Making a splash with water repellency

A 'splash' is usually heard when a solid body enters water at large velocity. This phenomena originates from the formation of an air cavity resulting from the complex transient dynamics of the free interface during the impact. The classical picture of impacts on free surfaces relies solely on fluid inertia, arguing that surface properties and viscous effects are negligible at sufficiently large velocities. In strong contrast to this large-scale hydrodynamic viewpoint, we demonstrate in this study that the wettability of the impacting body is a key factor in determining the degree of splashing. This unexpected result is illustrated in Fig.1: a large cavity is evident for an impacting hydrophobic sphere (1.b), contrasting with the hydrophilic sphere's impact under the very same conditions (1.a). This unforeseen fact is furthermore embodied in the dependence of the threshold velocity for air entrainment on the contact angle of the impacting body, as well as on the ratio between the surface tension and fluid viscosity, thereby defining a critical capillary velocity. As a paradigm, we show that superhydrophobic impacters make a big 'splash' for any impact velocity. This novel understanding provides a new perspective for impacts on free surfaces, and reveals that modifications of the detailed nature of the surface -- involving physico-chemical aspects at the nanometric scales -- provide an efficient and versatile strategy for controlling the water entry of solid bodies at high velocity.Comment: accepted for publication in Nature Physic

Capillary-scale solid rebounds:Experiments, modelling and simulations

A millimetre-size superhydrophobic sphere impacting on the free surface of a quiescent bath can be propelled back into the air by capillary effects and dynamic fluid forces, whilst transferring part of its energy to the fluid. We report the findings of a thorough investigation of this phenomenon, involving different approaches. Over the range from minimum impact velocities required to produce rebounds to impact velocities that cause the sinking of the solid sphere, we focus on the dependence of the coefficient of restitution, contact time and maximum surface deflection on the different physical parameters of the problem. Experiments, simulations and asymptotic analysis reveal trends in the rebound metrics, uncover new phenomena at both ends of the Weber number spectrum, and collapse the data. Direct numerical simulations using a pseudo-solid sphere successfully reproduce experimental data whilst also providing insight into flow quantities that are challenging to determine from experiments. A model based on matching the motion of a perfectly hydrophobic impactor to a linearised fluid free surface is validated against direct numerical simulations and used in the low Weber number regime. The hierarchical and cross-validated models in this study allow us to explore the entirety of our target parameter space within a challenging multi-scale system

Enhanced Laminar Convective Heat Transfer using Microstructured Superhydrophobic Surfaces

For many centuries, researchers have investigated the complex interactions between a solid surface and a fluid in motion relative to the surface. For many cases, the classical no slip boundary condition holds true. However, there are a subset of situations where this assumption is not valid, and slip between the surface and fluid must be considered. One such example is a micropatterned, superhydrophobic surface, which has been shown to enable slip resulting in a decrease in drag and pressure loss for both laminar and turbulent flow. The hydrodynamic effects of these surfaces have been studied in depth, but the effects on heat transfer are largely unknown. The primary goal of this research effort was to explore the effects of slip flow on laminar convective heat transfer resulting from micropatterned, superhydrophobic surfaces. The first step toward achieving the research goal was to develop a model to study first order effects, predict the effect of slip flow on heat transfer, and design the experimental setup. The general momentum equation for Poiseuille flow was solved using modified boundary conditions consistent with slip flow, and the resulting velocity profile was input into the thermal balance equation which was numerically solved. The model assumed hydrodynamic slip but not thermal slip nor a temperature jump at the boundary, and as a result, it predicted a net increase in heat transfer performance. For the experimental portion of the study, laminar Poiseuille flow in a parallel plate configuration with a constant temperature boundary condition at 273 K using an ice bath was studied. Four sets of copper sample plates measuring 15 cm by 3.8 cm were fabricated with different surface condition: 1) uncoated smooth, 2) hydrophobic coated smooth, 3) uncoated micropatterned, and 4) hydrophobic coated micropatterned. The micropattern was a laser machined array of 25 \uf06dm x 25 \uf06dm microridges oriented in the streamwise direction. Contact angle measurements were made on all of the test samples to ensure the coated plates were hydrophobic and the uncoated plates were not. From the experimental results, several observations and conclusions were made. First, only the micropatterned, superhydrophobic coated sample achieved a slip state with an average slip length of 0.3 mm. Second, hydrodynamic slip was observed without the accompaniment of thermal slip since the heat transfer performance for the superhydrophobic sample was as good as or better than the baseline sample for all flow rates tested. Finally, it was concluded that micropatterned superhydrophobic surfaces reduce pressure loss and improve heat transfer as seen by the improved efficiency factor, which is the ratio between the Nusselt number and the friction loss

