STUDY OF DROPLETS INTERACTIONS ON SOLID SURFACE FOR MANUFACTURING APPLICATIONS

Droplet impact behaviors on a solid substrate is a complicated process that involves multiple phenomena on a small scale within a short time. This phenomenon has wide applications in the industrial, medical, and scientific fields. To understand the complicated mechanism, we select the oscillation process as the marked phase to observe and analyze the dynamical balancing between different forces and conservation of energies and compare it to the typical damped harmonic oscillator (DHO) model to examine the character of variables as viscosity, surface tension, and wettability. From the testing results, we created a precise model for post-impact dynamics and described how the solid wettability (which is demonstrated as the contact angle) and the surface tension (which is presented as the weber number) helped control the oscillation dynamics; then further studies reveal the influence of weber number on oscillation frequencies and make a capillary pressure related assumption about satellite droplet separated from the main body. Finally, the latest study reveals the viscosities’ influence on the dynamics. By creating the overall functions of the Weber and Reynolds numbers, we created a model that can represent the oscillation process under a wide range of conditions

Corrosion of One-Step Superhydrophobic Stainless-Steel Thermal Spray Coatings

As most superhydrophobic coatings are made of soft materials, the need for harder, more robust films is evident in applications where erosional degradation is of concern. The work herein describes a methodology to produce superhydrophobic stainless-steel thermal spray coatings using the high-velocity oxygen fuel technique. Due to the use of a kerosene fuel source, a carbon-rich film is formed on the surface of the thermal spray coatings, lowering the surface energy of the high-energy metallic substrates. The thermal spray process generates a hierarchical micro-/sub-micro-structure that is needed to sustain superhydrophobicity. The effect of spray parameters such as particle velocity and temperature on the coating’s hydrophobicity state was explored, and a high particle velocity was shown to cause superhydrophobic characteristics. The coatings were characterized using scanning electron microscopy, profilometry, X-ray photoelectron spectroscopy, static water contact angle measurements, water droplet roll-off measurements, and water droplet bouncing tests. The corrosion behavior of the coatings was studied using potentiodynamic polarization measurements in order to correlate water repellency with corrosion resistance; however, all coatings demonstrated active corrosion without passivation. This study describes an interesting phenomenon where superhydrophobicity does not guarantee corrosion resistance and discusses alternative applications for such materials

Manipulating droplet jumping behaviors on hot substrates with surface topography by controlling vapor bubble growth: from vibration to explosion

A major challenge in surface science is rapid removal of sessile liquid droplets from a substrate with complex three-dimensional structures. However, our understanding of interfacial phenomena including droplet wetting dynamics and phase changes on engineered surfaces remains elusive, impeding dexterous designs for agile droplet purging. Here we present a surface topography strategy to modulate droplet jumping behaviors on micropillared substrates at moderate superheat of 20-30 {\deg}C. Specifically, sessile droplets usually dwell in the Wenzel state and therefore the micropillar matrix functions as fin array for heat transfer enhancement. By tuning the feature sizes of micropillars, one can adjust the vapor bubble growth at the droplet base from the heat-transfer-controlled mode to the inertia-controlled mode. As opposed to the relatively slow vibration jumping in seconds, the vapor bubble growth in the inertia-controlled mode on tall-micropillared surface leads to droplet out-of-plane jumping in milliseconds. Such rapid droplet detachment stems from the swift Wenzel-to Cassie transition incurred by vapor bubble burst (explosion), during which the bubble expanding velocity can reach as fast as ~4 m/s. Vapor bubble growth in a droplet and bubble-burst-induced droplet jumping have been less explored. This study unveils the underpinning mechanisms of versatile jumping behaviors of boiling droplets from a hot micro-structured surface and opens up further possibilities for the design of engineered surfaces that mitigate potential damage of vapor explosion or alleviate condensate flooding

Tuning the interaction of droplets with liquid-repellent surfaces: fundamentals and applications

