Browsing by Author "Ras, Robin, Prof., Aalto University, Department of Applied Physics, Finland"

Now showing 1 - 9 of 9

Critical study of contact angle goniometry
(2024) Huhtamäki, Tommi
School of Science | Doctoral dissertation (article-based)
In this thesis contact angle goniometry as a characterization method for solid surface wettability is critically examined. Contact angle goniometry is a powerful and versatile tool for measuring wetting properties, facilitating the study of microscale surface properties by macroscopic an optical method. The apparent ease which with contact angles can be measured have ensured its position as the golden standard of wetting characterization. Measuring meaningful contact angles and interpreting the data correctly is far from simple, however. Real solid surfaces exhibit a range of stable contact angles. Collecting meaningful data requires knowledge to recognize the contact angles which can be reproducibly measured, on how to perform the measurements in a reliable manner and on interpretation of the results. Publication I provides a method for reliable measurement of the receding contact angle. The validity of the method is evaluated both theoretically, and by a wide range of experimental evidence. Publication II proposes a growth model for the synthesis of silicone nanofilaments - a class of superhydrophobic 1-dimensional polysiloxane nanostructures. Theoretical model for pressureinduced film expansion is provided. Publication III introduces a protocol for contact angle measurements that can be applied to a wide variety of samples. Instructions on minimizing both systematic and random errors are provided, along with troubleshooting for most common problems encountered. Publication IV quantifies the error in contact angle measurement caused by misplacement of the baseline - the line between the solid and liquid/gas in the 2D-images analyzed. Special emphasis is given on the error for superhydrophobic surfaces. Publication V expands on publications IV by quantifying the error caused by the optical system used in contact angle goniometry. User error of contact angle measurements is also meas-ured.
Droplet friction on heterogeneous surfaces
(2024) Lepikko, Sakari
School of Science | Doctoral dissertation (article-based)
The ability to control friction between a solid and a liquid is becoming more and more important in various existing applications as well as in novel ones. However, the mechanisms behind this liquid-solid friction are not yet sufficiently understood. This thesis compiles research performed in three publications where origin of the liquid-solid friction of water droplets is examined for various surfaces with properties ranging from hydrophilic to superhydrophobic and from a regular surface structure to a stochastic one. Publication I examines how molecular level heterogeneity of a surface affects the contact line friction between the surface and water. This is performed by preparing and characterizing self assembled monolayers with varying level of molecular coverages of hydrophobic alkyl tails on hydrophilic silicon wafer substrates. The results show that the low- and high-coverage surfaces with least chemical heterogeneity have the lowest friction while the intermediate-coverage surfaces with most heterogeneity have the highest friction. Publication II focuses on the relation of the contact line friction and the liquid-solid contact fraction of superhydrophobic surfaces. The friction is shown to scale with the contact fraction, being lowest with the lowest contact fraction, and a mathematical model is provided to describe this relation. The model works over wide ranges of friction and contact fraction values, both extending almost over three orders of magnitude. Another important message is that conical microstructures can be used to create surfaces with an extremely low liquid-solid contact fraction that results in an extremely low contact line friction. Publication III explores how superhydrophobic surfaces with stochastic roughness in the nanoand micrometre scales affect the liquid-solid friction. The wetting characterization shows that the friction is time dependent such that static droplets have time to adapt to the surface roughness while moving droplets do not have such time. This adaptation increases the liquid-solid contact fraction, which causes the increased contact line friction. Effectively, this creates a static friction barrier that pins static droplets to the surface but does not restrict the movement of already mobiledroplets. The obtained results of Publications I-III help determining critical surface parameters when designing functional surfaces for applications where low friction between a solid and a liquid is needed.
Dynamics of Magnetic Droplets on Liquid-Repellent Surfaces
(2020) Latikka, Mika
School of Science | Doctoral dissertation (article-based)
Ferrofluids are colloidal suspensions of superparamagnetic nanoparticles in a carrier liquid. The combination of liquid properties and a strong magnetic response makes ferrofluids versatile materials exhibiting fascinating phenomena. The main focus of this thesis is on the intersection of ferrofluids and highly water-repellent surfaces. The basic properties of ferrofluids and the limitations of contact angle goniometry are experimentally investigated, and novel field-induced instabilities and ferrofluid-based wetting characterisation methods are demonstrated. Publication I is an in-depth comparative analysis of two ferrofluids with electrostatic and electro-steric stabilisation. Structure, colloidal stability, magnetic and flow properties of the fluids are investigated with a range of advanced techniques. Publication II reviews recent literature on wetting of ferrofluids, which is important for microfluidics applications and magnetic actuation. The Publication discusses contact angle goniometry, the most common wetting characterisation method, and the challenges caused by the magnetic-field-induced deformation of ferrofluid droplets. Publications III and IV examine the challenges of contact angle goniometry on highly water-repellent surfaces. In the superhydrophobic wetting regime, an error of just a single pixel in locating the solid-liquid interface leads to large inaccuracies in the measured contact angles, rendering this technique poorly suited for these surfaces. Publications V and VI describe wetting characterisation methods for superhydrophobic surfaces based on oscillating magnetic droplets. A water droplet with a tiny amount of superparamagnetic nanoparticles is brought to an oscillatory motion in a parabolic magnetic field on the surface under investigation. The wetting properties can be measured from the rate at which the oscillations decay due to the dissipative forces. Publications VII and VIII investigate magnetic-field-induced ferrofluid droplet splitting on water-repellent surfaces. The field-induced instability is used to create self-assembled droplet populations, which can be magnetically actuated. Reversible switching between static and dynamic self-assembly is demonstrated using an oscillating field, and novel concepts for microfluidics and interfacial tensiometry are introduced. This thesis improves our understanding on the basic properties of ferrofluids and examines the limitations of contemporary wetting characterisation methods. Building on this foundation, it introduces novel phenomena and concepts, which can help the development of advanced ferrofluid-based applications.
