Browsing by Author "Vuckovac, Maja"
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Item 3D Printing of Superhydrophobic Objects with Bulk Nanostructure(WILEY-V C H VERLAG GMBH, 2021-11-11) Dong, Zheqin; Vuckovac, Maja; Cui, Wenjuan; Zhou, Quan; Ras, Robin H.A.; Levkin, Pavel A.; Department of Applied Physics; Department of Electrical Engineering and Automation; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Robotic Instruments; Karlsruhe Institute of TechnologyThe rapid development of 3D printing (or additive manufacturing) technologies demands new materials with novel properties and functionalities. Superhydrophobic materials, owing to their ultralow water adhesion, self-cleaning, anti-biofouling, or superoleophilic properties are useful for myriad applications involving liquids. However, the majority of the methods for making superhydrophobic surfaces have been based on surface functionalization and coatings, which are challenging to apply to 3D objects. Additionally, these coatings are vulnerable to abrasion due to low mechanical stability and limited thickness. Here, a new materials concept and methodology for 3D printing of superhydrophobic macroscopic objects with bulk nanostructure and almost unlimited geometrical freedom is presented. The method is based on a specific ink composed of hydrophobic (meth)acrylate monomers and porogen solvents, which undergoes phase separation upon photopolymerization to generate inherently nanoporous and superhydrophobic structures. Using a desktop Digital Light Processing printer, superhydrophobic 3D objects with complex shapes are demonstrated, with ultralow and uniform water adhesion measured with scanning droplet adhesion microscopy. It is shown that the 3D-printed objects, owing to their nanoporous structure throughout the entire volume, preserve their superhydrophobicity upon wear damage. Finally, a superhydrophobic 3D-printed gas-permeable and water-repellent microfluidic device and a hierarchically structured 3D-printed super-oil-absorbent are demonstrated.Item Characterization of surface wetting properties using Scanning Droplet Adhesion Microscopy(2021-08-24) Sahaoui, Mohamed; Vuckovac, Maja; Perustieteiden korkeakoulu; Ras, RobinItem Contact angle goniometry: From wetting theory to instrument automation(2024-02-29) Saine, Reetta; Vuckovac, Maja; Perustieteiden korkeakoulu; Martikainen, Jani-PetriWetting is a multiscale phenomenon describing the behavior of a liquid in contact with a solid surface, which has significance in numerous biological systems and technological applications. Traditionally, surface wettability is characterized by a contact angle, a quantity arising from the interfacial interactions of a liquid, a gas and a surface. The predominant method used to measure contact angles is contact angle goniometry, where an optical subsystem captures the profile of a droplet. Despite its apparent simplicity, the method is highly susceptible to errors and has inherent limitations, which proposes a need for more sophisticated experimental setups than those commercially available. Incorporating automation and developing control software become crucial in instrumentation development, allowing instrument capabilities to be pushed beyond the current state-of-the-art. Increased precision and reporoducibility of the measurement techniques advances wetting characterization, improving our understanding of surface properties and aiding the design of functional surfaces. The aim of this thesis is to develop operational control software for a custom-made contact angle goniometer. Contact angles are discussed from a theoretical perspective as a background for the technique, and an overview of the experimental methodology is given. The custom-made setup and the implemented control software are introduced. To demonstrate the operability of the instrument, three surfaces with distinct wetting properties are measured. The results are validated against those measured with a commercial instrument and the performance of the software is evaluated. The control software developed in this thesis allows intentional interaction between the user and the measurement and meets the responsibilities for conducting reproducible and reliable contact angle measurements. As a final result, the experimental setup is operational and the conducted measurements are comparable to those performed with a commercial instrument. In addition to being more convenient to operate, it demonstrates more favorable features than a commercial one. Overall, the setup appears promising and holds potential for advanced wetting measurements.Item Droplet Probe