2018 Fall.Includes bibliographical references.Liquid-repellent surfaces can be broadly classified as non-textured surfaces (e.g., smooth slippery surfaces on which droplets can slide easily) and textured surfaces (e.g., super-repellent surfaces on which liquid droplets can bead up and roll off easily). The liquid repellency of smooth slippery surfaces can be adjusted by tuning the surface chemistry. The liquid repellency of super-repellent surfaces can be adjusted by tuning the surface chemistry and surface texture. In this work, by systematically tuning the surface chemistry and surface texture and consequently the surface wettability of solid surfaces, the interaction of droplets of various liquids on liquid-repellent surfaces has been investigated. Based on this understanding, the following phenomena/applications have been investigated/developed: (i New methodology to sort liquid droplets based on their surface tension: By tuning the surface chemistry and surface texture of solid surfaces, we tuned the mobility of liquids with different surface tension on super-repellent surfaces. Utilizing this, we fabricated a simple device with precisely tailored domains of surface chemistry that can sort droplets by surface tension. (ii) New approach to detect the quality of fuel blends: By tuning the surface chemistry of solid surfaces, we investigated the interaction of fuel blends with liquid-repellent surfaces. Based on the understanding gained, we fabricated a simple, field-deployable, low-cost device to rapidly detect the quality of fuel blends by sensing their surface tension with significantly improved resolution. (iii) Novel materials with improved hemocompatibility: By systematically tuning the surface chemistry and surface texture and consequently the surface wettability of solid surfaces, we investigated the interaction of blood with super-repellent surfaces. Based on the understanding gained, we fabricated super-repellent surfaces with enhanced hemocompatibility. (iv) Advanced understanding of droplet splitting upon impacting a macroscopic ridge: By systematically tuning the ridge geometry, we investigated the interaction of impacting water droplets with super-repellent ridges. Based on the understanding gained, we demonstrated the scaling law for predicting the height from which water droplets should fall under gravity onto a super-repellent ridge for them to split into two smaller droplets

A coating-free superhydrophobic sensing material for full-range human motion and microliter droplet impact detection

Traditional waterproofing strategies (e.g. plastic seals, superhydrophobic coatings) of strain sensors greatly limit their sensing performance (e.g., sensitivity, working-range, and working-life). Here a unique ultra-stretchable, coating-free superhydrophobic material is developed for high-performence strain sensing in harsh environments. This material integrates high sensitivity (GF of 2.1 to 214), wide sensing range (up to 447% strain), low resolution (<0.2% strain), dynamic durability (over 10,000 stretching cycles at 50% strain), and ultra-robust superhydrophobicity (mechanically, chemically, thermally, and UV impervious) in a single system, outperforming most of reported waterproof sensors. Such remarkable sensing materials can detect full range human movement, pulse rate and vocal fold vibration. The sensing material is designed to be superhydrophobic throughout its bulk material for work in harsh environments (water, corrosive liquid, high humidity, etc.). More importantly, the superhydrophobicity enables the highly sensitive sensor to detect microliter droplets impact with minimized energy loss. Thus, this sensing material should find many potential applications in wearable electronics, measurement platform, rainfall monitoring and intelligent irrigation system

Drop Impact on a Solid Surface

International audienceA drop hitting a solid surface can deposit, bounce, or splash. Splashing arises from the breakup of a fine liquid sheet that is ejected radially along the sub-strate. Bouncing and deposition depend crucially on the wetting properties of the substrate. In this review, we focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, and surface tension , but also the surrounding gas. The gas cushions the initial contact; it is entrapped in a central microbubble on the substrate; and it promotes the so-called corona splash, by lifting the lamella away from the solid. Particular attention is paid to the influence of surface roughness, natural or engineered to enhance repellency, relevant in many applications. 36

Developing Superhydrophobic Coatings for Mitigating Aircraft Icing using Plasma Spray Processes

