An inkjet printer can deposit a very small volume of liquid with high positional accuracy, high speed and low cost. As a maskless, non-contact additive patterning method, inkjet printing technology is increasingly being explored as an alternative to lithography, etching and vapour deposition processes to pattern electrical conductors and thin films with applications in printed electronic devices. The functional inks used in many of the applications involve non-linear viscoelasticity and their behaviours in the context of inkjet printing have not been fully understood. This thesis aims to characterise Newtonian and non-Newtonian properties of inkjet fluids and identify the key parameters affecting drop impact and spreading processes. Various fluid characterisation techniques such as the filament stretching rheometer and piezoelectric axial vibrator are explored. We propose an experimental method to assess the jettability of non-Newtonian inkjet fluids, without using an inkjet print head. The oblique collision of two continuous liquid jets leads to the formation of a thin oval liquid sheet bounded by a thicker rim which disintegrates into ligaments and droplets. Under certain conditions the flow structure exhibits a remarkably symmetrical 'fishbone' pattern composed of a regular succession of longitudinal ligaments and droplets. Good correlation was found between the maximum included angle of the fishbone pattern and the maximum ligament length in the jetting experiments, which suggests that a test based on oblique impinging jets may be useful in the development of fluids for ink jet printing. High-speed imaging is used to analyse the impact and spreading of sub-30 μm drops of diethyl phthalate or polystyrene solutions in diethyl phthalate on to smooth glass surfaces with controlled wettability at speeds from 3 to 8 m s-1, under conditions representative of drop-on-demand inkjet printing. Data on drop height and spreading diameter are generated with high time and spatial resolution, over eight orders of magnitude in timescale. The effects of fluid viscosity and elasticity, which significantly affect jetting performance, are negligible throughout the whole deposition process, with no significant difference between spreading curves. The values of the fluid surface tension and the substrate wettability also have no effect on the kinematic, spreading or relaxation phases, but a marked influence on the wetting phase, in terms of the speed of expansion of the contact diameter and the final spreading factor.

This dissertation describes experiments on drop dynamics. It is split into two main parts: The first investigates the breakup of liquid during the impact of a drop on a pool surface, with focus on the smallest and fastest splashed satellite droplets. The second part studies the much slower coalescence of two minute water droplets in oil inside a micro-channel, with applications to separation of water droplets from crude oil emulsions. First, we study drop-on-liquid impacts in high-speed experiments with extreme time and spatial resolutions using up to 5 million frames-per-second video imaging. This is used to identify and explain two primary mechanisms which produce the smallest and fastest splashed secondary droplets, i.e. ejecta sheets and singular jets. Using a novel 25-m-tall vacuum tube we generate very large impact velocities, to reach regimes in parameter-space never studied before. During the earliest stage of the impact a fast-moving horizontal ejecta sheet emerges from the neck between drop and pool. The breakup of this sheet forms a myriad of micron-size droplets. The ejecta bending is dominated by air resistance, which we investigate under reduced ambient pressures and successfully model based on Bernoulli suction which pulls down the ejecta to hit the pool surface. The ejecta can initially bend up or down depending on the relative viscosities of the drop and pool, bending up if the pool is less viscous. Singular jets are produced by the collapse of drop-impact craters for deep pools, when a dimple forms at the bottom of the crater focusing the energy into a micron-sized region, with jetting velocities over 100 m/s. We use Gerris to study the fine details, obscured in the experiments. In the second part, we study the coalescence of water droplets inside an oil emulsion, developing an empirical relation between the coalescence interaction time tc and the modified shear-rate. This is done by tracking 3-D drop trajectories inside a microchannel, with two perpendicular high-speed cameras. For droplets in crude oil, we implement near-infrared visualization in an innovative device to quantify demulsifier efficiency, using mono-disperse micro-droplets.

Flying insects face challenging conditions such as rainfall, fog, and dew. In this theoretical and experimental thesis, we investigate the survival mechanisms of the mosquito, Anopheles, through particles of various size. Large particles such as falling raindrops can weigh up to fifty times a mosquito. Mosquitoes survive such impacts by virtue of their low mass and strong exoskeleton. Smaller particle sizes, as present in fog and insecticide, pose the greatest danger. Mosquitoes cannot fly through seemingly innocuous household humidifier fog or other gases with twice the density of air. Upon landing, fog accumulates on the mosquito body and wings, which in small quantities can be shaken off in the manner of a wet dog. Large amounts of dew on the wings create a coalescence cascade ultimately folding the wings into taco shapes, which are difficult to dry. The insights gained in this study will inform the nascent field of flapping micro-aerial vehicles.