for Characterization of Advancing and Receding Contact Angles of Single Fibers(2023) Freitas Vieira, Arthur; Vuckovac, Maja; Schlapp-Hackl, Inge; Hummel, Michael; Zhou, Quan; Department of Electrical Engineering and Automation; Department of Applied Physics; Department of Bioproducts and Biosystems; Haliyo, Sinan; Boudaoud, Mokrane; Qasaimeh, Mohammad A.; Fatikow, Sergej; Robotic Instruments; Soft Matter and Wetting; Biopolymer Chemistry and Engineering; Center of Excellence in Life-Inspired Hybrid Materials, LIBERCharacterizing the wetting properties of fibers is crucial for many research and industry applications, including textiles for water-oil separation and composite materials. Those fibers are often soft, typically tens of micrometers in diameter but millimeters in length, making manipulation and characterization difficult. Contact angles of single fibers are usually determined by droplet shape analysis or force-based Wilhelmy method. However, these methods are unable to accurately measure contact angles above 60∘ or ensure reliable control of the liquid-fiber interaction process, especially for soft fibers prone to bending. Consequently, reliable characterization of the advancing and receding contact angles of single fibers remains a challenge. Here we report a novel method for characterizing the advancing and receding contact angles of both soft and rigid single fibers using a millimeter-sized droplet probe affixed to a disk and a numerical model of the system. By analyzing side-view images, we extract key geometrical parameters of the disk-droplet-fiber system, which, when used in detailed simulations, allows estimating the contact angle of fibers ranging from 20∘ to 140∘ . We applied this method to characterize three distinct micro-fibers: a highly hydrophilic rigid borosilicate glass fiber, a mildly hydrophilic soft PET fiber, and a rigid hydrophobic tungsten wire coated with a commercial super-repellent coating.Item Droplet splitting on superhydrophobic wires(2015-02-20) Jokela, Aleksi; Vuckovac, Maja; Perustieteiden korkeakoulu; Ras, RobinItem Dynamics of Superhydrophobic Surfaces(Aalto University, 2018) Vuckovac, Maja; Ras, Robin, Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Ras, Robin, Prof., Aalto University, Department of Applied Physics, FinlandItem Force sensor design for scanning droplet adhesion microscope(2023-12-12) Turkki, Valtteri; Vuckovac, Maja; Cenev, Zoran; Perustieteiden korkeakoulu; Ras, RobinCurrent knowledge of wetting on macroscale is rather good, but on the other hand it has many assumptions that might not hold for more complex surfaces that have for example defects. By studying smaller scales, it is possible to understand better how these complex features affect the wetting properties. This allows for example developing better surface coatings that can have longer lifetime or other enhanced properties. One instrument that allows characterizing wetting of surfaces in fine detail is Scanning Droplet Adhesion Microscope (SDAM). It uses a small, roughly 1 microlitre, droplet connected to a force sensor and by moving the sample surface in and out of contact with the droplet the adhesion forces with the sample can be measured. SDAM offers better wetting characterization properties than any usual technique, such as contact angle goniometry and the possibility of scanning the surface properties with micron resolution makes SDAM an ideal tool for studying the complex surfaces. The challenge is that as the features of interest in the surface get smaller, also the measured forces decrease, and the resolution of the force sensor becomes the limiting factor. In case of superhydrophobic surfaces the forces can go as low as few nanonewtons. This thesis work describes a new cantilever sensor design for SDAM to replace the previous micro-electromechanical sensor. The problem is first approached analytically with Timoshenko beam theory to find the desired cantilever dimensions after which the cantilever geometry is optimized using COMSOL finite-element simulations. Finally, the new force sensor is constructed and integrated to the SDAM setup and everything is connected under a new control software. The new force sensor was tested with three different sample surfaces to demonstrate the concept and test the new control software. The initial goal of 1 nN force resolution was achieved with simulations, but due to challenges in the cantilever manufacturing process and lab environment noise the demonstration measurements were performed with draft cantilevers. Those have the finalized geometry, but they are from different material and thus have worse