Browsing by Author "Nurmi, Heikki A."

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  • Long-term stability of aerophilic metallic surfaces underwater
    (2023-12) Tesler, Alexander B.; Kolle, Stefan; Prado, Lucia H.; Thievessen, Ingo; Böhringer, David; Backholm, Matilda; Karunakaran, Bhuvaneshwari; Nurmi, Heikki A.; Latikka, Mika; Fischer, Lena; Stafslien, Shane; Cenev, Zoran M.; Timonen, Jaakko V.I.; Bruns, Mark; Mazare, Anca; Lohbauer, Ulrich; Virtanen, Sannakaisa; Fabry, Ben; Schmuki, Patrik; Ras, Robin H.A.; Aizenberg, Joanna; Goldmann, Wolfgang H.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Aerophilic surfaces immersed underwater trap films of air known as plastrons. Plastrons have typically been considered impractical for underwater engineering applications due to their metastable performance. Here, we describe aerophilic titanium alloy (Ti) surfaces with extended plastron lifetimes that are conserved for months underwater. Long-term stability is achieved by the formation of highly rough hierarchically structured surfaces via electrochemical anodization combined with a low-surface-energy coating produced by a fluorinated surfactant. Aerophilic Ti surfaces drastically reduce blood adhesion and, when submerged in water, prevent adhesion of bacteria and marine organisms such as barnacles and mussels. Overall, we demonstrate a general strategy to achieve the long-term stability of plastrons on aerophilic surfaces for previously unattainable underwater applications.
  • Predicting plastron thermodynamic stability for underwater superhydrophobicity
    (2024-06-29) Tesler, Alexander B.; Nurmi, Heikki A.; Kolle, Stefan; Prado, Lucia H.; Karunakaran, Bhuvaneshwari; Mazare, Anca; Erceg, Ina; de Brito Soares, Íris; Sarau, George; Christiansen, Silke; Stafslien, Shane; Alvarenga, Jack; Aizenberg, Joanna; Fabry, Ben; Ras, Robin H.A.; Goldmann, Wolfgang H.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Non-wettable surfaces, especially those capable of passively trapping air in rough protrusions, can provide surface resilience to the detrimental effects of wetting-related phenomena. However, the development of such superhydrophobic surfaces with a long-lasting entrapped air layer, called plastron, is hampered by the lack of evaluation criteria and methods that can unambiguously distinguish between stable and metastable Cassie-Baxter wetting regimes. The information to evaluate the stability of the wetting regime is missing from the commonly used contact angle goniometry. Therefore, it is necessary to determine which surface features can be used as a signature to identify thermodynamically stable plastron. Here, we describe a methodology for evaluating the thermodynamic underwater stability of the Cassie-Baxter wetting regime of superhydrophobic surfaces by measuring the surface roughness, solid-liquid area fraction, and Young’s contact angle. The method allowed the prediction of passive plastron stability for over one year of continuous submersion, the impeding of mussel and barnacle adhesion, and inhibition of metal corrosion in seawater. Such submersion-stable superhydrophobicity, in which water is repelled by a stable passive air layer trapped between the solid substrate and the surrounding liquid for extended periods at ambient conditions, opens new avenues for science and technologies that require continuous contact of solids with aqueous media. (Figure presented.).
  • Super-Droplet-Repellent Carbon-Based Printable Perovskite Solar Cells
    (2024-07-10) Mai, Cuc Thi Kim; Halme, Janne; Nurmi, Heikki A.; da Silva, Aldeliane M.; Lorite, Gabriela S.; Martineau, David; Narbey, Stéphanie; Mozaffari, Naeimeh; Ras, Robin H.A.; Hashmi, Syed Ghufran; Vuckovac, Maja
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Despite attractive cost-effectiveness, scalability, and superior stability, carbon-based printable perovskite solar cells (CPSCs) still face moisture-induced degradation that limits their lifespan and commercial potential. Here, the moisture-preventing mechanisms of thin nanostructured super-repellent coating (advancing contact angle >167° and contact angle hysteresis 7°) integrated into CPSCs are investigated for different moisture forms (falling water droplets vs water vapor vs condensed water droplets). It is shown that unencapsulated super-repellent CPSCs have superior performance under continuous droplet impact for 12 h (rain falling experiments) compared to unencapsulated pristine (uncoated) CPSCs that degrade within seconds. Contrary to falling water droplets, where super-repellent coating serves as a shield, water vapor is found to physisorb through porous super-repellent coating (room temperature and relative humidity, RH 65% and 85%) that increase the CPSCs performance for 21% during ≈43 d similarly to pristine CPSCs. It is further shown that water condensation forms within or below the super-repellent coating (40 °C and RH 85%), followed by chemisorption and degradation of CPSCs. Because different forms of water have distinct effects on CPSC, it is suggested that future standard tests for repellent CPSCs should include rain falling and condensate formation tests. The findings will thus inspire the development of super-repellent coatings for moisture prevention.