This project was concerned with the development of a finite element model capable of simulating a drop-impact event of handheld radio devices. Handheld radios call for exceptional robustness and reliability due to their deployment in critical applications. The development of a drop-impact finite element model aims to provide greater understanding of impact behaviours, this insight would ultimately be used to develop more robust and optimised handheld electronic products. Before such analysis tools can be introduced into the product development cycle an understanding of finite element methods, of setup parameters for the finite element solver and the accuracy of simulation results must be considered. Experimental results were used throughout the project to validate the finite element models developed. A drop-impact test rig was designed and constructed to control both impact orientation and velocity of the handheld radios tested. Drop-impact modelling of a handheld radio is extremely challenging because of the complex interaction of the contacting surfaces, the complex stress-strain and damping characteristics of the materials, and the excitation of the high frequency modes. For this reason, the finite element model was developed in two stages: a simplified radio was used to develop the understanding of the above complexities and then the understanding implemented in a more detailed radio model. The mesh size of the finite elements, the elastic and the damping characteristics of the materials and the contact conditions for the simplified radio model were varied to understand their influence on the simulation results. The finite element input settings and parameters were altered to give better agreement with the experimental results of the simplified radio model. The detailed radio was subsequently modelled. The lessons learnt from the simplified radio model were applied to the analysis of the detailed radio assembly. Despite general agreements, there were some differences between the finite element and experimental results which was attributed to the high complexity of the model. The project delivered a workable finite element model capable of analysing the drop-impact event of handheld radio devices. Suggestions have been provided that would further improve the quality of the model.

Drop impacts play a key role in many industrial applications, from spray coating of surfaces, to splashing of fuel-droplets within combustion chambers. Splashing, or break-up during ink-jet printing, can cross-contaminate biological assays, or degrade the quality of ink-jet printed products. Crime scene studies of blood splatter can give vital clues for the police. Spreading of plant diseases between nearby leaves by splashing depends on the velocity and trajectory of secondary droplets. In this dissertation, I study the early dynamics of splashing and the dynamics of ejecta sheets under extreme impact conditions, using ultra-high-speed video imaging at up to 5 million fps. In the first part, I show the effect of the surface tension differences on the break-up of the Edgerton crown, I verify that individual droplets hit the crown wall and generated Marangoni holes, thereby causing the crown wall to rupture at multiple locations. In the second part, I investigate the splashing of a drop impacting onto a solid substrate with high impact velocity, I show that for sufficiently high Re, splashing can no longer be suppressed by only reducing the surrounding air pressure. Furthermore, I tracked the earliest splashed spray droplets to catch their maximum velocity. Surprisingly, the splashed droplets can travel at extremely high speed of up to 1 km/s, which is 50 times faster than the impact speed. The influence of viscosity on the lamellar spreading along the substrate was investigated. I find that the intact lamella, following the fine spray, spreads as R(t) ~〖 t〗^(1/3) , while the maximum spreading radius of the drop was shown to be a strong function of viscosity, scaling as β_max∝〖Re〗^0.175. The data did not show a strong effect of surface tension on β_max over a wide range. Therefore, I concluded that surface tension at this parameter space does not play a major role in both splashing nor spreading. In the third part, I study extreme splashing dynamics of the Ejecta sheet when a drop impacts on a thin liquid film with very large impact velocities using the same device, at up to ~ 22 m/s. For this purpose, we have constructed a novel experimental device consisting of a 26-m-tall vacuum tube. I investigate the interplay between viscosity, the surrounding ambient air pressure, and surface tension, on the ejecta shapes and break-up. I show how the bending of the ejecta sheet is primarily produced by air-resistance. This is supported by an analytical and numerical model to quantify the effect of the surrounding air pressure on the sheet bending and touch-down.

This work uses high-speed video imaging to study the formation of filaments, during impact and rebounding of drops with polymer additives. We use PEO of different concentrations from 10 to 1000 ppm and study how drops rebound from various different surfaces: superhydrophilic, hydrophilic, hydrophobic, and superhydrophobic. Bouncing occurs for all surfaces at low impact velocities. We specifically focus on the phenomenon of the generation of polymer filaments, which are pulled out of the free surface of the drop during its rebounding from micro-pillared or rough substrates. We map the parameter regime, in terms of polymer concentration and impact Weber number, where the filaments are generated in the most repeatable manner. This occurs for regularly pillared surfaces and drops of 100 ppm PEO concentrations, where numerous separated filaments are observed. In contrast, for superhydrophobic coatings with random roughness the filaments tend to merge forming a branching structure. Impacts on inclined surfaces are used to deposit the filaments on top of the pillars for detailed study.