In-flight icing due to the presence of super-cooled water droplets is a major problem for aircraft operators. Accumulation of ice on the surface of wings, control parts and sensors can result in a range of problems including navigation issues, decreased efficiency, increasing fuel consumption, forced flight delays and cancelation and, if neglected or mismanaged, even fatal flight incidents. A significant potential solution for mitigating the icing problem is the use of superhydrophobic coatings i.e. coating that are extremely water repelling. Superhydrophobic coatings, by repelling the water droplets, can delay and in some cases prevent ice accumulation. Additionally, superhydrophobic coatings can facilitate ice removal by heating or vibration due to their non-stick properties. The superhydrophobicity of a surface is a result of the combination of the surface micro-texture and its surface energy which is determined by the chemistry of the surface. The major challenge facing the use of superhydrophobic coatings is the fact that low surface energy materials are mainly organic, polymeric compounds that suffer from poor durability, and in addition, micro-textured coatings are typically made by complex and expensive techniques. In this work atmospheric plasma spray (APS) and suspension plasma spray (SPS) which are flexible, scalable and efficient surface engineering techniques, are employed to develop micro-textured superhydrophobic coatings for anti-icing applications. In this research, APS and SPS TiO2 micro-textured coatings are developed. After treatment by a stearic acid solution in order to lower their surface energy, these coatings demonstrate hydrophobicity and superhydrophobicity to different extents. APS coatings that are produced using 10-80 micron-sized particle feedstock, although highly hydrophobic, lack the extreme water repellency known as water mobility, due to their relatively coarse micro-texture. In the SPS process, submicron-sized TiO2 particles in the form of a suspension are used as feedstock. The SPS coatings typically show superhydrophobicity with water contact angles higher than 150°. The coatings produced using an ethanol-based suspension demonstrated extreme hydrophobicity and a water droplet impacting on their surface bounces back and detaches from them easily. The parameters influencing the SPS process are further studied and optimized to achieve coatings with hierarchical surface micro-texture i.e. a surface with a primary micron-sized and a secondary submicron-sized micro-texture. After optimization of the process, the SPS TiO2 coatings show extreme superhydrophobicity with water contact angles as high as 170°, water sliding angles as small as 1.3° and a contact angle hysteresis as small as 4°. These superhydrophobic SPS TiO2 coatings demonstrate promising results in terms icing performance and durability

