Gustav Graeber

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Graeber

First Name

Gustav

ORCID

0000-0003-0658-6084

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Publications 1 - 10 of 10

Freezing Physics and Derived Surface Nano-Engineering for Spontaneous Deicing
Graeber, Gustav (2019)
Droplet freezing is important both in nature and in technology. In this thesis I investigate the fundamentals of freezing water droplets and derive design criteria for the development of intrinsically ice-repellent materials. Such icephobic surfaces could improve the performance and safety of a multitude of technical processes in energy and transport. This includes for example heat exchangers, where ice built-up reduces thermal transport, and airplane flight, where freezing of water on airfoils can result in catastrophic events. The thesis consists of three individual studies. In the first study we investigated how the environmental conditions during droplet freezing affect the freezing outcome. We found that evaporatively or convectively supercooled water droplets resting on solid substrate can self-remove during freezing. This phenomenon, which we termed self-dislodging, requires that the heat removal from the droplet’s free surface dominates the heat removal through the solid substrate. Consequently, the freezing front moves from the outside of the droplet towards the center and from the top to the bottom, resulting in a solid ice shell with an unsolidified core and an unfrozen droplet-substrate interface. We observed experimentally that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, due to the volumetric expansion associated with freezing, semi-frozen water is displaced towards the droplet-substrate interface lifting the freezing droplet away from the substrate. The combined effects of dewetting and lifting result in droplet self-removal. We found that the more the substrate is hydrophobic the more robust self-dislodging occurs. In the second study we examined how multiple water droplets interact during freezing in a low-pressure environment. Understanding droplet interactions during freezing is important as droplets do not appear in isolation, but always in groups. We found that the freezing of a supercooled droplet results in self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We suggest that as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble resulting in ice coverage of the solid surface. In the third study we introduced a controllable and upscalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture for improved repellency of viscous liquids. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7mPa s, which covers the viscosity of supercooled water down to -17 °C. Based on the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces that can repel supercooled water before it freezes and sticks to the surface. The three studies presented in this thesis address the topic of surface icing from three different angles, collaboratively covering a broad range of the problem. Only when taking into account the environmental conditions, freezing group dynamics and liquid solid interactions, robust icephobic surfaces can be designed in the future. With my thesis I contribute to this development process.
Leidenfrost droplet trampolining
Graeber, Gustav; Regulagadda, Kartik; Hodel, Pascal; et al. (2021)
Nature Communications
A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.
3D-Printed Surface Architecture Enhancing Superhydrophobicity and Viscous Droplet Repellency
Graeber, Gustav; Martin Kieliger, Oskar B.; Schutzius, Thomas M.; et al. (2018)
ACS Applied Materials & Interfaces
Spontaneous self-dislodging of freezing water droplets and the role of wettability
Graeber, Gustav; Schutzius, Thomas M.; Eghlidi, Hadi; et al. (2017)
Proceedings of the National Academy of Sciences of the United States of America
Morphing and vectoring impacting droplets by means of wettability-engineered surfaces
Schutzius, Thomas M.; Graeber, Gustav; Elsharkawy, Mohamed; et al. (2014)
Scientific Reports
Driven by its importance in nature and technology, droplet impact on solid surfaces has been studied for decades. To date, research on control of droplet impact outcome has focused on optimizing pre-impact parameters, e.g., droplet size and velocity. Here we follow a different, post-impact, surface engineering approach yielding controlled vectoring and morphing of droplets during and after impact. Surfaces with patterned domains of extreme wettability (high or low) are fabricated and implemented for controlling the impact process during and even after rebound —a previously neglected aspect of impact studies on non-wetting surfaces. For non-rebound cases, droplets can be morphed from spheres to complex shapes —without unwanted loss of liquid. The procedure relies on competition between surface tension and fluid inertial forces, and harnesses the naturally occurring contact-line pinning mechanisms at sharp wettability changes to create viable dry regions in the spread liquid volume. Utilizing the same forces central to morphing, we demonstrate the ability to rebound orthogonally-impacting droplets with an additional non-orthogonal velocity component. We theoretically analyze this capability and derive a We−.25 dependence of the lateral restitution coefficient. This study offers wettability-engineered surfaces as a new approach to manipulate impacting droplet microvolumes, with ramifications for surface microfluidics and fluid-assisted templating applications.