Designing Robust Hierarchically Textured Oleophobic Fabrics

Commercially available woven fabrics (e.g., nylon- or PET-based fabrics) possess inherently re-entrant textures in the form of cylindrical yarns and fibers. We analyze the liquid repellency of woven and nanotextured oleophobic fabrics using a nested model with n levels of hierarchy that is constructed from modular units of cylindrical and spherical building blocks. At each level of hierarchy, the density of the topographical features is captured using a dimensionless textural parameter D[subscript n][superscript *]. For a plain-woven mesh comprised of chemically treated fiber bundles (n = 2), the tight packing of individual fibers in each bundle (D[subscript 2][superscript *] ≈ 1) imposes a geometric constraint on the maximum oleophobicity that can be achieved solely by modifying the surface energy of the coating. For liquid droplets contacting such tightly bundled fabrics with modified surface energies, we show that this model predicts a lower bound on the equilibrium contact angle of θE ≈ 57° below which the Cassie–Baxter to Wenzel wetting transition occurs spontaneously, and this is validated experimentally. We demonstrate how the introduction of an additional higher order micro-/nanotexture onto the fibers (n = 3) is necessary to overcome this limit and create more robustly nonwetting fabrics. Finally, we show a simple experimental realization of the enhanced oleophobicity of fabrics by depositing spherical microbeads of poly(methyl methacrylate)/fluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) onto the fibers of a commercial woven nylon fabric.United States. Army Research Office (W911NF-13-D-0001

Capillary Forces in Partially Saturated Thin Fibrous Media

Capillarity is often exploited in self-cleaning, drag reducing and fluid absorption/storage (sanitary products) purposes just to name a few. Formulating the underlying physics of capillarity helps future design and development of optimized structures. This work reports on developing computational models to quantify the capillary pressure and capillary forces on the fibrous surfaces. To this end, the current study utilizes a novel mass-spring-damper approach to incorporate the mechanical properties of the fibers in generating virtual fibrous structures that can best represent fibrous membranes. Such virtual fibrous structures are then subjected to a pressure estimation model, developed for the first time in this work, to estimate the liquid entry pressure (LEP) for a hydrophobic fibrous membrane. As for accurate prediction (and not just estimation) of the capillary pressure, this work also presents an energy minimization method, implemented in the Surface Evolver code, for tracking the air–water interface intrusion in a hydrophobic fibrous membrane comprised of orthogonally oriented fibers. This novel interface tracking algorithm is used to investigate the effects of the membrane’s microstructure and wetting properties on its resistance to water intrusion (i.e., LEP). The simulation method developed in this work is computationally affordable and it is accurate in its predictions of the air–water interface shape and position inside the membrane as a function of pressure. Application of the simulation method in studying effects of fiber diameter or contact angle heterogeneity on water intrusion pressure is reported for demonstration purposes. Capillary forces between fibrous surfaces are also studied experimentally and numerically via the liquid bridge between two parallel plates coated with electrospun fibers. In the experiment, a droplet was placed on one of the polystyrene- or polyurethane-coated plates and then compressed, stretched, or sheared using the other plate and the force was measured using a sensitive scale. In the simulation, the liquid bridge was mathematically defined for the Surface Evolver finite element code to predict its 3-D shape and resistance to normal and shearing forces, respectively, in presence of the contact angle hysteresis effect. Despite the inherent non-uniformity of the fibrous surfaces used in the experiments and the simplifying assumptions considered for the simulations, reasonable agreement was observed between the experiments and simulations. Results reveal that both normal and shear force on the plates increase by increasing the liquid volume, or decreasing the spacing between the plates