패턴을 이용한 표면제어와 그 활용에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 화학생물공학부(에너지환경 화학융합기술전공), 2021. 2. 차국헌.표면은 구조적 또는 화학적 성질에 따라 고유의 특성을 갖으며, 외부에서 가해지는 힘에 의해 변형이 일어나기도 한다. 표면을 나노 단위로 관찰 가능한 전자현미경 시스템이 발전함에 따라 표면에서 발생하는 위와 같은 현상에 대한 연구는 지난 수십 년간 활발하게 진행되었다. 연잎, 소금쟁이 등 자연계에서 발견된 독특한 성질을 나타내는 초발수표면은 마이크로 구조 위에 미세한 나노 구조로 이루어진 계층 구조에 의한 특성으로 나타나는 현상인 것이 밝혀지면서 이를 모방하여 공학적으로 이용해보고자 하였고, 표면에 발생하는 주름, 균열 등은 구조적 실패 혹은 파손으로 간주되어 이를 제어하고자 하였다. 패터닝은 표면에서 발생하는 현상을 체계적으로 제어하고 분석할 수 있는 플랫폼을 제공한다. 패턴의 크기, 종류, 간격 등을 나노 단위로 자유롭게 조절하여 표면의 구조적 특성을 제어할 수 있기 때문이다. 표면에 패턴을 제작하는 방법으로는 빛과 마스크를 이용한 광학 리소그래피, 화학적 특성을 이용한 식각, 그리고 PDMS몰드를 이용한 소프트 임프린팅 방법 등이 알려져 있다. 초발수표면은 자가세정유리, 자가세정자동차, 얼지 않는 표면 등과 같이 산업적으로 이용 가치가 부각되면서 이를 구현하기 위해 다양한 연구가 진행되었다. 특히, 계층구조를 제작함에 있어 공학적으로 초발수현상을 체계적 분석하고자 패턴을 활용하는 방법이 적용되었다. 하지만, 대부분 발표된 연구는 계층구조를 제작하는 과정이 복잡하여 산업적으로 이용하기에는 한계가 있으며 따라서, 초발수표면을 간단하게 제작하여 산업분야에 적용시킬 수 있는 방법에 대한 연구가 필요한 때이다. 한편, 표면에 다양한 형태의 힘이 가해지면 이를 완화시키기 위해 변형이 일어난다. 그 중 인장응력에 의해 발생하는 균열현상은 불규칙적이고 시스템이 복잡하기 때문에 조절하기 힘들 뿐만 아니라 대부분 결점으로 여겨지면서. 이를 방지하고자 하는 연구에 집중되어 왔다. 특히 가뭄 현상과 같이 용매가 증발함에 따라 발생하는 균열은 고분자 박막에서 발생하는 균열현상보다 더욱 복잡하고 불균일하게 형성되기 때문에 현재까지 이를 제어하는 방법에 대한 연구는 거의 진행되지 않아 이에 대한 연구가 필요하다. 본 학위논문에서는 표면에서 발생하는 현상 중 패턴을 적용 하여초발수 표면과 균열 제어에 관한 체계적인 연구를 제시한다. 산업적으로 이용 가능한 초발수표면을 간단하게 제작하는 방법을 연구하였고, 초발수표면에 성질이 다른 화학적 패턴을 각인하여 물방울 충격 역학의 기계적 조절에 대한 연구를 하였다. 또한 결점으로 여겨졌던 건조 콜로이드 박막에서 발생하는 균열 현상을 패턴을 이용하여 체계적으로 제어하였으며, 더 나아가 균열로 생성되는 조각의 크기까지 자유롭게 조절하는 방법을 제시하여 공학적으로 균열을 유용하게 활용할 수 있는 연구를 제시한다. 제 1장에서는 스프레이시스템을 활용하여 대량 면적에도 간단히 초발수표면을 구현하는 방법에 대한 연구를 제시하였다. 상온에서도 빠른 반응을 유도하기 위해 에폭시-사이올 반응을 도입하여 실리카나노입자와 바인더, 용매로 이루어진 콜로이드 용액을 제조하고 각 요소의 비율이 접촉각, 투명성, 표면 구조에 미치는 영향을 연구하였다. 또한 다양한 표면에 적용시켜 범용적인 표면에 초발수효과를 나타내는 것을 확인하였다. 제 2장에서는 초발수표면에 친수성을 가진 비대칭 화학적 패턴을 각인시켜 물방울의 거동이 조절 가능한 표면을 제작하였다. 꼭지점이 있는 패턴을 설계하여 물방울이 표면에 충격하고 다시 튀어 오를 때, 꼭지점 방향으로 튀어 오르도록 유도하였다. 패턴의 각도와 충돌 거리를 조절하여 방향성의 정도에 영향을 주는 요인을 찾아내었고, 이를 기하학적 이론 값과 비교하여 차이가 없는 것을 확인하였다. 이와 같은 표면은 물리적 구조가 없는 평평한 표면에서도 화학적 패턴에 의해 물방울의 거동이 조절되었고, 나아가 물방울을 꼭지점과 같은 특정 위치에 모을 수 있다는 것을 확인하였다. 제 3장에서는 제 1장에서 표면을 제작하는 과정에서 발생한 균열 현상을 마이크로패턴을 이용하여 제어하는 방법을 제시하였다. TiO2로 이루어진 콜로이드 필름에 소프트 임프린팅 기법을 이용하여 프리즘 혹은 피라미드 마이크로패턴을 각인시키고, 소결 과정을 통해 필름의 유기물을 모두 제거하면 필름에 부피 수축이 발생되고 스트레스는 각 패턴의 가장자리에 집중되고 균열이 이곳에 생성되도록 유도하였다. 균열 집중 현상의 정도는 필름의 두께, 나노 입자의 크기, 가열 속도에 영향을 받는 것을 확인하였다. 특히 필름 두께는 생성되는 균열 조각의 면적에도 영향을 주었으며, 각 균열 조각은 정사각형 피라미드로 구성된 장점을 활용하여 면적을 쉽게 정량화 하였고, 필름 두께와 면적이 갖는 스케일링 관계를 확인하였다. 제 4장에서는 균열을 제어하는 것에서 더 나아가 균열 조각의 크기까지 자유롭게 조절하여 조각을 기판에서 떼어내어 마이크로 블록을 대량으로 만들 수 있는 플랫폼을 개발하였다. 서로 다른 높이를 갖는 라인 혹은 필라 패턴이 있는 기판을 사용하여 기판이 균열에 미치는 영향을 분석하였다. 제어된 사이즈의 균열 조각은 기판에서 떼어내어 마이크로블록으로 사용할 수 있었고, 이를 활용하는 방법을 제시하였다. 이러한 연구 결과는 패턴을 활용하여 표면을 제어할 수 있는 방법을 제시하고, 이를 통해 결점으로 여겨졌던 균열 현상을 공학적으로 응용 가능하도록 영역을 넓혀 줄 것을 기대한다.The surface properties are affected by the physical structures or chemical characteristics of the surface. As the electron microscope system advanced to allow observing in nanoscale, the fundamental researches of surface properties conducted last few decades. The superhydrophobic surfaces, which was unique property found in lotus leaf, was revealed due to the effect of hierarchical structure, many researchers try to imitate them. Patterning method was introduced to fabricate hierarchical structure to systematic