Freezing-induced wetting transitions on superhydrophobic surfaces
Lambley, Henry; Graeber, Gustav; Vogt, Raphael; et al. (2023)
Nature Physics
Supercooled droplet freezing on surfaces occurs frequently in nature and industry, often adversely affecting the efficiency and reliability of technological processes. The ability of superhydrophobic surfaces to rapidly shed water and reduce ice adhesion make them promising candidates for resistance to icing. However, the effect of supercooled droplet freezing—with its inherent rapid local heating and explosive vaporization—on the evolution of droplet–substrate interactions, and the resulting implications for the design of icephobic surfaces, are little explored. Here we investigate the freezing of supercooled droplets resting on engineered textured surfaces. On the basis of investigations in which freezing is induced by evacuation of the atmosphere, we determine the surface properties required to promote ice self-expulsion and, simultaneously, identify two mechanisms through which repellency falters. We elucidate these outcomes by balancing (anti-)wetting surface forces with those triggered by recalescent freezing phenomena and demonstrate rationally designed textures to promote ice expulsion. Finally, we consider the complementary case of freezing at atmospheric pressure and subzero temperature, where we observe bottom-up ice suffusion within the surface texture. We then assemble a rational framework for the phenomenology of ice adhesion of supercooled droplets throughout freezing, informing ice-repellent surface design across the phase diagram.
Honeycomb-structured metasurfaces for the adaptive nesting of endothelial cells under hemodynamic loads
Bachmann, Bjoern J.; Giampietro, Costanza; Bayram, Adem; et al. (2018)
Biomaterials Science
Scaling of Planar Sodium-Nickel Chloride Battery Cells to 90 cm² Active Area
Lan, Tu; Svaluto-Ferro, Enea; Kovalska, Natalia; et al. (2025)
Batteries & Supercaps
High-temperature sodium-nickel chloride (Na−NiCl2) batteries offer a competitive solution for stationary energy storage due to their long-term stability, high energy efficiency, and sustainable raw materials. However, scaling up this technology faces challenges related to the costly integration of tubular Na-β′′-alumina ceramic electrolytes into hermetically sealed battery cells. Alternative cell designs with a planar Na-β′′-alumina ceramic electrolyte have been a focus of research for many years, and a series of achievements were made on cell design, on reduction of the operating temperature, and on the analysis of electrochemical reaction mechanisms. However, the data presented in these reports was derived from laboratory-scale cells with small area (1–5 cm²). To date, there has been no research conducted on enlarging planar cells to an economically viable size. Here we report the fabrication of large planar Na-β′′-alumina electrolytes and their integration into planar Na−NiCl₂ cells with 90 cm² active area and >7 Ah capacity. Our cell design enabled cycling at 300 °C for three months, transferring a cumulative capacity of 323 Ah. We discuss design and engineering considerations for large planar high-temperature cells emphasizing the need for cell stacking to compete with tubular Na−NiCl₂ batteries in terms of mass-specific energy.
Cascade Freezing of Supercooled Water Droplet Collectives
Graeber, Gustav; Dolder, Valentin; Schutzius, Thomas M.; et al. (2018)
ACS Nano
Physics-based prediction of moisture-capture properties of hydrogels
Díaz-Marín, Carlos D.; Masetti, Lorenzo; Roper, Miles A.; et al. (2024)
Nature Communications
Moisture-capturing materials can enable potentially game-changing energy-water technologies such as atmospheric water production, heat storage, and passive cooling. Hydrogel composites recently emerged as outstanding moisture-capturing materials due to their low cost, high affinity for humidity, and design versatility. Despite extensive efforts to experimentally explore the large design space of hydrogels for high-performance moisture capture, there is a critical knowledge gap on our understanding behind the moisture-capture properties of these materials. This missing understanding hinders the fast development of novel hydrogels, material performance enhancements, and device-level optimization. In this work, we combine synthesis and characterization of hydrogel-salt composites to develop and validate a theoretical description that bridges this knowledge gap. Starting from a thermodynamic description of hydrogel-salt composites, we develop models that accurately capture experimentally measured moisture uptakes and sorption enthalpies. We also develop mass transport models that precisely reproduce the dynamic absorption and desorption of moisture into hydrogel-salt composites. Altogether, these results demonstrate the main variables that dominate moisture-capturing properties, showing a negligible role of the polymer in the material performance under all considered cases. Our insights guide the synthesis of next-generation humidity-capturing hydrogels and enable their system-level optimization in ways previously unattainable for critical water-energy applications.

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Gustav Graeber

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