Polymer wettability properties: their modification and influences upon water movement

Transcription

1 Dissertations Department of Chemistry University of Eastern Finland No. 139 (2016) Anna Kirveslahti Polymer wettability properties: their modification and influences upon water movement

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3 Polymer wettability properties: their modification and influences upon water movement Anna Kirveslahti Department of Chemistry University of Eastern Finland Finland Joensuu 2016

4 Anna Kirveslahti Department of Chemistry, University of Eastern Finland P.O. BOX 111, Joensuu, Finland Supervisors Professor Tapani Pakkanen, University of Eastern Finland Professor Mika Suvanto, University of Eastern Finland Referees Professor emeritus Helge Lemmetyinen, Tampere University of Technology Docent Esa Puukilainen Opponent Professor Jouni Pursiainen, University of Oulu To be presented with the permission of the Faculty of Science and Forestry of the University of Eastern Finland for public criticism in Auditorium BOR 100, Yliopistokatu 7, Joensuu, on October 28 th, 2016, at 12 o clock. Copyright 2016 Anna Kirveslahti (pdf) ISSN: Grano Oy Jyväskylä Joensuu 2016

5 ABSTRACT 3 The theory of wettability is widely known and there are many applications that use extreme wettabilities that have been inspired by nature. Some of these applications are already a part of our daily lives, but many of them are still being studied in order to discover the specific requirements needed for the surface and to meet the limits of industrial productivity. This study introduces new methods for the production of polymer surfaces with different wettabilities. The emphasis is on hydrophobic surfaces, but hydrophilic surfaces were also studied. For standard characterization, the static and dynamic contact angles and sliding angles show how the surface structures influence the movement of small water droplets. Speed measurements of underwater sliding for model boats provided information about the applicability of these surfaces in systems exposed to high water pressures and shear flows. Injection molded polypropylene surfaces with regular micro patterns and irregular nanostructures were studied. Pit structures are expected to be more durable than protruding structures, hence a method to produce pit structured polypropylene surfaces was aspired towards. Pit structured surfaces possessed nearly superhydrophobic properties. Water controlling anisotropic surfaces were also produced from polypropylene by injection molding. The water was steered off the surface via the use of a smaller micro-nano structured channel bordered by higher structures. When using a microstructured channel bordered by micro-nanostructures, water was captured in the channel. Hierarchical paint surfaces were produced by mixing particles in the paint composition. Four different painting procedures were explored via the use of spray and rod painting. Two of these techniques provided surfaces with as high a contact angle as 165. Rod painting produces long lasting durable surfaces, whereas, spray painting allows the coating of surfaces with various shapes and sizes and is easily repainted. Plasma treatments effectively altered the wettability properties of paint surfaces. The method is simple, but there is an observable decay in the surface over time after this treatment. Plasma treatment is a suitable method when there is no need for a long-term impact on the surface. The studied methods proved to be easy and rather straightforward and show promising application for mass production. Plasma treated and spray painted coating surfaces in the contact angle range of were tested via the sliding speed measurements. Hydrophilic surfaces showed no influence on the sliding speed, whereas superhydrophobic surfaces had conflicting impacts. Hydrophobic surfaces decrease or increase the speed, depending on the models Reynolds number.

6 LIST OF ORIGINAL PUBLICATIONS This dissertation is a summary of the following original publications I IV. 4 I. Rasilainen, T.; Kirveslahti, A.; Nevalainen, P.; Suvanto, M.; Pakkanen, T. A. Modification of polypropylene surfaces with micropits and hierarchical micropits/nanodepressions, Surface Science, 604, 2010, II. Rasilainen, T.; Kirveslahti, A.; Nevalainen, P.; Suvanto, M.; Pakkanen, T. A. Controlling the movement of water droplets with micro- and hierarchical micro/nanostructures, Surface Review and Letters, 18, 2011, 209. III. Kirveslahti, A.; Korhonen, T.; Suvanto, M.; Pakkanen, T. A. Hierarchical micronano coatings by painting, Surface Review and Letters, 23, 2016, IV. Kirveslahti, A.; Mielonen, K.; Ikonen, K.; Cui, W.; Suvanto, M.; Pakkanen, T. A. Underwater sliding properties: effect of slider shape and surface wettability, Surface Review and Letters, 23, 2016, The author had an important role in the experimental work in publications I and II. In publication III, the author has been responsible for the experimental work with guidance from the co-authors and prepared the manuscript. In publication IV, the author had an essential role in the plan and execution of the experimental procedures. The key ideas in publication IV derive from discussions between the author and co-authors. The author prepared manuscript for publication IV with comments and inserts from the co-authors.

7 CONTENTS 5 Abstract 3 List Of Original Publications 4 Contents 5 Symbols And Abbreviations 6 1 Introduction WETTABILITY OF SURFACES ALTERATION OF THE SURFACE WETTABILITY APPLICATIONS CHARACTERIZATION METHODS AIMS OF THE STUDY 14 2 Structured Polypropylene Surfaces FABRICATION OF STRUCTURED POLYPROPYLENE SURFACES PIT STRUCTURED POLYPROPYLENE SURFACES ANISOTROPIC PILLAR STRUCTURES 17 3 Hierarchical Coating Surfaces FABRICATION OF HIERARCHICAL COATING SURFACES SINGLE LAYERED SPRAY PAINTED SURFACES DOUBLE LAYERED SPRAY PAINTED SURFACES ROD PAINTED SURFACES PLASMA TREATED SURFACES 27 4 The Effect Of Surface Wettability On Sliding Speed In Water THE AVERAGE SLIDING SPEEDS OF THE KAYAK AND NON-KAYAK MODELS COMPARISON 30 5 Conclusions 32 Acknowledgements 33 References 34