analysis, however, most researches showed limitations for industrial application due to the complicated process. It was necessary to study how to create supernumerary surfaces by implementing hierarchies in a simple way. Meanwhile, mechanical instabilities such as wrinkles, folds and cracks arise when the surface is subjected to various kind of stresses. Among them, the crack phenomenon caused by tensile stress regarded as defect due to its random generation, research was focus on to prevent them. Especially in the drying colloidal film system, there has been little research on how to control it so far, because it is more complex and unevenly formed than the crack phenomenon occurring in the polymer thin film. Pattering provides a platform for systematically controlling and analyzing these phenomena occurred on surfaces. This is because the structural characteristics of the surface can be controlled by freely adjusting the size, type, and spacing of patterns in nano unit. The patterning methods are known as photo lithography, etching and soft imprinting, etc. This dissertation presents a systematic study of superhydrophobic surface and crack manipulation by applying pattern. A simple method to fabricate hierarchical structure was studied for applying industrial field and mechanical control of drop impact dynamics on patterned surfaces was analyzed. The surface crack in desiccation film was controlled by stress localization effect induced by micropatterns. Furthermore, we controlled the size of crack fragments with substrate effect to fabricate homogeneous microblocks and suggest that crack can be usefully utilized in the engineering field. In Chapter 1, we presented a study on fabricating method to transparent superhydrophobic surfaces in large areas. In order to induce a rapid reaction even at room temperature, epoxy-thiol click reaction was introduced into the binder to manufacture a colloid solution consisting of silica nanoparticles, binders and solvents. We investigated the effect of the proportion of each element on the contact angle, transparency and surface structure. Finally, we suggest the spray coating to applicable on a universal surface to coating with superhydrophobicity. In Chapter 2, an asymmetrical chemical pattern with hydrophilic properties was imprinted on the surface to control droplet after impacting the surface. A pattern with vertex was designed to induce droplets bouncing toward vertex after impacting the surface. By adjusting the angle of the pattern and the impacting distance from the vertex, we find factors that influence the degree of directionality, and compare them with geometric theoretical values. This idea provided the platform to collect the droplets on flat surface without any physical patterns in surface. In Chapter 3, we studied the method of controlling random cracks generated during surface fabrication process in Chapter 1 to well-ordered cracks via micropatterns (prisms and pyramids). Micropatterned films were fabricated by the soft imprinting technique with wet TiO2 nanoparticle pastes, followed by calcination to remove organic components. During the calcination, the volume shrinkage occurred in film and stress was concentrated on the edges of each pattern induced