Yes / Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.

No / There is an increasing interest in the use of Ground Source Heat Pumps (GSHPs) as a source of renewable energy in temperate countries. GSHPs coupled with buried heat collectors can harness the thermal energy from near-surface soils to provide the heating required for domestic properties. The performance of a GSHP system depends greatly on the thermal conductivity of the surrounding soils. Near-surface soils undergo cycles of drying and wetting due to, for example, the infiltration of rain water and/or fluctuations of the ground water table. Several parameters - including the properties of soil, suction head and saturation history - affect the thermal properties as well as the retention and flow of water. This paper presents results from a comprehensive laboratory investigation on sand samples with markedly different grain size distribution. Simultaneous measurements of thermal and hydraulic properties of the sands were taken under incremental increase/decrease in the suction head values to simulate cycles of drying and wetting. The results clearly suggest that the thermal conductivity is better expressed as a function of the matric suction head so as to reflect the saturation history. There has been almost five-fold increase in the measured value of thermal conductivity when the soil was wetted to a residual degree of saturation from being dry.

Interfacial phenomena are prevalent in various natural and engineered systems. A thorough understanding of these phenomena is essential for a complete understanding of processes such as phase transitions and interaction of liquid droplets with different surfaces. The insights gained from understanding interfacial behavior are pivotal in fields such as pharmaceuticals, microfluidics, material sciences, and environmental engineering. This dissertation aims to advance our understanding of interfacial behaviors, thereby facilitating the development of innovative technologies for applications in health, defrosting, and omniphobic surfaces. In Chapters 1 and 2, relevant background information and goals are provided to contextualize the research being presented in this dissertation. Chapter 3 introduces a novel metal-free alternative to conventional antiperspirants (containing aluminum salts and zirconium salts). We leverage the composition of human sweat (97% water and 3% minerals) and employ a hygroscopic substance near the outlet of an artificial sweat duct rig. This leads to complete diffusion and dehydration of sweat, forming a natural mineral plug within the artificial sweat duct that halts the flow. Chapter 4 examines the behavior of room temperature water droplets spreading on a flat icy substrate. The use of flat ice, as opposed to cold substrates, eliminates the nucleation energy barrier, enabling freeze front initiation as soon as the bulk temperature of the spreading drop reaches 0 C. Through scaling analysis, we identify distinct thermo-hydrodynamic regimes with varying Weber numbers. Chapter 5 presents a novel construct for lubricant-impregnated surfaces (LIS). To date, most of the investigations characterizing the wettability of LIS have focused on droplet mobility. We pioneer a lubricant-impregnated fiber (LIF) which exhibits unique droplet dynamics due to simultaneous exploitation of both, high mobility and high adhesion. Chapter 6 proposes an innovative approach for defrosting by exploiting the polarizability and natural thermo-voltage of frost sheets. By placing an actively charged electrode near the frost sheet, we observe that frost dendrites migrate towards the electrode. This technique, termed Electrostatic Defrosting (EDF), effectively removes up to 75% of the frost mass for superhydrophobic surfaces and 50% of the frost mass for untreated surfaces in less than 100 s. / Doctor of Philosophy / Raindrops falling on surfaces, pesticides being sprayed on crops, and frost forming on windshields—these seemingly unrelated phenomena all stem from fundamental water-structure interactions and phase change processes. We encounter these occurrences throughout nature, with some being enchanting, like water dancing on lotus leaves or morning dew sparkling on glass, while others can pose risks, such as condensation impairing visibility while driving. This dissertation aims to enhance our understanding of water-structure interactions by utilizing the phase changes of water (transitioning between vapor and ice). Through this exploration, we seek to develop innovative technologies for health, de-icing, and fog harvesting, highlighting the practical applications of such water-structure interactions. Through four distinct projects, we aim to unlock innovative solutions that enhance everyday life and address pressing environmental challenges. In the first project, we introduce a novel antiperspirant construct that utilizes sweat's own minerals to clog sweat ducts by vaporizing water with a hygroscopic material. The second project investigates droplet dynamics on ice, focusing on how freezing initiates at the contact line when droplets make contact. In the third project, we develop a new design for oil-impregnated surfaces by embedding fibers, characterizing droplet behavior on these curved surfaces. We envision these fibers being utilized in industrial fog harvesting systems, where water can be effectively collected through dropwise condensation. Finally, we present an innovative defrosting method that exploits naturally occurring thermovoltage in frost, using a positively charged electrode to facilitate the removal of frost sheets. Together, these projects illustrate the impact of water-structure interactions on technology and the environment.

