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    Mechanisch adaptive nanoporöse Metalle durch reversible Metallelektroabscheidung
    Nanoporous metals have attracted significant attention due to their nanostructure-enabled mechanical, optical, and electrochemical properties. Of particular interest are materials with reversibly tunable macroscopic mechanical behaviors, which are essential for adaptive systems capable of responding dynamically to external stimuli. Controlled, reversible modulation of properties such as stiffness, strength, and shape is highly desirable for next-generation smart and multifunctional devices, including soft robotics and artificial muscles. In the past two decades, advances in nanoscale architecture have enabled unprecedented control over mechanical properties through surface and interface effects, facilitated by the high surface-to-volume ratios inherent in these materials. While reversible tuning of strain, namely actuation, in nanoporous metals is well established, reversible tuning of other mechanical properties remains relatively underexplored, especially beyond noble-metal systems. Prior studies have largely focused on electrochemical surface charging and hydroxide adsorption as mechanisms for tunability. This project introduces an alternative and broadly applicable approach: reversible metal electrodeposition (RME). RME offers expanded tunability and compatibility with earth-abundant, cost-effective metals. This project will employ in situ mechanical testing to demonstrate significant, reversible modulation of stiffness and strength in nanoporous metals during RME. The work will also extend to hierarchical nanoporous architectures with dual-scale porosity to optimize the interplay between surface area, mass transport, mechanical performance, and structural integrity. To uncover the underlying mechanisms, we will combine in situ mechanical testing with microstructural characterization such as scanning electron microscopy, transmission electron microscopy and ultra-/small angle X-ray scattering methods. We will elucidate critical structure–property–function relationships by examining how features such as ligament size, coating thickness, hierarchical porosity, and interface characteristics influence the tunable mechanical response. Ultimately, this research aims to establish a hybrid metal–electrolyte nanomaterial system capable of rapid, large-amplitude mechanical switching that could enable a new class of adaptive structural materials and smart devices. Furthermore, we will explore electrodeposition as a fabrication pathway for core–shell nanoporous metal/metal nanocomposites, targeting the rare and valuable combination of high strength and tensile ductility.
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    EXC 3120 BlueMat - Research Topic B3: Tunable Hydraulic and Capillarity-Driven Water Flow
    In conventional porous media, hydraulic permeability and capillary pressure during imbibition are determined by static geometry, water–solid interaction, and fluid viscosity. RT B3 aims to integrate mechanisms for stimulus-dependent changes in the pore size and solid- liquid interaction, resulting in tunable permeability and hydrophilicity-induced changes of local capillary forces. Combining this strategy with multiscale 3D structuring and local interface modification will enable materials that can locally and reversibly switch imbibition ability and kinetics. Challenges include the fabrication of multiscale 3D structures and precise hydrophilicity change at local levels. This RT focuses on understanding dynamic fluid–solid interactions upon reversible tuning by external stimuli in 3D multiscale porous structures. Key questions include: How can we explore the water–solid interfacial structure and measure the impact of tunable pore size and hydrophilicity on fluid transport? How can we tailor the flow and permeability rates by adjusting multiscale porosity (“Murray vascularization”) and response to stimuli? How can we fabricate structured 3D (nano)porous materials with locally and reversibly tunable permeability and hydrophilicity?
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    EXC 3120 BlueMat - Research Topic A1: Hydrogen-Driven Nanoporous Materials
    We will develop a hydrogen-enabled acoustic metamaterial. The key parameter is the wavelength of sound, which needs to be (1) small (in the order of mm to cm) to enable miniaturization and (2) tunable to enable device switching. The wavelength is determined by the ratio of elastic modulus to mass density. As these parameters are usually strongly correlated, mate- rials with the required properties do not currently exist. A promising approach, demonstrated in preliminary research by the members of the RA A team, is np-Pd, which substantially changes its elastic compliance when alloyed with hydrogen. Thus, a metal/water hybrid nanomaterial achieves switching when hydrogen is relocated between its metal and water phases under the control of an electric potential bias. This new effect implies that one and the same metallic body can be reversibly switched between a hard-matter-like elastic response characteristic of metals and a supercompliant characteristic of soft matter, such as polymers or biological tissue. The key scientific questions are: How can the underlying materials design be shaped in terms of base metal composition and microstructure? How can the open-system elastic response in a nanoscale material with complex microstructural geometry be understood? How can processing routes for components that meet the requirements of Topic A4 be established?
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    EXC 3120 BlueMat - Research Topic A3: Water Absorption-Driven Nanoporous Materials
    We aim to establish and enhance mechanical actuation based on the reversible switching of soft, water-filled hydrogels in response to external triggers (e.g., pH, ionic strength, electrical potential, CO2). For the first time, we aim to realize such an actuation on both nm- (ligament/pore scale) and mm-scopic levels by embedding soft responsive hydrogels in a hard, porous matrix (np-Au from A2) with responsive hydrogels, thus combining reversible switchability in aqueous media with significant macroscopic response and local controllability. We will focus on cellulose-based hydrogels, known for their inherent responsiveness in aqueous environments [Don21]. The envisioned materials will refine and optimize hydrogel actuation for broader applicability in diverse devices. The precise control of mechanical properties of hydrogels needed for active control of actuation is a challenge: although fundamental mechanisms are understood, there is a lack of research aiming at the predictable control of overall hydrogel actuation, which is necessary to pave the way for the application of these sustainable materials in devices [Che22]. Key scientific questions include: How can we tune hydrogel/wall interactions for optimal integration of hydrogels into a porous metal matrix forming a hybrid material? How can we use engineered biofilm matrix-enzyme interactions to maximize the actuation of hydrogel/metal hybrid systems in response to external stimuli? How can we predict/model the hybrid material response and thus define optimal mechanical actuation?
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    EXC 3120 BlueMat - Research Topic A2: Electrolyte-Driven Nanoporous Materials
    Nanoporous metals, characterized by their high surface-to-volume ratio, have demonstrated promising surface-controlled chemo-mechanical coupling behaviors that can rival or surpass those of bulk functional materials. These behaviors include bidirectional actuation/sensing and switchable elastic compliance. However, there is a trade-off between amplitude and response time. Our goal is to significantly enhance functional performance by designing multiscale hierarchical nanoporous materials that harmonize large amplitude and rapid switching. Nanoscale ligaments at lower hierarchy levels provide high specific surface area for enhanced functional amplitude, while larger pore channels at upper hierarchy levels facilitate efficient mass transport, accelerating response time. Reducing the smallest ligament/pore sizes to single-digit nanometers will create a huge surface area, enabling (1) significant modulation of the elastic modulus and sound velocity in-operando and (2) substantial enhancement of actuation/sensing output signals through reversible electrochemical surface oxidation in an aqueous environment. This provides the basis for sustainable robotic materials for autonomous locomotion and self-reconfiguration. Key scientific questions include: How can we refine and stabilize the microstructure at the lowest hierarchical level while achieving scalable production of these materials in the form of monolithic and mechanically loadable bodies? How can we maximize both the switching kinetics and amplitude of output signals for bidirectional sensing/actuation and switchable mechanical/acoustic properties?
      60
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