resolution.Item Force-Based Wetting Characterization of Stochastic Superhydrophobic Coatings at Nanonewton Sensitivity(WILEY-V C H VERLAG GMBH, 2021-10-21) Hokkanen, Matti J.; Backholm, Matilda; Vuckovac, Maja; Zhou, Quan; Ras, Robin H.A.; Department of Applied Physics; Department of Electrical Engineering and Automation; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Robotic InstrumentsSuperhydrophobic coatings have extraordinary properties like self-cleaning and staying dry, and have recently appeared on industrial and consumer markets. The stochastic nature of the coating components and coating processes (e.g., spraying, painting) affects the uniformity of the water repellency across the coated substrate. The wetting properties of those coatings are typically quantified on macroscale using contact angle goniometry (CAG). Here, highly sensitive force-based methods, scanning droplet adhesion microscopy (SDAM), and micropipette force sensor (MFS), are used, to quantify the microscale heterogeneity in the wetting properties of stochastic superhydrophobic coatings with irregular surface topography that cannot be investigated by CAG. By mapping the wetting adhesion forces with SDAM and friction forces with MFS, it is demonstrated that even the best coatings on the market are prone to heterogeneities that induce stick–slip motion of droplets. Thus, owing to their high spatial and force resolution, the advantages of these techniques over CAG are demonstrated.Item Improving surface-wetting characterization(AMER ASSOC ADVANCEMENT SCIENCE, 2019-03-15) Liu, Kai; Vuckovac, Maja; Latikka, Mika; Huhtamäki, Tommi; Ras, Robin H.A.; Department of Applied Physics; Soft Matter and WettingItem Mapping microscale wetting variations on biological and synthetic water-repellent surfaces(2017-11-27) Liimatainen, Ville; Vuckovac, Maja; Jokinen, Ville; Sariola, Veikko; Hokkanen, Matti; Zhou, Quan; Ras, Robin; Department of Electrical Engineering and Automation; Department of Applied Physics; Department of Chemistry and Materials Science; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Microfabrication; Robotic InstrumentsDroplets slip and bounce on superhydrophobic surfaces, enabling remarkable functions in biology and technology. These surfaces often contain microscopic irregularities in surface texture and chemical composition, which may affect or even govern macroscopic wetting phenomena. However, effective ways to quantify and map microscopic variations of wettability are still missing, because existing contact angle and force-based methods lack sensitivity and spatial resolution. Here, we introduce wetting maps that visualize local variations in wetting through droplet adhesion forces, which correlate with wettability. We develop scanning droplet adhesion microscopy, a technique to obtain wetting maps with spatial resolution down to 10 µm and three orders of magnitude better force sensitivity than current tensiometers. The microscope allows characterization of challenging non-flat surfaces, like the butterfly wing, previously difficult to characterize by contact angle method due to obscured view. Furthermore, the technique reveals wetting heterogeneity of micropillared model surfaces previously assumed to be uniform.Item Oscillating droplet tribometer for sensitive and reliable wetting characterization of superhydrophobic surfaces(JOHN WILEY & SONS, 2022-07-27) Junaid, Muhammad; Nurmi, Heikki; Latikka, Mika; Vuckovac, Maja; Ras, Robin; Department of Applied Physics; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Center of Excellence in Life-Inspired Hybrid Materials, LIBERAccurate wetting characterization is crucial for the development of next-generation superhydrophobic surfaces. Traditionally, wetting properties are measured with a contact angle goniometer (CAG) suitable for a broad range of surfaces. However, due to optical errors and challenges in baseline positioning, the CAG method suffers from inaccuracies on superhydrophobic surfaces. Here we present an improved version of the oscillating droplet tribometer (ODT), which can reliably assess wetting properties on superhydrophobic surfaces by measuring the frictional forces of a water-based ferrofluid droplet oscillating in a magnetic field. We demonstrate that ODT has superior accuracy compared to CAG by measuring the wetting properties of four different superhydrophobic surfaces (commercial Glaco and Hydrobead coatings, black silicon coated with fluoropolymer, and nanostructured copper modified with lauric acid). We show that ODT can detect the small but significant changes in wetting properties caused by the thermal