Omniphobic surfaces, which repel polar and non-polar liquids alike, have proven of value in a myriad of applications ranging from piping networks, textiles, food and electronics packaging, and underwater drag reduction. A limitation of currently employed omniphobic surfaces is their reliance on perfluorinated coatings/chemicals, increasing cost and environmental impact and preventing applications in harsh environments. Thus, there is a keen interest in rendering conventional materials, such as hydrocarbon-based plastics, omniphobic by micro/ nanotexturing rather than via chemical makeup, with notable success having been achieved for silica surfaces with doubly reentrant pillars (DRPs). We discovered a critical limitation of DRPs – they catastrophically lose superomniphobicity in the presence of localized physical damages/defects or on immersion in wetting liquids. In response, we pioneered bio-inspired gas-entrapping microtextured surfaces (GEMS) architecture composed of doubly reentrant cavities (DRCs). DRCs are capable of robustly entrapping air when brought into contact with liquid droplets or on immersion, which prevents catastrophic wetting transitions even in the presence of localized structural damage/defects. This dissertation presents our multifaceted research on DRCs via custom-built pressure cells, confocal laser scanning microscopy, environmental scanning electron microscopy, contact angle goniometry, high-speed imaging, and upright optical microscopy. Specific accomplishments detailed in this thesis include: (i) the microfabrication protocols for silica GEMS developed at KAUST; (ii) the characterization of GEMS’ omniphobicity via apparent contact angles and immersion; (iii) the demonstration of ~ 1000,000,000% delays in wetting transitions in DRCs compared to those in simple cavities (SCs) under hexadecane; (iv) a proposal for immersion of surfaces as a criterion for assessing their omniphobicity in addition to apparent contact angles; (v) effects of surface chemistry, hydrostatic pressure, and cavity dimensions on Cassie-to-Wenzel transitions in DRCs and SCs; (vi) the demonstration of “breathing” (liquid-vapor) interfaces in GEMS under fluctuating hydrostatic pressures; and (vii) the demonstration of directional wetting transitions in DRCs (or cavities in general) arranged in one- and two-dimensional lattices. The last chapter in the thesis presents future research directions such as breathing surfaces capable of preempting vapor condensation and gas replenishment.

Improving water use efficiency is included in the Sustainable Development Goals of the United Nations. Ethiopia is a developing country struggling with food production as well as water scarcity. This study presents the results of a statistical analysis of changes in water productivity (i.e. yield versus water usage), wheat yield and irrigation amount by implementation of irrigation scheduling in Koga Irrigation Scheme, north-west Ethiopia. Highest water usage (570 mm), lowest water productivity (0.5 kg m-3) and lowest yield (2800 kg ha-1) were obtained for the control group (i.e. traditional irrigation scheduling, based on experience). All groups which implemented some irrigation scheduling displayed higher water productivity than the control group. The highest water productivity and yield was achieved with a soil moisture sensor (Chameleon) technology, with increases of 58 % and 32 % with respect to the control group, respectively. Nitrogen had a positive effect on both yield and water productivity, however, the interaction effects between applied nitrogen and implemented irrigation scheduling were considered insignificant. This study is concluding that implementation of irrigation scheduling should be a successful approach for improving yield as well as water productivity in Koga. / En förbättrad effektivitet i vattenanvändningen ingår i Förenta nationernas Globala mål för hållbar utveckling. Etiopien är ett utvecklingsland med utmaningar i såväl matproduktion som vattenbrist. Denna studie presenterar resultaten av en statistisk analys av förändringar i vattenproduktivitet (dvs skörd per vattenmängd), skörd och bevattningmängd genom implementering av verktyg för bevattningsplanering i Koga bevattningsområde, nordvästra Etiopien. Högsta vattenförbrukning (570 mm), lägsta vattenproduktivitet (0,5 kg m-3) och lägsta skörd (2800 kg ha-1) erhölls för kontrollgruppen. Alla grupper som infört någon typ av bevattningsplanering visade högre vattenproduktivitet än kontrollgruppen (dvs traditionell bevattningsplanering baserad på erfarenhet). Den högsta vattenproduktiviteten och skörden uppnåddes med en vattenfuktsmätare (Chameleon), med ökningar på 58 % respektive 32 % jämfört med kontrollgruppen. Kväve hade en positiv effekt på både skörd och vattenproduktivitet, men interaktionseffekterna mellan kväve och de implementerade bevattningsplaneringarna ansågs försumbara. Denna studie drar slutsatsen att införandet av någon typ av bevattningsplanering bör vara ett framgångsrikt tillvägagångssätt för att förbättra skörd samt vattenproduktivitet i Koga. / “Using Remote Sensing in support of solutions to reduce agricultural water productivity gaps” (Capacity development for increasing water productivity) (GCP/INT/229/NET)

This master’s thesis deals with measuring and evaluation of wetting for samples of different surface finishes, using a test coupon developed in cooperation with firm Gatema. It deals with surface finishes ENIG and immersion tin. For these samples deals with quality monitoring and periodic testing of these surface finishes. The solder using for tests is SAC305. Test methods are used for simulation of reflow soldering and wave soldering.

This work deals with flushing monitoring of spreading velocity on solder finishes Niau, OSP, Sn deposited on copper plated base material FR4 using digital cameras. After the reflow process is measured by length of spreading solder and size of wetting angle. It evaluates and compares the time dependence of velocity of spreading solder, length of solder spreading and wetting angle for NiAu, OSP, Sn finishes at different temperatures reflow and surface treatment.