Dynamics of Superhydrophobic Surfaces
(2018) Vuckovac, Maja
School of Science | Doctoral dissertation (monograph)
Functional Liquid-Fluid Interfaces Based on Hydrophobin Proteins: An Experimental Study for Medical Applications
(2024) Al-Terke, Hedar H.
School of Science | Doctoral dissertation (article-based)
Interfaces are everywhere around us. Any direct interaction occurs at the interface. This thesis explored the potential of functional interfaces to solve various medical application challenges. The four publications presented in this thesis highlight the different ways in which functional interfaces can be utilized to address these challenges. Publication I focused on the relationship between gravity, viscoelasticity, and the shape of water droplets coated with HFBI hydrophobin proteins. By studying the self-organization of hydrophobins at the air-water interface, it was found that a rigid layer is formed at a critical concentration, which affects the droplet morphology. This finding has significant implications for engineering and biomedical applications, as it provides a pathway for controlling the shape of droplets in various systems. Publication II presented a novel antibody extraction method using advanced materials and functional interfaces. By dividing an oil-based ferrofluid into daughter droplets under an external magnetic field, the surface area of liquid-liquid interfaces is increased, allowing for the functionalization and application of these interfaces as a substrate for antibody extraction. Publication III focused on the formation and characterization of protein-coated gas bubbles. By employing a micropipette aspiration technique, the mechanical properties of these bubbles were assessed, and a sealing parameter (Q) was determined to evaluate their gas permeability. These well-characterized bubbles have promising potential as ultrasound-enhanced contrast agents in various biomedical fields. They can be utilized for imaging purposes and targeted drug delivery, opening up new possibilities for medical diagnostics and therapies. Publication IV explored the utilization of hydrophobin protein functionalized bubbles to develop an advanced ultrasound molecular imaging probe. By functionalizing bubbles with a moiety part at their interface, they can attach to specific antigens and reveal diseased cells, such as cancer cells. This innovative approach holds great promise for improving the accuracy and sensitivity of molecular imaging techniques, enabling early detection and precise targeting of diseases. Overall, the findings presented in this thesis demonstrate the immense potential of functional interfaces in solving various medical application challenges. They provide valuable insights into the design and development of novel materials and techniques that can improve diagnostics, therapeutics, and imaging in the biomedical field.
Programmable and Responsive Superhydrophobic Surfaces
(2021) Al-Azawi, Anas
School of Science | Doctoral dissertation (article-based)
Progress in the field of superhydrophobic surfaces requires precise characterization techniques and synthesis of surfaces that exhibit robust non-wettability. In this thesis, microfabrication techniques are used to produce static as well as bioinspired responsive superhydrophobic structures. In addition, transverse droplet oscillations are implemented to accurately evaluate superhydrophobicity of micropillared surfaces. Variations in surface properties that influence the degree of superhydrophobicity were successfully probed by relating friction and viscous dissipation of dynamic droplets to pattern density and chemical coating. Additionally, controlling the normal force exerted on the water-based ferrofluid droplet allows the measurement of impalement pressure necessary to induce wetting transition for a droplet in motion. A new fabrication process was introduced for rapid prototyping of cilia-inspired magnetic micropillars. The fabricated array of sub-10 µm diameter pillars are based on polydimethylsiloxane (PDMS) loaded with carbonyl iron particles (CIP). Lubricating in silicone oil allowed controlled droplet motion at the sub-mm scale facilitated by fast actuation and superhydrophobicity of the oil infused PDMS magnetic micropillars. Lack of mechanical stability due to flexibility of the high aspect ratio PDMS micropillars restricted the application of the array to liquid media only. Thiol-ene based magnetic micropillar arrays were introduced to address the stability issue of the high-aspect ratio micropillars. The remarkable properties of thiol-ene including tunability of surface and mechanical properties allowed topography modification of the magnetic micropillars using photo-induced thiol-ene click coupling. Decorating the surface of the high aspect ratio thiol-ene micropillars with polyvinyltrimethoxysilane (PVTMS) colloidal micro- and nanoparticles enhanced the mechanical stability of the flexible micropillars without compromising powerful bending actuation. This allowed actuation of the micropillar arrays in air as well as in liquid media. The magnetic micropillars were rendered superhydrophobic by grafting hydrophobic self-assemnled monolayer onto the PVTMS micro- and nanoparticles that are covalently bonded to the surface. This enabled directed water droplet motion by repetitive bending and recovery of the micropillars. Combining mechanical stability with robust superhydrophobicity can lead to numerous practical applications of cilia-inspired thiol-ene magnetic micropillars.