8 SYMBOLS AND ABBREVIATIONS 6 PDMS AAO Al PP SEM PES PVDF PTFE HSiO 2 K NK Re VS Polydimethylsiloxane Anodized aluminum oxide Aluminum Polypropylene Scanning electron microscopy Polyester Polyvinyldifluoride Polytetrafluoroethylene Hydrophobic fumed silica Model kayak Model non-kayak Reynolds number Viscous sublayer

9 1 INTRODUCTION 7 Wettability is a property of a surface, which describes how easily the surface is covered by a fluid, most often water. [1-3] On fully wettable surfaces, the liquid spreads totally, whereas on a non-wettable surface it takes a spherical form. [1-3] A hydrophilic extreme, on which the water is spread, is called superhydrophilic and a hydrophobic extreme, on [2, 4] which water is spherical, is called superhydrophobic. Extreme wettabilities have many characteristic benefits over normal, slightly hydrophobic or hydrophilic surfaces. The most apparent applications are non-wettable fabrics [5] and easily wettable absorbant papers [6], but there are also other advantages. Most of these advantages are found from nature, where superhydrophlilic and [3, 7, 8] surperhydrophobic surfaces were first seen. Many plants and animals have superhydrophobic or superhydrophilic surfaces in their leaves or skin. [7] Figure 1 shows water drops on natural and artificial hydrophobic surfaces. Both superhydrophobic and superhydrophilic surfaces are used in antifogging purposes [9]. Other application examples of these surfaces include self-cleaning surfaces [10-12] inspired by the lotus leaf [7] ; adhesives inspired by the gecko lizard [13-15] ; water controlling surfaces [16] inspired by rice leaves and drag reducing surfaces inspired by sharkskin [17]. They are also used in water oil separation processes [18] and in optical applications for their antireflective [19, 20] and color [21] possibilities. These numerous applications make extreme wettabilities worthy of study. Figure 1 Water drops on hydrophobic surfaces.

10 1.1 WETTABILITY OF SURFACES 8 Surface free energy determines wettability and it is affected by surface chemistry and roughness. [1, 2] Both of them affect the area of the interface between the surface and the liquid. The surface is non-wettable if its surface energy is low enough to repulse the liquid more than it takes to overcome gravity, with the help of the surface tension of the [1, 2] liquid. Water is known for its high surface energy and cohesive forces, which create high surface tension to minimize the area of water-air interaction. To be wettable by water a surface has to overcome this surface tension. [1, 2] To overcome the surface tension a surface should have enough high surface energy so that it is favorable for the total system to cover the surface with water. [1] Liquid spreads on the surface to a degree that minimizes the total energy. [1] Water spreads easily to surfaces that are polar and may form dipole-dipole interactions or hydrogen bonds (hydrophilic surfaces) but repels nonpolar surfaces on which it is more favorable to have internal interactions [1, 4, 7] (hydrophobic surfaces). When a drop of water is placed on a surface, it will settle to a certain shape based on the wettability of the surface. By using the droplet shape, the wettability is characterized by the apparent contact angle, which is the angle of a tangent of a projection of a fluid drop with respect to the surface (Figure 2). [22] It is specific to the material, surface roughness, drop size and content, temperature and pressure [2]. The contact angle can have values from 0 180, 0 representing fully wetted surfaces and 180 representing totally nonwettable [2, 23] surfaces. Figure 2. The contact angle of the surface. The angle is shown in green.. Hydrophobic surfaces are characterized by a contact angle of 90 or more and hydrophilic surfaces are characterized by a contact angle of less than 90. [23] On a smooth surface, the chemical composition of the surface determines the wettability alone and it can be evaluated based on Young s equation, expressed in Equation 1 [3] and figure 2A. Equation 1. γ SV γ SL = γ LV cosθ Where γ SV, γ SL, and γ LV represent the interfacial tensions of the solid vapour, solid liquid, and liquid vapour interfaces repectively and the θ is the contact angle of the surface.

11 9 For a rough surface, Wenzel s equation (Equation 2) [1], using a roughness factor, is more appropriate. The roughness factor represents the anomaly of the surface area from the smooth surface. [1, 2] For an ideally smooth surface the roughness factor r is 1, and the equation corresponds to Young s equation. [1] The theory of Wenzel [22] states that the surface roughness enhances the hydrophobicity of a hydrophobic surface and the [1, 24] hydrophilicity of a hydrophilic surface. Equation 2. cos θ W = r cos θ Y Where θ W is Wenzel s contact angle and r is the roughness factor. In some cases, the surface is so rough that water does not fully penetrate all the surface pores and cavities, so air pockets are left under the drop. In these cases, Wenzel s model does not represent reality well, because the contact area between the drop and the surface is smaller than the total surface area under the drop [2]. Thus, the surface can be likened to a composite surface and the contact angle can be calculated with the Cassie-Baxter equation (Equation 3) [25], setting air as the other component of the composite. [25] Equation 3. cos θ CB = r f cos θ Y + f 1 Where θ CB is the contact angle in a Cassie-Baxter state, r f is the roughness factor and f is the wetted surface fraction. Figure 3. From right to left: water droplet on Young, Wenzel and Cassie-Baxter states. Based on the Cassie-Baxter theory, the higher the fraction of area of air that is under the droplet, the higher the contact angle. [3] This statement applies whether the surface is hydrophobic or not. [23] Hydrophobic surfaces are more prone to capture air, because they repel water more, whereas hydrophilic surfaces need deeper and narrower cavities so that the water cannot penetrate them. [25] A droplet on rough surfaces often adopts some intermediate state, which has both Cassie-Baxter and Wenzel state qualities. [23] It has been found that for wettability alteration, the physical structure of the surface should be in a micrometer or nanometer scale, moreover, a hierarchical combination of both is the key for a Cassie-Baxter state. [26]