restructuring of surfaces that are undetectable by CAG. Even more, unlike any other wetting characterization technique, ODT features an inverse sensitivity: the more repellent the surface, the lower the error of measurement, which was demonstrated by experiments and simulations.Item Oscillating Ferrofluid Droplet Microrheology of Liquid-Immersed Sessile Droplets(AMER CHEMICAL SOC, 2017-06-27) Backholm, Matilda; Vuckovac, Maja; Schreier, Jan; Latikka, Mika; Hummel, Michael; Linder, Markus B.; Ras, Robin H.A.; Department of Applied Physics; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Biorefineries; Biomolecular MaterialsThe damped oscillations of liquid-immersed ferrofluid sessile droplets were studied with high-speed imaging experiments and analytical modeling to develop a novel microrheology technique. Droplet oscillations were induced with an external magnetic field, thereby avoiding transients in the resulting vibrational response of the droplet. By following the droplet relaxation with a high-speed camera, the frequency and relaxation time of the damped harmonic oscillations were measured. We extend upon existing analytical theories to describe our liquid-immersed sessile droplet system, and directly quantify the droplet relaxation with the viscosity of the internal and external fluid as well as the interfacial tension between these. The easily controllable magnetic droplets make our oscillating ferrofluid droplet technique a potential candidate for high-throughput microrheology and tensiometry in the future.Item Probing adhesion on micropillar clusters using scanning droplet adhesion microscope(2022-09-29) Rapinoja De Carvalho, Daniel; Vuckovac, Maja; Perustieteiden korkeakoulu; Ras, RobinItem Probing surface wetting across multiple force, length and time scales(Nature Publishing Group, 2023-12) Daniel, Dan; Vuckovac, Maja; Backholm, Matilda; Latikka, Mika; Karyappa, Rahul; Koh, Xue Qi; Timonen, Jaakko V.I.; Tomczak, Nikodem; Ras, Robin H.A.; Department of Applied Physics; Soft Matter and Wetting; Living, Fluid, & Soft Matter; Active Matter; Center of Excellence in Life-Inspired Hybrid Materials, LIBER; King Abdullah University of Science and Technology; Agency for Science, Technology and ResearchSurface wetting is a multiscale phenomenon where properties at the macroscale are determined by features at much smaller length scales, such as nanoscale surface topographies. Traditionally, the wetting of surfaces is quantified by the macroscopic contact angle that a liquid droplet makes, but this approach suffers from various limitations. In recent years, several techniques have been developed to address these shortcomings, ranging from direct measurements of pinning forces using cantilever-based force probes to atomic force microscopy methods. In this review, we will discuss how these new techniques allow for the probing of surface wetting properties in far greater detail. Advances in surface characterization techniques will improve our understanding of surface wetting and facilitate the design of functional surfaces and materials, including for antifogging and antifouling applications.Item Super-Droplet-Repellent Carbon-Based Printable Perovskite Solar Cells(Wiley-VCH Verlag, 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; Department of Applied Physics; Center of Excellence in Life-Inspired Hybrid Materials, LIBER; New Energy Technologies; Soft Matter and Wetting; University of Oulu; Solaronix SA; Monash University AustraliaDespite 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.Item Toward vanishing droplet friction on repellent surfaces(National Academy of Sciences, 2024-04-15) Backholm, Matilda; Kärki, Tytti; Nurmi, Heikki; Vuckovac, Maja; Turkki, Valtteri; Lepikko, Sakari; Jokinen, Ville; Quéré, David; Timonen, Jaakko; Ras, Robin; Department of Applied Physics; Department of Chemistry and Materials Science; Living, Fluid, & Soft Matter; Active Matter; Soft Matter and Wetting; Center of Excellence in Life-Inspired Hybrid Materials, LIBER; ESPCISuperhydrophobic surfaces are often seen as frictionless materials, on which water is highly mobile. Understanding the nature of friction for such water-repellent systems is central to further minimize resistance to motion and energy loss in applications. For slowly moving drops, contact-line friction has been generally considered dominant on slippery superhydrophobic surfaces. Here, we show that this general rule applies only at very low speed. Using a micropipette force sensor in an oscillating mode, we measure the friction of water drops approaching or even equaling zero contact-line friction. We