The European Research Council (ERC) has awarded a Starting Grant of approximately two million euros to Dr. Karin Schwarzenberger of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). In her project InterTrace, she wants to directly visualize flows at gas-liquid interfaces and measure them in a reliable manner for the first time. Custom-developed tracer particles will help answer fundamental questions regarding mass transport in technological as well as natural systems – from rising gas bubbles to the invasive floating fern species Salvinia molesta .
To measure flows in liquids, researchers often use Particle Image Velocimetry (PIV): Cameras track the movement of tiny added particles in the fluid, calculating the velocity and direction of the flow. However, this method fails at interfaces, for instance, between water and air. Conventional tracer particles accumulate and clump together at such an interface, altering the very flow they are supposed to visualize. Yet interfaces play a decisive role in determining how quickly gases and other substances are exchanged between two phases.
Surface-active molecules, known as surfactants, play a key role in this process. If they are unevenly distributed at an interface, they cause variations in surface tension. This generates forces along the interface where liquid tends to flow from the region of lower surface tension toward the region of higher tension, thereby driving motion in the adjacent liquid. This phenomenon is known as the Marangoni effect.
“We know that the mobility of an interface can have a decisive influence on mass transfer. But so far, we have hardly been able to directly observe when an interface remains mobile, when it becomes effectively immobile, and what happens to mass transfer in the process,” Schwarzenberger explains. “This is precisely the gap we aim to close with InterTrace.”
Two tracer systems for field and laboratory use
InterTrace therefore relies on two complementary strategies. The team is developing easily detectable Janus macrotracers for measurements in the field and directly within ongoing processes. These tracers are hollow glass spheres measuring less than a millimeter in diameter whose surfaces possess two different properties. During polymerization in ambient air, a water-repellent cap forms spontaneously on the part of the sphere that protrudes from the water. This way, the researchers hope to control precisely how the particle is anchored at the interface.
Under varying gravitational conditions, they test whether the tracers actually follow the interface flow without exerting a noticeable impact on it. Thanks to a parabolic flight campaign, the researchers are able to switch between microgravity and hypergravity, thereby isolating buoyancy effects from other influences. The goal is to achieve particles that are nearly buoyancy-neutral and neither slide off nor converge on curved interfaces.
The project is also developing fluorescent microtracers for high-resolution lab investigations. The tracers’ electric charge is adjusted to make the particles repel each other instead of clumping together. Gas bubbles trapped in a countercurrent serve to simultaneously measure interface motion and gas transport. The researchers run experiments with soluble and insoluble surfactants to reveal the interactions between Marangoni forces and molecular transport barriers.
“With InterTrace, Schwarzenberger is breaking new ground in experimentation: She makes it possible to measure previously unknown, crucial processes of mass transport in nature and technology. The ERC Starting Grant gives her the leeway she needs to develop a sophisticated new methodology, from basic research all the way to field applications. It is precisely this combination of scientific daring, technological innovation, and societal relevance that distinguishes cutting-edge research at the HZDR,” says Prof. Sebastian M. Schmidt, Scientific Director of HZDR.
Saxony’s Minister of Science, Sebastian Gemkow, comments: “I would like to warmly congratulate Dr. Karin Schwarzenberger. Funding from the European Research Council is awarded only to outstanding researchers. This once again demonstrates the excellence and high level of expertise that Saxony has to offer at the European level. I wish her every success and exciting new discoveries in her field of research.”
Understanding gas exchange in invasive aquatic plants
An unusual field test will take Schwarzenberger’s team to Louisiana in the United States, where Salvinia molesta – considered an invasive species in many parts of the world – is spreading rapidly in swamps, lakes, and ponds. Originally native to Brazil, this species of floating fern forms dense mats over bodies of water, impeding their use and disrupting ecosystems. Fine hairs on the plant’s leaves retain a persistent layer of air, known as a plastron, submerged in the water. This enables photosynthesis underwater, which might be contributing to the plant’s exceptionally rapid growth.
The researchers suspect that temperature-induced differences in surface tension trigger flow along this air layer, significantly accelerating the exchange of oxygen, carbon dioxide, and nutrients. Using the new macrotracers, they will try to visualize this interface convection, first in the laboratory, then in three field measurement campaigns in natural bodies of water.
The team plans to build on these insights to investigate whether biocompatible surfactants can systematically reduce gas exchange. In the long term, the results could offer new ways to curb the growth of this invasive plant. At the same time, the project promises to provide fundamental insights into natural gas exchange processes and technical applications – in chemical reactors, aeration systems, bioprocesses, and more.
Grant funds new research team
The five-year project will launch on 1 March 2027. In addition to equipment and consumables, the grant covers two doctoral positions, one post-doc position and funds for project management. ERC Starting Grants support outstanding early-career researchers in establishing an independent research program and building their own team. The funding is provided as part of the EU’s Horizon Europe research program.
Further information:
Dr. Karin Schwarzenberger | Head of Interfacial Phenomena
Institute of Fluid Dynamics at HZDR
Phone: +49 351 463 36 627 | Email: k.schwarzenberger@hzdr.de
Media Contact:
Simon Schmitt | Head
Communications and Media Relations at HZDR
Phone: +49 351 260 3400 | Mobile: +49 175 874 2865 | Email: s.schmitt@hzdr.de
The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the Dresden High Magnetic Field Laboratory and the ELBE Center for High-Power Radiation Sources. HZDR is a member of the Helmholtz Association and has seven sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Rostock, Schenefeld near Hamburg) with almost 1,500 members of staff, of whom about 700 are scientists, including 200 Ph.D. candidates.