12 10 Surfaces with contact angles of over 150 are called superhydrophobic and under 5 superhydrophilic. The term superhydrophobic is, however, ambiguous; the term is often reserved for surfaces that enable droplets to slide on low inclination angles of the surface, but many surfaces possessing over a 150 contact angle have a tendency to pin droplets. [25, 27] Sometimes extra criteria based on the water sliding abilities of the surface are used to define the superhydrophobicity. [25] 1.2 ALTERATION OF THE SURFACE WETTABILITY The surface wettability is altered by altering the surface chemistry, or roughness, or both. Modification of the surface chemistry includes coatings and all kinds of surface treatments that change the surface chemistry permanently or reversibly, such as plasma treatment [28] or atomic [29-31] or molecular [31] layer deposition. The surface roughness is most often altered permanently by structuring the surface, but roughness can also be introduced by adding a renewable structured surface layer, such as a coating on the surface. [32-34] One simple method for physical surface structuring is abrasion of the surface, for example with sandpaper [35] or sandblasting [36]. Growing attention is being given to the durability of the surface structures. [10, 37, 38] Long protruding structures are easily destroyed and lower protrusions and hollow structures are found to be more suitable in practical applications. [39] There are many modification methods and the most suitable method is selected based on the criteria of the target application. Many suggestions for surface chemistry alteration contain multi-step processes and include several chemical and or heat treatments or have a limited array of target applications. [40-42] Some metal oxides such as TiO 2 and ZnO, form superhydrophilic surfaces induced by ultraviolet light. [38, 43] The electronic transitions formed in these surfaces create highly water-attracting chemistry, so that physical surface structuring is not required. [43, 44] The sol-gel method is effective in the production of photo-induced superhydrophilic surfaces [38, 43]. However, it has been discussed whether the term superhydrophilic should be used for smooth surfaces with a low contact angle or only for those with a surface roughness factor over 1. [4] Many different kinds of paint surfaces have been proposed for wettability modification. [10, 45] Paints are generally cost-efficient and suitable on most of the materials. Extra functions like corrosion inhibition [46] and appearance [45] can also be included in the same composition. However, paints generally have poor scratch resistance and they might wear out quickly compared to other materials [47]. Fortunately, in most applications the surfaces are easily repainted. Wettability is mostly altered by altering the surface chemistry of the paint film, but many studies also present surface structured paint films with a high surface area. The surface structure of these paints can be formed by self-assembly [48], wrinkling [49] or additional particles [34, 45, 47].

13 11 Anodization [50] and other electrochemical surface structuring methods [43] mostly used to structure metal surfaces are able to produce a variety of nanostructures. Micro and nanopolymer fibers have been produced with electrospinning [42, 43]. Organized surface structures can be produced by replicating physical structures on polymers by templating, which has shown to be a promising method also suitable for mass-production. The molding technique is selected based on the thermal properties of the selected polymer. Thermoplastics are often injection molded [51]. Examples of structured polymer surfaces with enhanced hydrophobic qualities produced by molding are PDMS [52], PP [16, 53], or Polyethylene [53] surfaces with pillar [53, 54], pit [54] or hogback [52] structures. Plasma treatment is fast and allows the immediate modification of the surface chemical and physical structure [55, 56]. Plasma coats [57] or etches [56] the surface or reacts with the surface atoms and molecules, forming new functional groups on the surface [55]. In simple plasma treatments, the plasma is introduced onto the surface without pretreatment or further modification [55]. Cavities are formed in etching if the treated material is heterogeneous by default or design [28]. Also, organized pillar structures have been produced by masking controlled areas of the surface before plasma etching. [20] Controlled surface structures with different localized properties have been produced with the use of stencils. [28, 58, 59] Oxygen etches and oxidizes the surface, leaving it hydrophilic and reactive [55]. Oxidized surfaces can then be further treated with another plasma gas or method. [56] Fluorine based gases can be used to obtain hydrophobic surfaces. [56] 1.3 APPLICATIONS The most famous applications of superhydrophobic surfaces are self-cleaning surfaces. When a superhdrophobic surface is watered, the water rolls off the surface, attaching to and carrying dirt particles away. [7] This self-cleaning requires low sliding angles and low surface energy. [3] A chemically inert surface repels everything, including dirt, which enhances the cleaning. [7] Self-cleaning surfaces may also proceed via the UV assisted organic dirt decomposition catalyzed by TiO 2, often used for superhydrophilic surfaces. [38] On superhydrophilic surfaces water moves fast and is also able to remove dirt by force, whereas on superhydrophobic surfaces water only carries the loose dirt away. [7] In water oil separation processes the hydrophilic, but oleophobic, filter can only pass water though, so the contaminant oil is left behind. [18] In optics, moth eye and butterfly inspired surfaces are researched for their antireflective [19, 20] and color [21] possibilities. Superhydrophobic surfaces have also been reported to have anti-icing properties [60]. In biological research, there is interest in extreme wettabilities regarding the enhancement of bacterial cultivation by inducing cell death among competing bacteria [61, 62], biological immobilization, [43] and water transportation in micro- or nano-channels in the studies of ion channels and sensors. [63]

