Tobias Armstrong

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Armstrong

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Tobias

ORCID

0000-0003-4805-9477

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Publications 1 - 6 of 6

Imparting scalephobicity with rational microtexturing of soft materials
Schmid, Julian; Armstrong, Tobias; Dickhardt, Fabian J.; et al. (2023)
Science Advances
Crystallization fouling, a process where scale forms on surfaces, is widespread in nature and technology, negatively affecting energy and water industries. Despite the effort, rationally designed surfaces that are intrinsically resistant to it remain elusive, due in part to a lack of understanding of how microfoulants deposit and adhere in dynamic aqueous environments. Here, we show that rational tuning of coating compliance and wettability works synergistically with microtexture to enhance microfoulant repellency, characterized by low adhesion and high removal efficiency of numerous individual microparticles and tenacious crystallites in a flowing water environment. We study the microfoulant interfacial dynamics in situ using a micro-scanning fluid dynamic gauge system, elucidate the removal mechanisms, and rationalize the behavior with a shear adhesive moment model. We then demonstrate a rationally developed coating that can remove 98% of deposits under shear flow conditions, 66% better than rigid substrates.
Designing Plastrons for Underwater Bubble Capture: From Model Microstructures to Stochastic Nanostructures
Wong, William S.Y.; Naga, Abhinav; Armstrong, Tobias; et al. (2024)
Advanced Science
Bubbles and foams are often removed via chemical defoamers and/or mechanical agitation. Designing surfaces that promote chemical-free and energy-passive bubble capture is desirable for numerous industrial processes, including mineral flotation, wastewater treatment, and electrolysis. When immersed, super-liquid-repellent surfaces form plastrons, which are textured solid topographies with interconnected gas domains. Plastrons exhibit the remarkable ability of capturing bubbles through coalescence. However, the two-step mechanics of plastron-induced bubble coalescence, namely, rupture (initiation and location) and subsequent absorption (propagation and drainage) are not well understood. Here, the influence of 1) topographical feature size and 2) gas fraction on bubble capture dynamics is investigated. Smaller feature sizes accelerate rupture while larger gas fractions markedly improve absorption. Rupture is initiated solely on solid domains and is more probable near the edges of solid features. Yet, rupture time becomes longer as solid fraction increases. This counterintuitive behavior represents unexpected complexities. Upon rupture, the bubble's moving liquid-solid contact line influences its absorption rate and equilibrium state. These findings show the importance of rationally minimizing surface feature sizes and contact line interactions for rapid bubble rupture and absorption. This work provides key design principles for plastron-induced bubble coalescence, inspiring future development of industrially-relevant surfaces for underwater bubble capture.
Nanostructured Surfaces Enhance Nucleation Rate of Calcium Carbonate
Armstrong, Tobias; Schmid, Julian; Niemelä, Janne-Petteri; et al. (2024)
Small
Nucleation and growth of calcium carbonate on surfaces is of broad importance in nature and technology, being essential to the calcification of organisms, while negatively impacting energy conversion through crystallization fouling, also called scale formation. Previous work studied how confinements, surface energies, and functionalizations affect nucleation and polymorph formation, with surface-water interactions and ion mobility playing important roles. However, the influence of surface nanostructures with nanocurvature-through pit and bump morphologies-on scale formation is unknown, limiting the development of scalephobic surfaces. Here, it is shown that nanoengineered surfaces enhance the nucleation rate by orders of magnitude, despite expected inhibition through effects like induced lattice strain through surface nanocurvature. Interfacial and holographic microscopy is used to quantify crystallite growth and find that nanoengineered interfaces experience slower individual growth rates while collectively the surface has 18% more deposited mass. Reconstructions through nanoscale cross-section imaging of surfaces coupled with classical nucleation theory-utilizing local nanocurvature effects-show the collective enhancement of nano-pits.
Three-Dimensional Metallic Surface Micropatterning through Tailored Photolithography-Transfer-Plating
Chen, Liyang; Schmid, Julian; Platek-Mielczarek, Anetta; et al. (2024)
ACS Applied Materials & Interfaces
Precise micropatterning on three-dimensional (3D) surfaces is desired for a variety of applications, from microelectronics to metamaterials, which can be realized by transfer printing techniques. However, a nontrivial deficiency of this approach is that the transferred microstructures are adsorbed on the target surface with weak adhesion, limiting the applications to external force-free conditions. We propose a scalable "photolithography-transfer-plating" method to pattern stable and durable microstructures on 3D metallic surfaces with precise dimension and location control of the micropatterns. Surface patterning on metallic parts with different metals and isotropic and anisotropic curvatures is showcased. This method can also fabricate hierarchical structures with nanoscale vertical and microscale horizontal dimensions. The plated patterns are stable enough to mold soft materials, and the structure durability is validated by 24 h thermofluidic tests. We demonstrate micropatterned nickel electrodes for oxygen evolution reaction acceleration in hydrogen production, showing the potential of micropatterned 3D metallic surfaces for energy applications.
Nanoengineering Scalephobic Surfaces for Liquid Cooling Enhancement
Schmid, Julian; Armstrong, Tobias; Denz, Niklas; et al. (2024)
Advanced Materials Interfaces