Development of Green Low Surface Energy (Superhydrophobic) Material for Various Surfaces

Superhydrophobic materials maintain air at the solid-liquid interface, when in contact with water and it can be defined as the tendency of a surface to repel water droplets. These surfaces possess high contact angles of at least 150°, low hysteresis contact angle <10o. Superhydrophobic surfaces have a wide range of application due to their self-cleaning, antifogging, anticorrosion, biomedical characteristics. A substantial body of research is based around the use of relatively expensive fluorocarbons and environmentally hazardous methods to obtain superhydrophobic surfaces. This study proposes an alternative method of fabricating superhydrophobic surfaces, through a cleaner and more cost-effective process which adopts highly branched hydrocarbon chains. To meet the objective of this study, superhydrophobic surfaces were fabricated, which make use of environmentally friendly, non-hazardous (i.e., when in contact with skin) as well as cost effective. We focus on the nanoparticles Al2O3 (13 nm) and SiO2 (10-20 nm), particularly on the impact of the nanoparticle sizes, properties, and shape. As a result of this alternative method, alumina (Al2O3) and silica (SiO2) nanoparticles were easily synthesized with the appropriate carboxylic acid and then spray coated unto different surfaces. After the fabrication process, a static contact angle of 153o were obtained for the functionalized Al2O3 (13 nm) nanoparticles with lanolin (1:5), showing low affinity with water and the droplet of water rolls off easily across the surface

Reducing the contact time of impinging droplets on non-wetting surfaces

This work examines the use of macro-textured surfaces to reduce the contact time between impinging liquid droplets and non-wetting surfaces. Six macro-texture geometries are evaluated for their impact on maximum deformation diameters and contact time. The geometries considered are a set of spokes extending radially out from the impact point. Spoke counts of n=1 to n=6 are evaluated. The six spoke geometry demonstrated the maximum reduction in contact time with a 49% measured contact time reduction compared to a flat plate. This study evaluates droplet impacts experimentally using high-speed video. Samples are traditionally machined aluminum surfaces made non-wetting though the use of the Leidenfrost effect. In conjunction with the experimental results, we develop an analytical model to predict the contact time reduction based solely on impact Weber number and texture geometry. Finally, this study considers the impact of the macro-texture geometries on the dispersion of daughter droplets. High spoke-count geometries were observed to produce more than one droplet per spoke. Here again we develop an analytical model to predict this phase doubling based on a quasi-steady-state Rayleigh Plateau instability approach

Nano-gefunctionaliseerde membraandistillatiemembranen voor drinkwaterproductie uit zout of brak water