evidence that dissipation then mainly stems from the viscous shearing of the air film (plastron) trapped under the liquid. Because this force is velocity dependent, it can become a serious drag on surfaces that look highly slippery from quasi-static tests. The plastron thickness is found to be the key parameter that enables the control of this special friction, which is useful information for designing the next generation of ultraslippery water-repellent coatings.Item Uncertainties in contact angle goniometry(ROYAL SOC CHEMISTRY, 2019-09-21) Vuckovac, Maja; Latikka, Mika; Liu, Kai; Huhtamäki, Tommi; Ras, Robin H.A.; Department of Applied Physics; Department of Bioproducts and Biosystems; Soft Matter and WettingThe most widely used method to quantify the wetting properties of surfaces is by measuring contact angles. Even though contact angle goniometry is a powerful technique for characterizing wetting properties, it is not accurate for very hydrophobic surfaces. As the technique relies on image processing, it has inherent errors due to optical limitations, especially near the three-phase contact line. This leads to uncertainties in the positioning of the baseline, which can cause large errors in the measured contact angles. In this paper, we systematically evaluate these errors both theoretically and experimentally, focusing on the importance of image resolution. For ∼9 microliter droplet, displacement of the baseline by a single pixel leads to errors of approximately ±0.5° to ±1° for contact angles up to 150°, and errors increase rapidly in the superhydrophobic regime, up to ±8°. The error in the contact angle can be slightly reduced by increasing the image resolution, but cannot be eliminated entirely.Item Viscosity-enhanced droplet motion in sealed superhydrophobic capillaries(AMER ASSOC ADVANCEMENT SCIENCE, 2020-10) Vuckovac, Maja; Backholm, Matilda; Timonen, Jaakko V.I.; Ras, Robin H.A.; Department of Applied Physics; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Active MatterIt is well known that an increased viscosity slows down fluid dynamics. Here we show that this intuitive rule is not general and can fail for liquids flowing in confined liquid-repellent systems. A gravity-driven, highly viscous glycerol droplet inside a sealed superhydrophobic capillary is moving more than 10 times faster than a water droplet with three-orders-of-magnitude lower viscosity. Using tracer particles, we show that the low-viscosity droplets are rapidly rotating internally, with flow velocities greatly exceeding the center-of-mass velocity. This is in stark contrast to the faster moving high-viscosity droplets with nearly vanishing internal flows. The anomalous viscosity-enhanced flow is caused by a viscosity-suppressed deformation of the droplet-air interface and a hydro- and aerodynamic coupling between the droplet and the air trapped within the micro/nanostructures (plastron). Our work demonstrates the unexpected role of the plastron in controlling fluid flow beyond the mere reduction in contact area and friction.Item Water droplet friction and rolling dynamics on superhydrophobic surfaces(Nature Publishing Group, 2020-09-15) Backholm, Matilda; Molpeceres, Daniel; Vuckovac, Maja; Nurmi, Heikki; Hokkanen, Matti; Jokinen, Ville; Timonen, Jaakko; Ras, Robin; Soft Matter and Wetting; Department of Applied Physics; Department of Chemistry and Materials Science; Active Matter; Department of Electrical Engineering and AutomationSuperhydrophobicity is a remarkable surface property found in nature and mimicked in many engineering applications, including anti-wetting, anti-fogging, and anti-fouling coatings. As synthetic superhydrophobic coatings approach the extreme non-wetting limit, quantification of their slipperiness becomes increasingly challenging: although contact angle goniometry remains widely used as the gold standard method, it has proven insufficient. Here, micropipette force sensors are used to directly measure the friction force of water droplets moving on super-slippery superhydrophobic surfaces that cannot be quantified with contact angle goniometry. Superhydrophobic etched silicon surfaces with tunable slipperiness are investigated as model samples. Micropipette force sensors render up to three orders of magnitude better force sensitivity than using the indirect contact angle goniometry approach. We directly measure a friction force as low as 7 ± 4 nN for a millimetric water droplet moving on the most slippery surface. Finally, we combine micropipette force sensors with particle image velocimetry and reveal purely rolling water droplets on superhydrophobic surfaces.