Crystallization fouling, a process where mineral scales form on surfaces, is of broad importance in nature and technology, negatively impacting water treatment and electricity production. However, a rational methodology for designing materials with intrinsic resistance to scaling and scale adhesion remains elusive. Here, guided by nucleation physics, this work investigates the effect of coating composition and surface structure on the nucleation and growth mechanism of scale on metallic heat transfer surfaces nanoengineered by large-area techniques. This work observes that on hydrophilic nanostructured copper, despite its significantly enlarged surface area compared to smooth surfaces, scale formation is substantially suppressed leading to sustained, efficient cooling performance. This work reveals the mechanism through thermofluidic modeling coupled with in situ optical characterization and show that surface bubble formation through degassing is responsible for generating local hot spots enhancing supersaturation. This work then demonstrates a scalephobic nanostructured surface which reduces the accumulated surface scale mass 3.5x and maintains an 82% higher heat transfer coefficient compared to superhydrophobic surfaces with corresponding energy conversion savings. This work not only advances the understanding of fouling mechanisms but also holds promise for practical applications in industries reliant on efficient heat transfer processes.
Nucleation Mechanisms of Mineral Scale on Nanostructures Toward Rational Scalephobic Surfaces
Armstrong, Tobias (2024)
Inorganic mineral scales like calcium carbonate and calcium sulfate are abundant materials used in various industries such as construction, agriculture, and medicine. These minerals are also inversely soluble in water and can form crystallization fouling layers, known as scale, on surfaces when they exceed their solubility limits. Fouling layers reduce the efficiency of industrial operations in the chemical industry, energy production, or water treatment. In addition to economic costs caused by reduced efficiencies, fouling stresses the sustainability of the interconnected and limited resources, water and energy—highlighting fouling as one of the critical challenges in the water-energy nexus. Mitigation strategies are essential, including surfaces preventing the fouling formation and facilitating the removal of existing fouling, also known as scalephobic surfaces. However, we currently cannot design such surfaces rationally due to a lack of fundamental knowledge about nucleation control mechanisms. The influence of nanostructures on the nucleation of mineral scales is not fully understood. It remains uncertain in both fundamental and complex applied environments. This thesis creates a scientific base for the influence of nanocurvature as a control mechanism on the nucleation of calcium carbonate, as well as the effects of nanostructures in complex applied environments. In the first part of the thesis, we investigate the isolated effect of surface nanocurvature on calcium carbonate nucleation and growth across various supersaturations. Nanoengineered surfaces alter the Gibbs free energy barrier, following predictions on size effects in classical nucleation theory. Our optical in situ measurements and ex situ spatial reconstructions demonstrate that these surfaces show magnitudes of higher nucleation rates unrelated to a surface area effect. Specifically, concave nanocurvatures, such as nano-pits, promote nucleation despite increasing the kinetic barrier through confinement in the nano-pits. No such enhancement is observed for microstructures without nanocurvature, highlighting its critical role in calcium carbonate formation. The orientation and polymorph of crystals on nanoengineered surfaces are equal to that on smooth surfaces, counterintuitive to findings on polymorphism in confined environments. The contact area of these crystals conforms to the underlying structure, resulting in interlocking structures and increased contact areas. These insights have profound implications for designing advanced surfaces. To prevent crystallization fouling, scalephobic surfaces should avoid concave nanocurvatures that favor nucleation. In the second part of the thesis, we systematically investigate calcium sulfate crystallization fouling using nucleation, adhesion, and heat transfer theories. A continuous flow fouling unit is employed to observe nucleation and growth on engineered heat transfer surfaces and assess fouling’s impact on heat transfer resistance. Microscopic optical in situ observations and heat transfer measurements reveal the substantial influence of persistent bubbles on heat transfer and scaling. We find that persistent bubbles worsen fouling by creating hot spots and contact lines, increasing local supersaturation and nucleation dynamics. Our experiments show that suppressing persistent surface bubbles is crucial in designing scalephobic surfaces to prevent fouling. Nanostructured, hydrophilic surfaces effectively inhibit persistent bubbles, enhancing heat transfer efficiency and reducing crystallization fouling. Even though these surfaces may contain disadvantageous nanocurvatures, as studied in the first part of this work, their ability to prevent persistent bubbles is crucial. This complexity highlights the potential of surfaces to address fouling by effectively preventing persistent bubbles. These insights are essential for designing effective scalephobic surfaces. In the third part of the thesis, we build on the previous part by investigating a highly water-affine, low-adhesion hydrogel coating under accelerated crystallization fouling conditions using the previous continuous flow unit. Hydrogels are potential coating materials that can inhibit persistent bubbles due to their high water affinity and show low adhesion underwater. The preliminary results show that the hydrogel allows extended operational periods compared to a rigid electropolished copper surface. Furthermore, in situ optical observations indicate that the improved performance is attributed to single crystal and crystal cluster detachment on the hydrogel surface, a phenomenon not previously observed in situ. The hydrogel’s enhanced cooling effectiveness highlights its potential for scalephobic surfaces. The understanding gained in this thesis on the nucleation mechanisms of mineral scale on nanostructures will provide valuable insights and is anticipated to offer additional design guidelines for engineered scalephobic surfaces. The findings presented have the potential to be applied across diverse fields, leading to innovative solutions in water treatment, energy production, and advanced materials, addressing critical challenges.

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Tobias Armstrong

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