Abstract in English and GermanThe reported PhD research study was conceived from real water problems experienced by a rural community in South Africa (SA). Specifically, water quality in the Nandoni Dam situated in the Vhembe District, Limpopo Province, South Africa was assessed in order to determine its fitness for use, following complaints by community members using this water for drinking and domestic purposes. The dam supplies water to 55 villages with approximately 800 000 residents. At the inception of the study, there was little scientific information relating to the quality of the water in the dam. Water samples from various sites across the Nandoni Dam, a primary source of domestic water supply in the region, were collected through each season of the year over a period of 12 months to ascertain the concentrations of dissolved salts in the dam. Additionally, harmful polycyclic aromatic hydrocarbons (PAHs) and phenols were assessed. The concentrations of the ions contributing to water salinity were generally lower than the brackish water bracket (i.e. 500 – 30 000 mg/L) but too high for potable water. The concentration of the phenols was relatively higher than the threshold limit of drinking water. Therefore, the water sourced from the Nandoni Dam was found not suitable for human consumption and therefore required integrated water resource management, as well as robust and cost-effective water treatment especially since the salinity of the water was high even after treatment by a water treatment plant sourcing water from the dam. In an attempt to develop a suitable energy-efficient technology or system for complete removal of salts (desalination) from the salty water (including brackish water), electrospun polyvinylidene fluoride (PVDF) nanofibre membranes were synthesised and evaluated for removal of salts using the Direct Contact Membrane Distillation (DCMD) process. The nanofibre membranes were synthesised with combined high mechanical stability, porosity, and superhydrophobicity to prevent fouling and wetting while maintaining high salt rejection and water flux. Organically functionalised silica nanoparticles (f-SiO2NPs) were embedded on PVDF nanofibre membranes using an in-situ electrospinning technique for superhydrophobicity enhancement. These modified membranes displayed Young’s modulus of ~43 MPa and showed highly porous properties (~80% porosity, 1.24-1.41 μm pore sizes) with superhydrophobic surfaces (contact angle >150°). Membranes embedded with octadecyltrimethoxysilane (OTMS), and chlorodimethyl-octadecyl silane (Cl-DMOS), octadecyltrimethoxysilane (ODTS)-modified SiO2NPs were the most efficient; rejecting >99.9% of NaCl salt, with a water flux of approximately 30.7-34.2 LMH at 60°C, thus indicating their capacity to produce potable water. The superhydrophobic membranes were coated with a thin layer consisting of carboxylated multiwalled carbon nanotubes (f-MWCNTs) and silver nanoparticles (AgNPs) to reduce membrane fouling. The AgNPs and f-MWCNTs were uniformly distributed with size diameters of 28.24±1.15 nm and 6.7±2.1 nm respectively as evidenced by transmission electron microscopy (TEM) micrographs. The antibacterial AgNPs embedded in the PVDF nanofibre membranes inhibited the growth of Gram-positive Geobacillus stearothermophilus and Staphylococcus aureus as well as Gram-negative Pseudomonas aeruginosa and Klebsiella pneumoniae indicating their potential to prevent biofilm formation. Fouling tests were conducted using bovine serum albumin (BSA), sodium alginate, colloidal silica, and thermophilic bacteria effluent as model organic, inorganic, and bio-foulants, respectively, using DCMD. The uncoated membranes were characterised by a flux decays ranging from 30% to 90% and salt rejection decays ranging from 1.4% to 6.1%. Membrane coating reduced the flux and salt rejection decays to 10–24% and 0.07–0.75%, respectively. Although the initial flux decreased from 42 to 16 LMH when using coated membranes, the resistance of these coated membranes to water flux and salt rejection decays indicated that coating could be a suitable one-step solution for fouling mitigation in DCMD. The major challenge would be to design the MD membranes with architectures that allow a high-water flux to be maintained i.e., a highly porous layer. Furthermore, the volatile compounds bearing hydrophobic groups were pretreated to reduce their fouling capacity on PVDF nanofibre membranes. In this study, polyacrylonitrile (PAN) and polyethylene-imine (PEI) functionalised-PAN nanofibre membranes were synthesised and evaluated as a pretreatment for the removal of chlorophenol and nitrophenol from solutions. Under optimised