization to evaluate various superhydrophobic coatings for the needles and anti-fog coatings for the optical window. Their stability in time was evaluated after they were exposed to Ethanol.
We developed a cleaning protocol and tested it by surface tension measurements, which is essential for maintaining the integrity of our experiments in the DROPCOAL project. We tested one valve assembly by deliberately contaminating it with to determine the optimal number of flushes. To clean the needles, internal channels, and valves, we rinsed them with water to remove contamination, and for each rise, measurements of surface tension were performed to determine the concentration of the contaminant.
A long time was invested in finding the best materials from which the needles were to be constructed. Some tests in this case were performed at INFLPR premises, by classical optical characterization in different custom experimental setups, but also by methods such as XPS and SEM.
Perspectives
The DROPCOAL project has opened up new possibilities, set to be explored starting in 2025. One upcoming milestone is the deployment of a modified DROPCOAL payload aboard the Nyx capsule, which will be launched on a suborbital flight providing approximately 30 minutes of microgravity. Unlike previous experiments on the ISS, this setup will use a higher 50% ethanol solution, replacing the prior 10% concentration. The device will autonomously perform six coalescence experiments under controlled conditions.
The second aspect is the analysis of the data obtained during the ISS mission, to understand the binary droplet coalescence process in microgravity.