14 12 Gecko inspired adhesives are both strong and long lasting, yet easily detached. [14] The large surface area allows many weak interactions between the adhesive and the counter surface. [15] The attachment requires that the surface structures of adhesives conform to the counter surface, unless there will be less contact surface area and the dry friction will be lower. [13, 14] Owing to the lower contact area, the adhesive is easily detached when the surface fine structure is aligned at the correct angle. [14] Hydrophobic surface chemistry eases the detachment and allows lowered friction, whereas, hydrophilic surface chemistry produces even stronger adhesives. [15] However, the adhesive energy is mostly accounted for by the physical, rather than the chemical structure of the surface. [15] In water, superhydrophobic and superhydrophilic [64] surfaces have been reported to have drag reducing effects. Water striders legs [65] and sharks skin [17] are examples of nature s drag reducing surfaces. Hydrophilic surfaces attract water and are known to absorb a viscous film of water on their surface when immersed in water. [26] This film, also called the viscous sublayer, has been thought to potentially reduce friction, because the viscous sublayer moves with the surface and the friction drag is mainly between the viscous sublayer of water and bulk water. [64] In normal conditions, normal surfaces have a noslip condition, where the viscous sublayer is not moving against the surface, but with it. [26] Hydrophobic surfaces repulse water and the viscous sublayer is more mobile, disabling the no-slip condition. [26, 66] When immersed in water, superhydrophobic surfaces maintain a layer of air, called plastron, between the surface structures. [67, 68] This air layer effectively reduces drag as the surface structures only interact with water through their protrusions. [26, 69] Despite this knowledge, some studies [70, 71] report drag increase with hydrophobic surfaces and there seems to be no general understanding on whether extremely hydrophilic surfaces effect the drag or not [64, 70]. Aljallis et. al. [71] suggested that the plastron could be released from the surface in high shear flow and water pressure, which explains why all high contact angle surfaces with low sliding angles are not drag reducing under water. It has also been noticed that sub-micro nanoscale surface structures are more effective in drag reduction and store air more efficiently [26, 39, 72] than higher structures. 1.4 CHARACTERIZATION METHODS The wettability of a surface is most commonly characterized by the static contact angle (described in section 1.1), often measured by placing a drop of a certain volume on a surface and observing its shape as a function of time. [22] Photographing the drop eases the analysis. The contact angle can also be determined from the meniscus of the surface placed partly under water. [2] The static contact angle is most suitable for predicting the surface behavior under static conditions and situations where the surface is exposed to only small volumes of water. [22] However, it can also be used as a pretest to estimate the surface quality, effectiveness of certain treatments conducted and possible applicability to more demanding applications. Still, the method itself is inconclusive for applications that require sustainability under moving water, exposure to large water volumes, or high water pressures. [22]

15 13 Dynamic contact angles are more suitable to the evaluation of surfaces heading under moving water. In the dynamic contact angle measurements, a small drop of water is introduced onto the surface. The volume of the drop is first increased at a steady speed and then reduced. [73] The droplet is photographed during this process. As a result, the advancing and receding contact angles are obtained from the volume increasing and reducing phases, respectively. [2] The advancing contact angle, which represents how easily the droplet can expand or move forward on the surface, is often nominated as the highest possible contact angle. [2, 22, 73] In turn, the receding contact angle representing how easily the droplet sticks on the surface is nominated as the lowest possible contact angle of the surface. [22] The difference of the two values gives the hysteresis of the surface [22], which is an important tool for the evaluation of the drag reducing, water [2, 3, 24, 73] transporting and self-cleaning properties. A low hysteresis ( 10 ) is often required for a superhydrophobic surface. [24] However, dynamic contact angles are not useful for the characterization of extremely hydrophilic surfaces and they also only accurately describe how small drops behave on the surface. [22] When the wettability is adjusted for surfaces exposed to large volumes and high pressures of water their performance should also be evaluated in such conditions. In addition to simulations [74, 75], such experiments are conducted with towing tanks [71] or other non-standard methods. [72, 76, 77] Most commonly, the sample is kept steady and a water flow is let passed it, while pressure drop or flow speed is observed. [35, 74, 78] In addition, methods using motorized specimens moving in a horizontal pool [79] and specimens falling vertically to a water pool [64, 76] have been presented. Plates [71, 72], balls [76, 80] and miniature boats [64, 79, 81] have been used as sample specimens. As standards for hydrophobic surfaces, hydrophilic [70] or uncoated undefined [71] samples have been used. The full spectrum of wettabilities has not been studied comparably. Turbulent [26, 82] flows are also less studied than laminar ones. Although the shape of the application is often known, the surface influence is rarely studied on a target shape. It is not clear whether the shape effects the performance of the surface.

16 AIMS OF THE STUDY This thesis summarizes studies that explore the production and characterization methods of micro-, nano- and hierarchical micro-nanostructured polymers with varying wettabilities. The aim is to provide modification methods in order to obtain structured injection molded polypropylene surfaces and hierarchically structured paint surfaces. The specific interest in these surfaces is their functionality regarding water movement control and drag reduction. The following topics will be covered: The production of superhydrophobic polypropylene surfaces without protrusions, but by using pit structure to obtain more durability. Suitable structures and channel dimensions for water steering or capturing channels produced from polypropylene. The possibility to adjust the wettability of paint surfaces gradually by using micro- and nanoparticles as additives and the effects of the choice of application method on the surface finish and wettability. How do the studied surfaces maintain their expected drag reducing properties underwater? And more generally, do hydrophobic or hydrophilic surfaces have drag-reducing properties, and how does the Reynolds number of the sliding model affect these properties?