experimental conditions, adsorption capacities ranging from 27.3 – 38.4 mg/g for PAN and PEI-modified nanofibres, respectively, were recorded. The PEI-functionalised nanofibres showed a high potential as a pretreatment step to be integrated to MD process. Ultimately an integrated water desalination system was developed. This involved a pretreatment filter (pore size ~100 μm) containing PEI-functionalised PAN nanofibre materials to reduce particulates and large molecules of dissolved organic/inorganic compounds from the water to be treated. In this research, it was observed that the pre-treatment step was not sufficient in removing all traces of compounds causing fouling of the superhydrophobic PDVF nanofibre membranes. As such, coating of the membranes with a thin hydrophilic layer and coupled with the filtration pretreatment step was found to provide fouling-resistance properties, high salt rejection, and low flux decays on brackish water collected at an estuary in Belgium and the Nandoni Dam in South Africa, demonstrating the potential of the MD separation process towards potable water recovery from brackish water.Het onderzoek in dit proefschrift was gebaseerd op concrete waterproblemen die een landelijke gemeenschap in Zuid-Afrika (SA) ervaart. In het bijzonder werd de waterkwaliteit in het Nandoni-reservoir in het Vhembe-district in de provincie Limpopo in Zuid-Afrika onderzocht, om te bepalen of dit water geschikt is voor gebruik, na klachten van leden van de gemeenschap die dit water gebruiken als drinkwater en voor huishoudelijk gebruik. Het reservoir levert water aan 55 dorpen met ongeveer 800.000 inwoners. Bij het begin van het onderzoek was er weinig wetenschappelijke informatie over de kwaliteit van het water in het reservoir. Watermonsters van verschillende locaties in het reservoir, dat een primaire bron van drinkwater is in de regio, werden gedurende verschillende seizoenen van het jaar verzameld over een periode van 12 maanden, om de concentraties van de meest voorkomende ionen in het reservoir te bepalen. Bovendien werden de concentraties van schadelijke polycyclische aromatische koolwaterstoffen (PAK's) en fenolen gemeten. De concentraties van de ionen die bijdroegen aan het zoutgehalte van het water waren in het algemeen lager dan de drempel om het water als brak water te bestempelen (dat wil zeggen 500 – 30 000 mg/l), maar waren te hoog voor drinkwater. De concentratie van de fenolen was hoger dan de limiet voor drinkwater. Daarom bleek het water afkomstig van het Nandoni reservoir niet geschikt voor menselijke consumptie. Een beter geïntegreerd waterbeheer is dus nodig om deze bron voor drinkwater te beschermen, naast een robuuste en kosteneffectieve waterbehandeling. Deze waterbehandeling moet vooral het zoutgehalte van het water naar beneden halen, maar ook de concentraties van fenolen. In een poging om een geschikte energie-efficiënte technologie of een systeem voor de volledige verwijdering van zouten (~ontzilting) uit brak water te ontwikkelen, werden elektrisch gesponnen polyvinylideenfluoride (PVDF) nanovezelmembranen gesynthetiseerd en beoordeeld op verwijdering van zouten met behulp van Direct Contact Membraandestillatie (DCMD). De nanovezelmembranen hadden een gecombineerde hoge mechanische stabiliteit, porositeit en superhydrofobiciteit, die hielp om vervuiling (fouling) en vloeistofintrede in de poriën (wetting) te voorkomen, terwijl een hoge zoutverwijdering en hoge waterflux doorheen de membranen gehandhaafd bleven. Organische gefunctionaliseerde silica-nanodeeltjes (f-SiO2NP's) werden nadien geïncorporeerd in de PVDF nanovezelmembranen met behulp van een in-situ elektrospinning techniek om zo een nog grotere superhydrofobiciteit te bekomen. Deze gemodificeerde membranen hadden een degelijke treksterkte (Young's modulus van ~ 43 MPa) en waren zeer poreus (~ 80% porositeit, 1.24-1.41 μm poriegrootte). Het oppervlak van de membranen vertoonde inderdaad superhydrofobe eigenschappen (contacthoek met water > 150 °). De membranen ingebed met octadecyltrimethoxysilaan (ODTS) SiO2NP's waren het meest efficiënt: ze toonden een zoutretentie van> 99.9% voor NaCl, bij een waterflux van ongeveer 30.7-34.2 l/(m².h) bij 60 ° C (ten opzichte van 20°C in het permeaat), wat aangeeft dat ze in staat zijn om drinkbaar water te produceren. De superhydrofobe membranen werden nadien ook gecoat met een dunne laag bestaande uit gecarboxyleerde multiwall-carbon nanotubes (f-MWCNT's) en zilver nanodeeltjes (AgNP's), in een poging om membraanvervuiling te verminderen. De AgNP's en f-MWCNT’s hadden uniforme diameters van respectievelijk 28,24 ± 1,15 nm en 6,7 ± 2,1 nm (zoals bleek uit transmissie-elektronenmicroscopie (TEM)). De antibacteriële AgNP's ingebed in de PVDF-nanovezelmembranen remden de groei van Gram-positieve Geobacillus stearothermophilus en Staphylococcus aureus bacteriën, evenals Gram-negatieve Pseudomonas aeruginosa en Klebsiella pneumoniae bacteriën. Dit toont het potentieel van deze membranen om biofilmvorming te voorkomen. Vervuilingsproeven (in DCMD) werden uitgevoerd met behulp van runderserumalbumine (BSA), natriumalginaat, colloïdaal silica, en thermofiele bacteriën - als respectievelijk organische, anorganische en biologische vervuiling. De niet-gemodificeerde membranen werden gekenmerkt door een fluxverval, met een daling van de flux met 30% tot 90%, naast een daling van de zoutretentie met 1.4% tot 6.1%. Bij de gecoate membranen daalde de flux slechts met 10-24% en de zoutverwijdering slechts met 0.07-0.75% respectievelijk. Hoewel de initiële flux ook afnam (van 42 naar ± 16 l/(m².h)) bij het gebruik van gecoate membranen, toonde de hogere weerstand tegen vervuiling van deze gecoate membranen aan dat deze coating een geschikte oplossing zou kunnen zijn tegen vervuiling in DCMD. Bovendien kan de synthese in één stap verlopen. De grootste uitdaging zal echter zijn om MD-membranen te ontwerpen waarbij de coating de oorspronkelijke waterflux/de porositeit van de membranen niet teveel verlaagt. Daarnaast werden gemodificeerde PVDF nanovezels geproduceerd om de verwijdering van vluchtige, hydrofobe stoffen (zoals fenolen) door adsorptie aan deze vezels te verhogen. Er werden polyacrylonitril (PAN) en polyethyleen-imine (PEI) gefunctionaliseerde PAN nanovezels gesynthetiseerd, waarna deze geëvalueerd werden als adsorbens (en dus voorbehandeling voor de membraanstap) voor chloorfenol en nitrofenol. Onder geoptimaliseerde experimentele omstandigheden werden adsorptiecapaciteiten tussen respectievelijk 27.3 en 38.4 mg / g voor PAN- en PEI-gemodificeerde nanovezels gemeten. De PEI-gefunctionaliseerde nanovezels vertoonden een hoog potentieel als een voorbehandelingsstap voor de hierboven beschreven DCMD. Tenslotte werd ook een geïntegreerd waterontziltingssysteem ontwikkeld. Dit systeem bestond uit een voorbehandelingsstap met PEI-gefunctionaliseerde PAN-nanovezels (in de vorm van een membraan met poriegrootte ~100 μm), gevolgd door een gemodificeerde DCMD stap. De voorbehandeling diende om deeltjes en grote opgeloste organische verbindingen uit het te behandelen water te verwijderen voor de DCMD-stap. In dit onderzoek werd waargenomen dat de voorbehandelingsstap niet voldoende was om alle organische contaminanten te verwijderen die vervuiling veroorzaakten op de superhydrofobe PDVF nanovezelmembranen in de DCMD-stap. Toch bleek coating van de DCMD membranen met een dunne hydrofiele laag (gekoppeld aan de voorbehandelingsstap) een voldoende bescherming tegen vervuiling te bieden zodat de zoutretentie en waterflux van deze membranen hoog bleef. De combinatie van voorbehandeling – gemodificeerde DCMD werd succesvol getest op water uit de Schelde en uit het Nandoni reservoir, waarmee het potentieel van de technologie om drinkwater uit brak water te produceren werd aangetoond.School of SciencePh.D. (Applied Biological Science : Environmental Technology

The growth and shrinkage of water droplets at the oil-solid interface.

HYPOTHESIS: The mechanism for the spontaneous formation of water droplets at oil/solid interfaces immersed in water is currently unclear. We hypothesize that growth and shrinkage of droplets are kinetically controlled by diffusion of water through the oil, driven by differences in chemical potential between the solid substrate and the aqueous reservoir. EXPERIMENTS: The formation, growth and shrinkage of water droplets at an immersed oil/solid interface are investigated theoretically and experimentally with three silicone oils. The surface is hydrophobic and the droplets formed are truncated spheres with radius, a, less than 10 μm. The expansion and contraction of the droplets can be controlled by adjusting the difference in chemical potential. The growth kinetics are modelled in terms of water migration through the oil layer which predicts a2∝t. FINDINGS: This is the first study of possible mechanisms for the formation of such interfacial droplets. Several possible causes are shown to be unfavourable, negligible, or are eliminated by careful experiments controlling key parameters (such as oil viscosity, substrate chemistry). The rate constant for mass transport is proportional to difference in chemical potential and an estimate shows dissociation of surface groups on the substrate provides a driving chemical potential of the right magnitude.National Natural Science Foundation of China (21872078