This dissertation talks about the dynamics of the drop impact in two parts, the impact of the drop on the deep liquid pool with singular jet and sound emission, and the bouncing drop with filaments on the superhydrophoic solid surface. First, we use experiments and simulations to study drop impacts on a deep liquid pool, with a focus on fine vertical jetting and underwater sound emission from entrapped bubbles, during the rebounding of the hemispherical crater. The much larger parametric complexity introduced by the use of two immiscible liquids, compared to that for the same liquid, leads to an extended variety of compound-dimple shapes. The fastest jet occurs from the rebounding of a telescope dimple shape without bubble pinch-off, at around 45 m/s, which leaves a toroidal micro-bubbles from the air-cusp at the base of the dimple. The finest jets have diameter of only 12 µm. A new focusing mechanism for singular jetting from collapsing drop-impact craters is then proposed based on high-resolution numerical simulations. The fastest jet is confined in a converging conical channel with the entrained air sheet providing a free-slip outer boundary condition. Sound can be emitted from the oscillation of the entrapped dimple-bubble, while the tiny bubble from the initial impact is induced to oscillate with the entrapped bubble, triggering the double crest of the acoustic signal. We track the compression of the bubble volume from the high-speed imaging and relate it to the hydrophone signal. In the second part, we investigate the impact of a polymeric drop on a superhydrophobic solid substrate with micropillar structure. The drop spreads on the substrate, wets the tops of the pillars, and rebounds out of the superhydrophobic soild surface. Numerous liquid filaments are stretched from the liquid drop to the attached adjacent pillars, and minuscule threads would be left on the top of the pillars using the inclined superhydrophobic solid surface. The well-organized exposed polymer threads are left on the top of the pillars after solvent evaporation. The thickness of the deposition of filament bundles using the bouncing method are thinner than those formed by drop evaporation or drop rolling from SEM (scanning electron microscope) observation.

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.

Solid surfaces featuring a spatial variation of surface wettability along particular directions on their surface, referred to as wettability gradient surfaces, are becoming increasingly important in practical applications such as enhancement of boiling and condensation heat transfer and separation of immiscible liquids in smart micro-fluidic devices. With the aid of an external energy input, such as mechanical vibration or impact kinetic energy, a liquid drop on such surfaces gets propelled towards more wettable region on the surface. A fundamental study of impact dynamics of liquid drops on such solid surfaces is relevant in understanding their effectiveness. The present thesis reports a combined experimental and theoretical study on the impact dynamics of liquid drops on solid surfaces comprising a smooth portion and a groove-textured portion separated by a junction line (dual-textured surfaces). Three different dual-textured surfaces – two made of intrinsically hydrophilic stainless steel and one of intrinsically hydrophobic poly-di-methyl-siloxane (PDMS) – are considered. Liquid drops, with Weber number (We) in the range 1–100, are impacted on the junction of the dual-textured surfaces and the entire impact dynamics across the junction is captured using a high speed video camera. Experiments of drop impact on the homogeneous surface portions of dual-textured surfaces (far away from the junction) are also conducted. The temporal variation of drop contact radius measured from the junction line on smooth and groove-textured portions of the dual-textured surfaces exhibits four distinct stages – primary spreading, primary receding, secondary spreading on more wettable surface portion, and final equilibrium – with the final outcome being the bulk movement and deposition of liquid drop away from the junction towards the more hydrophilic surface portion. Secondary parameters characterizing each of these different stages are extracted from these measurements and a one-to-one comparison between dual-textured and homogenous surfaces is presented. A significant effect of dual-texture nature is seen on the receding process of impacting drops. On the dual-textured surfaces, the receding velocity of impacting drop on the groove-textured portion is always greater than that on the smooth portion. The asymmetry in drop receding results in a drop drift velocity towards the more wettable surface portion leading to an enhanced secondary drop spreading on the more wettable smooth portion. The drop drift velocity shows a decrease with We at low We and remains almost constant at higher We after a particular value of We. Correspondingly, the ratio of the maximum drop spread factor achieved during the secondary spreading (βm2) to that during the primary spreading (βm) is seen to decrease with We at low We and remains constant at higher We. Owing to the differences in the static equilibrium wetting difference, βm2/βm is more on the stainless steel dual-textured surfaces than on the PDMS dual-textured surface. The presence of dual-texture results in a higher final spread on more wettable smooth portion and smaller final spread on less wettable textured portion of the dual-textured surfaces and this difference decreases with We. The difference in final spread factors between the smooth and textured portions is more on the stainless steel dual-textured surfaces than the PDMS dual-textured surface. The bulk drop movement (ξ), characterized in terms of distance measured from the junction to the final drop center, decreases with We at low We and remains constant at higher We on the stainless steel dual-textured surfaces whereas it remains constant at low We and decreases at higher We on the PDMS dual-textured surface. ξ on the PDMS dual-textured surface is always less than that on the stainless dual-textured surface due to the lower wetting difference across the junction of the former. Comparison of the trends of secondary parameters with the predictions from theoretical models reported in literature showed a lack of agreement. This is due to various physical processes encountered by impacting drop on the groove-textured surface, identified through experiments of drop impact on homogeneous groove-textured surfaces, such as (i) convex shape of liquid-vapor interface near contact line at maximum spreading, (ii) impregnation of drop liquid into the grooves during impact, and (iii) contact line pinning of spreading drop at the asperity edges of surface texture, as well as the wetting difference in dual-textured surfaces. The inclusion of these physical processes under conventional energy conservation approach is seen to predict the experimentally observed trends of maximum drop spread factor on the groove-textured portions. A force balance model, applied to the liquid drop configuration at the beginning of drop receding on the dual-textured surfaces, predicts the qualitative trend of ξ with We on all surfaces. Drop liquid impregnation into the grooves of textured portion at We > Wecr (critical We corresponding to transition from Cassie to impaled state) is proposed as a possible physical mechanism to account for the explanation of the specific trends of ξ with We. A theoretical model formulated using force balance at the three phase contact line beneath impacting drop on groove-textured surface is presented for the prediction of Wecr.