17 2 STRUCTURED POLYPROPYLENE SURFACES 15 Micro-, nano-, and hierarchically combined micro-nano structures were produced. Various different microstructures were designed, where the pit/pillar diameter, depth/height, and their spacing were varied in order to obtain optimal structures, containing water repellent and self-cleaning surfaces. Surfaces also suitable for water movement control were studied. 2.1 FABRICATION OF STRUCTURED POLYPROPYLENE SURFACES Structured polypropylene (PP) discs were produced by injection molding with an inverse structured mold insert. Figure 4 represents the different fabrication processes of structured polypropylene surfaces. Figure 4. The production scheme for micro and nano structured polypropylene surfaces. For microstructures, the mold insert was produced with a micro-working robot, which employs tungsten carbide needles to create pits with a controlled diameter and depth in desired patterns on electropolished aluminum foil (Step 1). The micro structured aluminum foil was then cut to suitable shape for injection molding (Step 2). This mold insert was then either used directly in the polypropylene injection molding (Step 3) to produce micropillar structures or used as a mold insert in epoxy cold mounting (Step 4).

18 16 Cold mounting of epoxy produces inverted mold inserts that are then replicated in the injection molding of polypropylene (Step 5) as pit structures on the polypropylene surface. To produce hierarchically combined micro and nanostructures, the microstructure was first processed on the aluminum foil and the microstructured foil was then nanostructured by anodization (Step 6). The as prepared and cut (Step 7) anodized aluminum oxide membrane (AAO) was then used as a mold insert for polypropylene injection molding (Step 8) or for the cold mounting of epoxy (Step 9). The cold mounted epoxy insert was replicated in the injection molding of polypropylene (Step 10) as microand nanopits on the polypropylene surface. Steps 8, 9 and 10 represent similar procedures for nanostructured samples as steps 3, 4 and 5, respectively. Sample designs The diameter of one pillar or pit was varied by using different sized tungsten carbide needles and the depth and spacing could be changed from the settings of the robot. For pit surfaces, five kinds of 1 1 cm micro-structured areas were fabricated and three kinds of mold inserts were further processed to obtain hierarchical micro-nano structures. Four types of anisotropic pillar structures were designed, all of which consisted of two identical pillar structured rectangular areas separated by a rectangular area with a different kind of surface structure. In most designs, the middle structure was lower than the two side structures forming a channel that could steer water droplets along it, however structures with a higher middle structure were also designed. The structured areas were 1 1 cm or cm and the widths of the middle area between the two identical areas were systematically changed to find an optimal width. The different structure types were formed as different combinations of the micro, nano and hierarchical micro-nano structures. The formation of a nanostructure in the anodization process could be prevented selectively by masking desired areas with nailpolish prior to the anodization (structures with two hierarchical micro-nano structures separated by an unstructured area). After anodization, the protective mask was carefully wiped off with acetone and cotton pads. 2.2 PIT STRUCTURED POLYPROPYLENE SURFACES The replication of the structures on the mold insert in the epoxy cold mounting was successful. Overall, the surface structures of the epoxy insert were also well replicated to the polypropylene samples. The obtained pits were often slightly wider than the original pits in the Al or AAO foil, hence the interpit distance was shortened. The nanostructures were not as well replicated as the microstructures. Only shallow nanopits were obtained, instead of the hollow structures formed on the Al in the anodization process. However, the obtained surfaces were fully covered by this regular nanopit structure. More defects were obtained, when the diameters of the surface micropits were

19 17 under 30 µm. These defects arise from the increased adhesion between the mold insert and the ready-made polypropylene disc due to the high contact area. The high adhesion requires more force to detach the sample. Figure 5 presents a SEM image of a micropit suface with a pit diameter of µm, an interpit distance of µm and the depth of pits being about µm. The dimensions were measured from SEM images. The produced structures can be compared to an unstructured, smooth, injection molded PP disc having a static water contact angle of 105 and a contact angle hysteresis of about 16. The tested microstructures showed contact angles of between 118 and 136 correlating with the roughness factor of the surfaces. The actual diameters and distances of the pits affect to a certain level whether the droplet is in a Cassie-Baxter or Wenzel state. In this case, all of the micropit surfaces are estimated to be in the Wenzel state. Moreover, the hysteresis of the contact angle is higher with larger pits and longer interpit distances. The contact angle hysteresis for micropit structures varied between 16 23, with a deviation of about 6. Figure 5. SEM images of pit structured polypropylene surface. Similar trends were seen with combined micro-nano structured surfaces. The static contact angles varied between Showing close to superhydrophobic properties, their contact angle hysteresis was also only 11 18, with a deviation of about 6. The nanostructure strongly affects the surface contact angle. Based on the contact angle values it can be estimated that a water drop on the combined micro-nanopit structure is in a transient Cassie-Baxter state. The water partly penetrates the surface structures but air pockets are partly retained. If the nanopits would replicate their entire depth, they would probably elevate the contact angle even higher and increase the resiliency under abrasive conditions. Deeper hollows would, however, make the structure more fragile. 2.3 ANISOTROPIC PILLAR STRUCTURES For the anisotropic pillar structure contact angle measurements, the water drop was placed onto the middle structure so that it was still supported by the side structures. For reference, a contact angle of the water drop resting fully on the side structure was used.