In this dissertation physical phenomena relevant to (i) an interface formed between two fluids and a solid phase (wetting line) and (ii) an interface between three fluids (triple contact line) were investigated. In the former case, the wetting line (WL) phenomena which encompass the wetting line energy (WLE) or pinning, the wetting line velocity (WLV), and the contact angle hysteresis, were studied using a micropump based on electrowetting on dielectric (EWOD). In the latter case, the interfacial phenomena such as the air film lubrication effect and the liquid free surface deformation were taken into account to explain the dual equilibrium states of water droplets on liquid free surfaces. EWOD was implemented to devise a pumping method for a continuous flow in a microchannel. An active micropump with a simple layout and no moving parts is designed and fabricated which has on demand flow on/off capability. The micropump is based on droplet/meniscus pressure gradient generated by EWOD. By altering the contact angle between liquid and solid using an electric field a pressure gradient was induced and a small droplet was pumped into the channel via a uniform flow rate. A surface tension based propellant method was introduced as a low power consumption actuation method in microfluidic devices. For an initial droplet volume of 0.3µL and a power of 12nW a constant flow rate of 0.02µL/sec was demonstrated. Sample loading on-demand could be achieved by regulating an electric potential. Unexpectedly, the flow rate of the pump was found to be constant in spite of the changes in the droplet’s radius, which directly affects the pump’s driving pressure. iv The WL phenomena were studied in details to unravel the physical concept behind the micropump constant flow rate during the operation. An interesting observation was that the shrinking input droplet changes its shape in two modes in time sequence: (i) in the first mode its contact angle decreases while its wetting area remains constant due to the pinning, (ii) in the second mode the droplet’s WL starts to move while its contact angle changes as a function of its velocity. Contact angles were measured for the droplet advancing and receding WLs at different velocities to capture a full picture of contact angle behavior due to pinning and WLV effects. These results are also relevant to the meniscus inside the channel. The changes on the contact angle caused by the presence of EWOD at the bottom of the channel were studied in detail. The EWOD based micropump was used as a platform to study the contribution of the pinning and WLV effects on its constant flow rate. The effects of the WLE on the static contact angle and the WLV on the dynamic contact angle in the pump operation were investigated. Also the effect of EWOD voltage on the magnitude and uniformity of the micropump flow rate was studied. Dynamic contact angles (as a function of pinning and WLV) were used to accurately calculate the pressure gradient between the droplet and the meniscus and estimate the flow rate. It was shown that neglecting either of these effects not only results in a considerable gap between the predicted and the measured flow rates but also in an unphysical instability in the flow rate analysis. However, when the WLE and WLV effects were fully taken into account, an excellent agreement between the predicted and the measured flow rates was obtained. v For the study of the TCL between three fluids, aqueous droplets were formed at oil-air interface and two stable configurations of (i) non-coalescent droplet and (ii) cap/bead droplet were observed. General solutions for energy and force analysis were obtained and were shown to be in good agreement with the experimental observations. Further the energy barrier obtained for transition from configuration (i) to (ii), was correlated to the droplet release height and the probability of non-coalescent droplet formation. Droplets formed on the solid surfaces and on the free surface of immiscible liquids have various applications in droplet-based microfluidic devices. This research provides an insight into their formation and manipulation.

Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.   Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.   The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed. / <p>QC 20160329</p>

Millions of years of evolution have led to a wealth of highly adapted functional surfaces in nature. Among the most fascinating are superhydrophobic surfaces which are highly water-repellent and shed drops very easily owing to their chemical hydrophobicity combined with micropatterning. Superhydrophobic materials have attracted a lot of attention due to their practical applications as ultra-low friction surfaces for ships and pipes, water harvesters, de-humidifiers and cooling systems. At small length scales, where surface tension dominates over gravity, these surfaces show a wealth of phenomena interesting to physicists, such as directional flow, rolling, and drop bouncing. This thesis focuses on two examples of dynamic drop interactions with micropatterned surfaces and studies them by means of a lattice Boltzmann simulation approach. Inspired by recent experiments, we investigate the phenomenon of the self-propelled bouncing of coalescing droplets. On highly hydrophobic patterned surfaces drop coalescence can lead to an out-of-plane jump of the composite drop. We discuss the importance of energy dissipation to the jumping process and identify an anisotropy of the jumping ability with respect to surface features. We show that Gibbs' pinning is the source of this anisotropy and explain how it leads to the inhibition of coalescence-induced jumping. The second example we study is the novel phenomenon of pancake bouncing. Conventionally, a drop falling onto a superhydrophobic surface spreads due to its inertia, retracts due to its surface tension, and bounces off the surface. Here we explain a different pathway to bouncing that has been observed in recent experiments: A drop may spread upon impact, but leave the surface whilst still in an elongated shape. This new behaviour, which occurs transiently for certain impact and surface parameters, is due to reversible liquid imbibition into the superhydrophobic substrate. We develop a theoretical model and test it on data from experiments and simulations. The theoretical model is used to explain pancake bouncing in detail.