Orientadores: Edvaldo Sabadini, Marcelo G. de Oliveira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-04T14:22:34Z (GMT). No. of bitstreams: 1 Alkschbirs_MelissaInger_D.pdf: 20703558 bytes, checksum: 093e76dc07818947b87478f9a799ad47 (MD5) Previous issue date: 2004 / Resumo: O efeito da redução de atrito hidrodinâmico em soluções poliméricas nas estruturas produzidas pelo impacto de gotas foi determinado por meio da análise de imagens obtidas utilizando uma câmera CCD e um programa de tratamento de imagens. O impacto de uma gota contra uma superficie líquida causa o fenômeno conhecido como splash, onde duas principais estruturas são formadas: a coroa e o jato Rayleigh. A altura máxima atingida pelo jato Rayleigh foi utilizada como parâmetro para determinar a redução de atrito hidrodinâmico, proporcionada pela presença de pequenas quantidades (ppm) de polímeros de elevada massa molar presentes em solução. A capacidade redutora de atrito hidrodinâmico do poli( óxido de etileno ), PEO, o mais eficiente agente redutor de atrito, foi estudada em função da qualidade do solvente, da temperatura, da concentração, da massa molar e da flexibilidade intrínsica da cadeia polimérica. As modificações decorrentes de alguns destes fatores sobre o raio de giração do polímero e, conseqüentemente sobre o tempo de relaxação e a viscosidade elongacional, são responsáveis pelas modificações morfológicas observadas no jato Rayleigh. Estudos temporais da evolução do splash também foram desenvolvidos, onde se procurou correlacionar a taxa de deformação do líquido com o tempo de relaxação da cadeia polimérica. O presente trabalho mostra, de forma inédita, que é possível utilizar o splash nos estudos sobre a redução de atrito hidrodinâmico / Abstract: The presence of very small amounts (ppm) of high-molecular weight polymers in a solution produces high levels of drag reduction in a turbulent flow. This phenomenon, termed as the Toms Effect, was studied using images of the impact of a small drop against shallows liquid surfaces, both liquids containing a drag reducer agent. After the impact a crown and a cavity are created and the collapse of these structures impels a liquid column, named as Rayleigh jet. This phenomenon is termed splash. The amplitude reached by the Rayleigh jet was used to estimate the energy of the drop stored in the liquid; therefore, the maximum height of the jet allow us to determine the percentage of drag reduction. The results were discussed in terms of different parameters such as polymer concentration, molecular weight in the poly(ethylene oxide), PEO, the most efficient drag reducer agent in aqueous system. The splash in aqueous polymeric solution is dominated by the elongational viscosity and therefore, the polymer relaxation time has an important role in the process. We consider that the main contribution of this work to the drag reduction field is the new approach proposed to investigate this old hydrodynamic phenomenon / Doutorado / Físico-Química / Doutor em Quimica