20 18 The directionality of the contact angle is typical for anisotropic surfaces, hence the contact angles were measured from two different directions; parallel to the channel and perpendicular to it. Figure 6 represents the sample types and their contact angles in parallel and perpendicular directions. The contact angle range gives the lowest receding contact angle and the highest advancing contact angle. The width of the middle zone was changed and the given contact angles summarize all of the structures, which explains the wide range of contact angles on one surface type. Even though there are quite high contact angles even when the channel is unstructured, it is evident that the receding contact angles stay low without a middle structure. When the channel is structured, the receding contact angle values become higher and the hysteresis becomes smaller. Figure 6. Representation of anisotropic polypropylene surfaces. The results from the dynamic contact angle measurements from the lowest receding angle to the highest advancing one are presented inside of each drop. The wider the unstructured area the lower is the contact angle in the perpendicular direction. In the parallel direction the contact angle first becomes higher as the unstructured area widens, but on a wide enough unstructured area the drop starts to sink and can no more be supported by the pillars of the side structures. A 2 8 µl droplet on an up to 1290 µm wide channel could be still supported by the side structures, but on wider channels the parallel contact angle also becomes lower. The trend is similar in all the structures. However, when the channel is also structured, the channel can be wider and the droplet can still be supported from the sides. In addition, the droplet maintains the spherical shape in the structured channels, while in the unstructured channels droplets tend to spread. When the channel is also micro-nanostructured, the droplet on the channel slides off the surface in very low inclination angles. However, the micro-nano structure of the channel has to be low enough compared to the side structures, so that the droplet slides along the channel, and does not move on to the side structures.

21 3 HIERARCHICAL COATING SURFACES 3.1 FABRICATION OF HIERARCHICAL COATING SURFACES 19 Coating surfaces with nano-, micro- and combined micro nano structures were produced from mixtures of base coating and nano-, micro- or both particles. The mixing was eased by using toluene as a solvent. The idea was to obtain surfaces covered with different sized particles, forming a hierarchical surface structure. Four different procedures employing spray-, rod- or a combination of both painting techniques, were explored for the paint application. The techniques were named single layer (S) or double layer (SS) spray painting, or rod (R) or rod-spray painting (RS), according to the employed application methods. Polyester (PES) and Polyvinyldifluoride (PVDF) were used as binders and each surface contained only one binder. Painting procedures are presented in Figure 7. Table 1 lists the produced samples with their particle loadings and sample codes. Figure 7. The painting procedures. Spray and rod painting of coating mixtures with polytetrafluoroethylene and hydrophobic silica particles. The substrate (A) was first primed with the basecoat by rod painting (B) and then further spray painted with a mixture containing micro- or nanoparticles or both (C, G and H respectively).

22 20 Part of the spray painted microparticle surfaces were recoated with a thin layer of a nanoparticle containing paint mixture (D), which produced double layered spray painted surfaces. However, rod painted surfaces (E) were coated straight onto the substrate plate and part of them were further spray painted with a mixture containing nanoparticles (F). For the rod painting, the paint mixture was applied to the surface with a laboratory paint rod (RD specialities, wire wound rod), whereas for the spray painting a spray gun (Air Gunsa AZ40, Anest IWATA Group) was employed. Table 1. Hydrophobic hierarchical coating samples Application method PTFE (w- %) HSiO 2 (w- %) Sample name Base-coated 0 0 Base-coated 0 0 S S S S02 Single layer spray painted samples (S) 0 5 S S S S S95 Double spray samples, layer 1 microparticles, layer 2 nanoparticles (SS) Rod painted surfaces (R) Rod painted PTFE surfaces (layer 1) with spray painted nanoparticle surfaces (layer 2) (RS) 6 2 SS SS SS SS SS R R R RS RS RS RS RS RS95 In addition, to extend the applicability of the coatings for curved surfaces also, the S type surfaces were also produced on surfaces that were base coated by spray painting. Hydrophilic surfaces were produced by using PES as a base coating and TiO 2 nanoparticles as additives. Furthermore, PES and PVDF spray painted surfaces were plasma treated with CF 4, O 2 or both as a plasma gas.

23 21 Base coated PES and PVDF surfaces were used as a reference to compare the effects of different treatments. Figures 8 A and B are SEM images of PES and PVDF base coated surfaces, respectively. Based on the SEM images, it is verified that the base coatings are very even and uniform. The contact angle of the base coated PES surface is slightly hydrophilic, 78 and that of the PVDF surface slightly hydrophobic, 101 Figure 8. The SEM images of base coated PES (A) and PVDF (B) surfaces, which were used as a reference for treated surfaces. 3.2 SINGLE LAYERED SPRAY PAINTED SURFACES Hydrophobic surfaces For the hydrophobic single layered spray painted surfaces, PTFE and hydrophobic silica particles were used. Different amounts and combinations of the two particles were tested. Mixtures with silica were sprayed thinner, as it was found that thicker mixtures are prone to fracture. Figure 9 SEM images of the singlelayer spray painted surfaces. Top left represents a blank test sample, top right sample with PTFE particles, bottom left sample with nanosilica and bottom right sample with both PTFE and nanosilica particles.

24 22 Figure 9 presents a set of SEM images of the obtained PVDF surfaces. Figure 9A presents the blank test sample of PVDF. The blank test samples were produced similarly to the particle samples, except no particles were added to the mixture. The surface of the PES blank test is similar to the PES base coating, but the PVDF blank test clearly has a different structure from the PVDF base. The structure formed on the PVDF blank test was due to the low temperature drying of the PVDF. The use of particles clearly affects the surface topography and when two particles are mixed together with the binding agent, both particles can be seen in the SEM images. From Figure 9 it can be estimated that the size of the PTFE particles is around 400 nm, while the size of the silica particles is in the range of nm. The relative loading of the particles can be estimated based on SEM images. When the loading of the PTFE and silica particles are close to equal, both particles are visible in relatively even proportions. Figure 10 presents the static water contact angles of the single layer spray painted surfaces. An unmodified polyester surface is slightly hydrophilic, but the addition of even a small amount of hydrophobic particles to the mixture produces hydrophobic surfaces. The higher particle loading results in higher contact angles only up to certain point, after which the contact angle seems to set to a slightly lower value than the highest. The same trend in the contact angles was obtained by using PVDF as a binding agent. PVDF is hydrophobic per se, so the effect of hydrophobic particles upon the contact angle is not as dramatic with low loadings as it is with PES. The highest contact angles with both binding agents were obtained with S65 coatings that had close to equal amounts of the two particles. 2 Contact angle ( ) PES PTFE 0 wt.% PTFE 6 wt.% PTFE 9 wt.% PVDF PTFE 0 wt.% PTFE 6 wt.% PTFE 9 wt.% HSiO 0 wt.% HSiO 2 2 wt.% HSiO 5 wt.% Figure 10. The static water contact angles of S -type samples. On the left CA for PES based and on right for PVDF based samples.