  • Superhydrophobic Lubrication: Gas–Liquid Bilayer Reduces the Friction Between Two Solids
    (2022-02-14) Nurmi, Heikki A.; Yu, Cunming; Toptunov, Dmytro; Ras, Robin H.A.; Jokinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Lubrication is one of the most important ways to reduce the effect of friction, which is the single largest cause for energy losses in society. Typically, friction reduction is done by lubrication with petroleum-based oils, while technology focus is shifting toward environmentally-friendly green lubrication. Lowest friction coefficients with water-based lubrication have previously been achieved with smooth surfaces such as silicon carbide and silicon nitride or polyzwitterionic polymer brushes with typical coefficients of friction in the order of 0.002. Here, a novel concept for green lubrication using a bilayer of water and ambient air acting as the lubricant between a hydrophilic and superhydrophobic surface is shown. This method achieves superlubricity with friction coefficients down to 0.002 as measured with oscillating tribometer and tilting stage. In addition, possible applications for superhydrophobic lubrication such as tunable lubrication and a 2D mouse treadmill, are shown.
  • Superhydrophobic metrology and applications
    (2022) Nurmi, Heikki A.
     School of Science | Doctoral dissertation (article-based)
    Superhydrophobic surfaces repel water and exhibit useful properties like self-cleaning, anti-fogging, anti-icing, and anti-fouling. Current characterization techniques have trouble grading the superhydrophobicity of samples due to limits in force and optical resolution. Thus, more suitable measurement techniques are needed to grade superhydrophobic surfaces. The current standard method, contact angle goniometer (CAG), is compared to two force-based measurement techniques: oscillating droplet tribometer (ODT) and micropipette force sensor (MFS). In addition, the properties of superhydrophobic surfaces are explored for lubrication. Publication I compares the sensitivity of the MFS and CAG on measuring superhydrophobic surfaces. In this publication, MFS measures the contact angle hysteresis force and CAG measures the contact angles of superhydrophobic surfaces. For superhydrophobic surfaces, the MFS can easily distinguish even slight differences between the samples, while the CAG cannot differentiate the samples from each other. Publication II studies the sensitivity of the ODT and CAG on measuring superhydrophobic surfaces. In this publication, the viscous and contact angle hysteresis force is measured using ODT and the contact angles with CAG. For the measured superhydrophobic surfaces, the ODT can differentiate all the samples and even find slight differences in heat treated nanostructured copper samples, while the CAG cannot distinguish the samples from each other. Publication III uses the MFS in an oscillating mode, which is based on the model of the ODT. In this publication, levitating carbonated water and MilliQ water are used to study the viscous losses in the air layer between the droplet and the surface in superhydrophobic surfaces. A mathematical model is constructed to explain the losses in the system, and it is validated using experimental data. Publication IV explores the lubrication properties of a the superhydrophobic surface. A slippery air-water bilayer forms between superhydrophobic surface and a water layer. This bilayer is used to lower dissipation forces between two solids. These dissipation forces are measured using oscillating tribometer and tilted plane. Extreme level of lubrication is demonstrated at low velocities (v<1 m/s) and pressures (50 Pa) using this system with friction coefficients at the order of 0.001, which is on par with the state-of-the-art lubrication methods. This thesis demonstrates the accuracy of ODT and MFS for characterizing superhydrophobic surfaces and the limited suitability of CAG for characterizing superhydrophobic surfaces. The need for these type of measurement devices will increase, as more superhydrophobic surfaces and application enter the market. These accurate measurement devices will be critical for utilizing and commercializing the useful properties of superhydrophobic surfaces.