cracks to be produced. The degree of stress localization effect was affected by film thickness, nanoparticle size, and heating rate. In particular, film thickness also affected the area of the crack fragments. Utilizing the advantages of our experimental system, we could use a pyramid as a unit to accurately quantify the area of fragments by simply counting the number of pyramids in isolated cracks to identify the scaling relationship between film thickness and area. In Chapter 4, we developed a platform to fabricate various size of pyramidal microblocks by cracking over patterned surfaces. We studied the substrate effect on control cracks and manipulate the fragmentation of TiO2 microscopic pyramids by cracking on prepatterned substrates that have different depths in the substrate. The homogeneous mesoporous microblocks with multifunctionality and various sizes for versatile applications by detaching them from the substrate. This crack engineering method can be used to economically produce a large number of mesoporous microblocks with tunable sizes and functionalities. We believe these studies suggest the ways to control surface properties by utilizing patterns and can give guide to the new strategies on manipulating cracks in desiccation crack system.Contents Chapter 1. Large-Scale Transparent Hydrophobic Surfaces Fabricated by Spray Coating Formulation 1 1.1. Introduction 1 1.2. Experimental Section 3 1.3. Results and Discussion 5 1.3.1. Design of the One-Step Transparent Superhydrophobic Coating Formulation through Spray Process 5 1.3.2. Development of Rapid Curing Binder System in Ambient Condition through Thiol Epoxy Reaction 7 1.3.3. Effects of the Ratio of Binder and Silica NPs on Surface Morphology and Transparency 10 1.3.4 Demonstrate to the Universal Surface by Spray Coating Formulation 15 1.4. Conclusion 17 Chapter 2. Mono Directional Bouncing of Droplets Impacting on Asymmetric Hydrophilic Patterned Surfaces 18 2.1. Introduction 18 2.2. Experimental Section 20 2.3. Results and Discussion 22 2.3.1. Fabrication of Vertex Hydrophilic Patterns on Hydrophobic Surfaces 22 2.3.2. Drop Impacting on Asymmetric Hydrophilic Patterned Surfaces 24 2.3.3. Mechanical Analysis 29 2.4. Conclusion 33 Chapter 3. Control of Surface Cracks by Stress Localization Induced Micropatterns 34 3.1. Introduction 34 3.2. Experimental Section 36 3.3. Results and Discussion 40 3.3.1. Imprinting Effect on Crack Manipulation by Stress Localization Effect 40 3.3.2. Degree of Stress Localization Effect in Film Thickness 45 3.3.3. Effect of Temperature on Crack Propagation 49 3.3.4. Effect of Particle Size and Heating Rate on Critical Cracking Thickness(CCT) 51 3.3.5. Analysis of Crack Fragment Phenomena above the Critical Film Thickness 54 3.4. Conclusion 64 Chapter 4. Tiled Microblocks Obtained on Crack Template with Substrate Patterns 65 4.1. Introduction 65 4.2. Experimental Section 68 4.3. Results and discussion 71 4.3.1. Crack Manipulation Induced by Substrate Effect 71 4.3.2. Control of the Crack Initiation Tips Depending on the Substrate Effect 80 4.3.3. Fabrication of Size and Porosity Controlled Microblocks 84 4.3.4. Multifunctional TiO2 Microblocks 90 4.4. Conclusion 93 Conclusions 94 Bibliography 97 국문 초록 109Docto