25 23 The dynamic water contact angles of a set of PVDF samples were measured to characterize the behavior of moving water on the surface. Table 2 shows the advancing and receding contact angles along with the water hysteresis, which is defined as the difference between the two. Table 2. Dynamic contact angles for PVDF-S samples Sample Advancing contact angle ( ) Receding contact angle ( ) Hysteresis ( ) S ± ± 2 11 ± 1 S ± ± 4 16 ± 4 S ± ± ± 10 All of the surfaces had very low hysteresis, even if they were not superhydrophobic. Only the S65 sample can be characterized as superhydrophobic based on the fact that both its advancing and receding contact angles are higher than 150. However, the deviation of the receding contact angle causes the hysteresis to be, on average, higher than 10, which suggests that the superhydrophobicity is not very uniform. To achieve higher applicability a set of surfaces were also produced on a spray painted base surface. Table 3 presents the static water contact angles for the S-type PVDF samples with a sprayed base coat. The hypothesis was that the application technique of the base coat would not affect the finished surface. However, it was noticed that these samples had even higher contact angles than samples with a rod painted base coat. The difference between the contact angles of differently base coated surfaces might arise from the application method of the base coat or because the PVDF was diluted with a small amount of toluene prior to spraying. These coatings were further analyzed in the diving speed measurements. Table 3. Particle modified PES and PVDF surfaces Coating Particle amount (w -%) Contact angle ( ) Untreated PES 78 ± 1 Untreated PVDF 101 ± 1 PVDF blank test ± 2 PVDF + PTFE ± 3

26 24 Hydrophilic surfaces Hydrophilic surfaces of PES and TiO 2 nanoparticles were produced. The production of surfaces with high TiO 2 loadings is challenging and the coating thickness has to be precise to obtain required durability. Furthermore, TiO 2 has photocatalytic activity, which may facilitate the degradation of the paint over time. The produced coatings were only for research purposes and they were analyzed within two working days after production. The prepared coating surfaces were stored in a dark place to minimize exposure to UV-light. Sufficient durability was ensured by blowing compressed air (1 bar) on the surface before any measurements were taken. Figure 11. A SEM image of hydrophilic paint surface with TiO2 nanoparticles. Figure 11 presents a SEM image of a PES surface with 15 w.t % TiO 2. The size of a single TiO 2 particle was around 20 nm, but most of the particles occur in agglomerates of nm. The surfaces were uniform and undamaged. In the contact angle measurements, the TiO 2 paint surfaces were found to have a contact angle of about 20 with a 4 standard error from the mean. 3.3 DOUBLE LAYERED SPRAY PAINTED SURFACES The double layered spray painted surfaces were produced in two steps on the base coated surface. The first paint layer contained microparticles and the second layer on top of the first, contained nanoparticles. Figure 12. SEM image of a double layer spray painted sample. The surface fracture is evident even in lower magnifications. Figure 12 presents an example of a typical SEM image of a double layered spray painted sample. It is evident that the surface is fractured and there are many large cracks on the paint surface. The contact angles measured from these surfaces were lower than corresponding single layered surfaces, produced by mixing all reagents in the same paint batch. The reason for the lower contact angles could be because of the surface fracture or because the microparticles are almost completely covered by the paint layer with nanoparticles and hence their effect is shaded by the top layer.

27 ROD PAINTED SURFACES Rod painting is a method suitable for thick coating mixtures, mixtures as diluted as in the spray painting cannot be successfully applied with a painting rod. Thus, more PES and PVDF and less toluene was used in the coating mixtures for rod painting. Only hydrophobic surfaces were produced. Coating mixtures with PTFE particles were applied to a clean substrate plate with a painting rod. Nano silica particles were introduced to the dried surface in a paint mixture by spray painting. SEM images of rod painted surfaces are shown in Figure 13. The surface of PES based samples is distinctly different from PVDF based samples. Rod painted PES based PTFE surfaces have craters of µm diameters. It was observed that the PES mixtures with high loadings of PTFE tend to foam, which was not seen with PVDF mixtures. When the foamed mixture is applied on the surface by rod painting the air bubbles at the surface break and form craters. When the PTFE samples are spray painted with nano silica mixtures, the craters become partly filled. Overall, the rod painted surfaces were less abundant with PTFE particles than spray painted surfaces, because the thicker coating layers tend to hide most of the particles inside it. The hidden particles do not affect the contact angle, but they might provide better durability; even if the surface is scratched, there will be new particles at the newly formed surface. Figure 14 presents the static water contact angles for rod painted surfaces. Figure 13. Rod-spray painted PES (At the top) and PVDF (at the bottom) surfaces. Craters formed onto the PES surfaces, forming hierarchy in three stages. PVDF surfaces only had a hierarchical surface from the particles.