Studies on Wettability - From Fundamental Concepts and Nanofibrous Materials to Applications
(2013) Korhonen, Juuso T.
School of Science | Doctoral dissertation (article-based)
Water is among the most vital substances on earth. Despite being an everyday element, prominently interesting phenomena occur when water is in contact with a surface. Superhydrophobic surfaces are ones that do not wet. In the extreme, they repel water to such a large degree that water does not stick to even nearly horizontal planes. Nowadays, many researchers pursue the magnificent examples set by nature, such as the extraordinary water repellency of a lotus leaf. Even though the basis of the study of wetting dates back to the 1800s, many elementary concepts remain unexplored. This thesis combines fundamental notions of wetting to experimental material science and demonstrates applications based on these materials, ranging from memory devices and sensors to pellets, which facilitate environmental clean-up. Publication I studies the fundamental concepts of surface wetting characterization and introduces a quantitative method for measuring the receding contact angle using the sessile-drop method. Theory and experiments together with calculations evaluate the validity of the developed model, and good agreement between theory and experiments is found. Furthermore, a novel definition for superhydrophobicity is proposed. Publication II introduces a growth model for the synthesis of silicone nanofilaments, which are one-dimensional nanostructures used to create superhydrophobic surfaces. In contrast to the previous studies, the present model explains the break of symmetry occurring in the initial phase of the growth, which has so far been implicitly assumed. Publication III demonstrates a hierarchically rough surface, which exhibits bi-stable superhydrophobic wetting states under water. A rapid local wetting transition occurs simply by locally applying pressure or suction onto the surface. Theoretical considerations explain the phenomena and a simple display demonstrates the concept. Publications IV and V introduce nanocellulose aerogels coated by atomic layer deposition with inorganic thin films. These materials are further employed as a resistive/capacitive humidity sensor and for oil spill cleanup from a water surface. The porosity and high surface area of the structures together with the wetting properties of the inorganic coating account for the observed properties. In addition, the study evaluates different drying methods for the nanocellulose aerogels based on the aggregation of the fibrils. Combining basic principles of wetting and superhydrophobicity to novel materials, as shown in this study, can lead to applications from myriad fields of technology. The concepts and applications demonstrated hopefully inspire future research towards many wetting-based applications.
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.
Wetting Characterization of Hydrophobic Opaque Surfaces and Micro Fibers
(2024) Vieira, Arthur
School of Electrical Engineering | Doctoral dissertation (article-based)
Wetting plays a key role in everyday phenomena, from the adhesion of sand particles in sandcastles to the visibility of windshields of cars under the rain. Of particular interest are the surfaces that repel water, a.k.a. hydrophobic. Characterizing the degree of hydrophobicity is essential for the development of advanced materials, which is typically done by measuring advancing and receding contact angles. However, wetting in real surfaces is often defined by irregularly shaped solid-liquid interfaces and multiple contact angles, which are far from the idealized cases that underly most measurement techniques. While there have been significant advances in wetting characterization, techniques that accurately quantify the liquid-solid interface are lacking. Contact angle goniometry is the gold standard. However, its resolution falls short in the superhydrophobic regime, which has spurred the development of numerous alternative techniques. Despite their innovations, these are either limited to transparent surfaces, demand specialized equipment, or involve complex experimental procedures. This thesis presents new wetting characterization techniques based on a transparent droplet probe, with a focus on superhydrophobic surfaces. The methods allow quantifying the advancing and receding contact angles on flat opaque superhydrophobic and hydrophobic surfaces, as well as cylindrical soft micro fibers that are either hydrophobic or hydrophilic. Firstly, the transparent probe allows direct visualization of the contact line from which the mean advancing and receding contact angles can be measured with an experimental uncertainty as low as 0.2 °, near 180 °. Secondly, a technique based on the finite element method is described that allows measuring the contact angle along irregularly shaped contact lines. The capability is also used to map advancing and receding contact angles on micro-patterned surfaces, with an unprecedented spatial resolution of 3 μm. The technique can distinguish contact angles that vary only 1 ° between zones of the same micro-patterned surface, near 180 °. Thirdly, a force-based method is presented that allows distinguishing the two key adhesion force components acting on the wetting interface: force due to internal droplet pressure, i.e. Laplace pressure; and the force due to surface tension. The method combines the force information with top-view and side-view images to provide a full description of the droplet-sample interaction, including an alternative way to accurately estimate the contact angle near 180 °. Lastly, finite-element-method analysis is combined with side-view imaging to characterize the wetting properties of single-fibers. The method is used to estimate advancing and receding contact angles of both soft and rigid fibers, and in both hydrophilic and hydrophobic regimes.