Interactions between liquid jets and soap film

The interaction between thin jets of water and soap films have been studied. The experiments conducted for this thesis are the first done on jet - soap film interaction with different liquids in the two components. The results are very different from earlier studies where the same liquid was used for both components. Two behaviors were found for the interaction, depending on the incident angle between the incoming jet and soap film and the We number. For low incident angles the jet is bent by the soap film, making it possible to define an index of refraction for the soap film. The bending of the jet done by the soap film increases rapidly when approaching a We number of zero. For We numbers higher than five the jet passes through the film without changing direction. For high incident angles and low We number (≤ 1) the jet rebounds on the film, in analogy with total reflection in optics. The lifetime of the film is measured for different jet speed leading to the conclusion that there is mass exchange between the jet and film. This makes experiments impossible to perform at We ≥ 17, corresponding to a speed of 1.2 m/s. At this speed the soap film breaks before measurements can be done.Felicia Ullstad Såphinnor och vattenstrålar Såphinnor är ett väl studerat fenomen inom fysiken. De studeras som ett skolboksexempel på tunnskiktsinterferens och används mycket ofta för att demonstrera att minimering av ytor. Tunna vätskestrålar är också ett välutforskat område, då de kan användas till allt från bestrutning, i bläckskrivare till att skära i stål. Kombinationen av såphinnor och tunna vätskestrålar är däremot knappt undersökt tidigare. Vi har skjutit tunna vattenstrålar med olika fart och vinkel mot såphinnan. I figur 1 demonstreras vad som händer när farten på vattenstrålen ändras. Bilderna är tagna snett under såphinnan och den inkommande vattenstrålen kan ses svagt genom hinnan tillsammens med en reflekterad bild av den utgående vattenstrålen. Figur finns i supplementary material. Figur 1: A: Vattenstråle som går igenom såphinnan. B: Längre fart på vattenstrålen ger en större utgående vinkel. C: Ännu lägre vattenfart gör att strålen absorberas tillfälligt i hinnan. D: Lägre fart gör att strålen reflekteras mot hinnan De olika fenomen som uppkommer i figur 1 kan förklaras främst med hjälp av ytspänning, hydrofobicitet och den inkommande strålens hastighet. När vattenstrålens fart sjunker så är det lättare för strålen att interagera med såphinnan. Vid tillräckligt låga farter kan strålen inte passera genom hinnan utan fastnar i hinnan eller reflekteras. Reflektionen är ett fenomen som inte studerats tidigare och tros bero på att såphinnans yta är hydrofobisk vilket tillåter strålen att studsa mot hinnan. Hydrofobiciteten tros även vara orsak till varför det kan vara svårt att observera hur vattenstrålen fångas av hinnan. Handledare: Johan Zetterberg Examensarbete 15 hp, vt 2013 Fysiska institutionen, Lunds Universite

Anti-Icing Properties of Femtosecond Laser Surface Processed Material

In this thesis, the use of femtosecond laser surface processing (FLSP) to enhance the anti-icing properties of a commonly used aircraft alloy, Al 7075-O Clad is described. By changing the surface morphology through FLSP and the surface chemistry through siloxane vapor deposition, the wettability of Al 7075-O Clad was altered. Condensation and the subsequent freezing of condensates on FLSP Al 7075-O Clad was studied. Both structure height and surface wettability were shown to play a role in the delay of freezing. Freezing occurred on the FLSP superhydrophilic surface faster than on the unprocessed Al 7075-O Clad surface, however, freezing was delayed for all superhydrophobic FLSP surfaces. Tall structure height FLSP functionalized surfaces delayed freezing time longer than short structure height FLSP functionalized surfaces although all were superhydrophobic. It was shown that FLSP functionalized surfaces were able to delay freezing by up to 530 seconds compared to unprocessed Al 7075-O Clad. Self-propelled condensate jumping on FLSP surfaces occurs during the condensing process. The self-propelled jumping phenomena provides a means to promote anti-icing of materials, especially where jumping drops can be swept away in flow conditions The dynamics of supercooled water droplet impact onto FLSP and unprocessed surfaces was also studied. Imaging of supercooled water droplet interaction dynamics on a solid Al 7075-O Clad cold substrate for a droplet diameter below 160 µm is shown for the first time. Results indicate that microscale supercooled water droplets at low velocities will stick and freeze to unprocessed Al 7075-O Clad surfaces, while FLSP surfaces will repel droplets under similar conditions. A method for estimating the cooling of small falling water droplets in an environment of about -16 °C is described. This method gives insights for determining the temperature of supercooled droplets for the range of droplet diameters used in the experimental studies included in this paper. In addition, a way to estimate the nucleation site of a supercooled droplet by extrapolation of dendrite front velocity is provided. Advisor: Dennis Alexande