28 Contact angle ( ) PTFE 0 wt. % 26 PTFE 7 wt. % PTFE 0 wt. % PTFE 6 wt. % PTFE 9 wt. % HSiO 0 wt. % HSiO 2 wt. % HSiO 5 wt. % PES PVDF Figure 14. The static water contact angles of Rod painted surfaces. On the right for PES based and on the left for PVDF based samples. While PES based surfaces show a gradual increase in the contact angle with a growing amount of either of the particles, PVDF based surfaces show little to no change in the contact angles with a growing amount of PTFE. These trends are mostly due to the PTFE induced foaming of PES paint. The formed craters add roughness and provide one more hierarchy scale to the surface structure. PVDF is less prone to foam, so no craters were formed and the contact angle only indicates the roughness effect and change in the surface energy that directly stems from the PTFE particles. The direct influence of PTFE particles was negligible, since most of the particles were left underneath the surface. In addition, PVDF is more hydrophobic than PES per se, which is why lowering the surface energy will not have as dramatic an effect on the contact angle. A larger amount of silica nanoparticles increases the contact angles of both PES and PVDF based surfaces. The contact angles of PES based rod painted surfaces were found to be unaltered by time after four years of storage.

29 PLASMA TREATED SURFACES Spray painted PES and PVDF base surfaces were plasma treated to study an easy and straightforward method for producing surfaces with various different wettabilities. Plasma treatments with O 2, CF 4 or both plasma gases were performed. Figure 15 shows a SEM image of the O 2 and CF 4 plasma treated surface. The plasma treatments produced small random surface structures. O 2 plasma treatment slightly etched the surface, while CF 4 as a plasma gas formed small piled structures on the surface. Table 4 presents the static water contact angles of plasma treated PES and PVDF surfaces. Figure 15. A SEM image of a Plasmatreated PVDF surface. Table 4. The static water contact angles of plasma treated coated surfaces Coating + Plasma gas Treatment time (min) Contact angle ( ) Untreated PES 78 ± 1 Untreated PVDF 101 ± 1 PVDF + O 2 + CF ± 2 PVDF + O 2 + CF ± 2 PVDF + O ± 2 PES + O ± 1 PES + O ± 1 The O 2 plasma treatment lowered the surface contact angle of both PES and PVDF surfaces. Both PES and PVDF surfaces become hydrophilic with the O 2 plasma treatment, which is found to result from the oxidation of the surfaces. Superhydrophilic PES surfaces with a contact angle of only 3 were obtained. PVDF surfaces were also treated with CF 4 gas in order to obtain hydrophobic surfaces. However, contact angles of only up to 130 were obtained. After 5 minutes of CF 4 treatment, no significant changes in the contact angles were observed with longer treatment times. When desiring a more hydrophobic finish, more complicated methods should be used.

30 4 THE EFFECT OF SURFACE WETTABILITY ON SLIDING SPEED IN WATER 28 Since contact angle hysteresis and sliding angles can only describe hydrodynamic drag by means of small water drops, a system for the analysis of surfaces on a representative test body in their functional environment of use with larger water masses was developed. In the sliding speed measurements, coatings with different wettabilities were tested on model boats in order to study the wettability influence on hydrodynamic drag. The vertical test system measures the falling speed of a model boat in water. Change in the total drag can be evaluated by comparing the speeds of differently coated model boats with each other. Two different models were designed to have different flow characteristics. Fluids flow differently around objects with different shapes and speeds with respect to the fluid speed. Reynolds number describes the characteristics of a fluid flow. Re of around describes laminar flows and 200 onwards describe the gradual changes to turbulent wake and boundary layer separation. [83] A Kayak Model shape (hereafter referred to as K) was designed based on a kayak cross-section against the water surface and then processed into a cylindrical symmetrical shape, which has a Reynolds number of an Olympic kayak ( (v= 5 m/s)), which is in the critical flow range. The Non-Kayak Model (hereafter referred to as NK) is more curved and smaller and its Reynolds number is about (v= 4 m/s). The model boats were base coated and further treated with a plasma or particle modified coating layer to produce samples with a wide range of wettabilities. Figure 16 represents the modification methods. The mass and diameter of the coated model boats were monitored to ensure an even and repeatable finish and to rule out any mass and volume differences between the differently coated model boats. This allows us to address the differences in the sliding speeds of one model to arise from the different surface chemistry and structure only. The two models have different masses and speeds by nature, so they cannot be directly compared, but the trends of the relation between contact angle and the sliding speed can be compared. Figure 16. Spray painting model K and plasma treatment of model NK

31 29 The test system comprises of a vertically oriented 5 meters long acrylic tube filled with water and two photogates 4.1 m apart from each other and a timer connected to the photogates for recording the time. The center-symmetrical model boats are attached to the system with a steering line, which runs through a longitudinal inner tube inside the model boat. The steering line is then tightened by a steel weight at the bottom of the tube and a winch at the top of the tube. In the measurement, the model boat slides along the centered steering line along the tube and its time is recorded with a 1 ms level of precision. The test system is described in more detail in Publication IV. The average sliding speeds of coated model boats for two different models were plotted against the surface contact angles given by the static water contact angle measurements conducted for planar samples. The speed difference in the sliding speed measurements not only depends on the hydrodynamic drag, it also depends on the pressure drag differences. However, it is important to consider both pressure- and hydrodynamic drag. 4.1 THE AVERAGE SLIDING SPEEDS OF THE KAYAK AND NON- KAYAK MODELS The average speed of uncoated NK and K was 3.38 m/s and 3.98 m/s, with standard deviations of 0.02 m/s and of 0.03 m/s, respectively. The speeds of uncoated model boats are within error limits equal to the speeds of models coated with hydrophilic coatings. Figure 17 presents the correlation of the static water contact angle and the average sliding speed of models K and NK. For model K, plasma-treated and particle modified paints were studied. The different surface treatments seem to be comparable with each other, and only the contact angle of the surface is an important factor in relation to speed differences. Figure 17 The sliding speeds of model NK (A) and model K (B) versus the contact angle of the surface.