Ultrahydrophobic surfaces are hydrophobic, i.e. difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°; this is referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can rebound like an elastic ball, or pancake. In 1805, Thomas Young defined the contact angle θ by analysing the forces acting on a fluid droplet resting on a solid surface surrounded by a gas. Γ S G = γ S L + γ L G cos θ where γ S G = Interfacial tension between the solid and gas γ S L = Interfacial tension between the solid and liquid γ L G = Interfacial tension between the liquid and gasθ can be measured using a contact angle goniometer. Wenzel determined that when the liquid is in intimate contact with a microstructured surface, θ will change to θ W ∗ cos θ W ∗ = r cos θ where r is the ratio of the actual area to the projected area. Wenzel's equation shows that microstructuring a surface amplifies the natural tendency of the surface.
A hydrophobic surface becomes more hydrophobic when microstructured – its new contact angle becomes greater than the original. However, a hydrophilic surface becomes more hydrophilic when microstructured – its new contact angle becomes less than the original. Cassie and Baxter found that if the liquid is suspended on the tops of microstructures, θ will change to θ C B ∗ cos θ C B ∗ = φ – 1where φ is the area fraction of the solid that touches the liquid. Liquid in the Cassie-Baxter state is more mobile than in the Wenzel state, it can be predicted whether the Wenzel or Cassie-Baxter state should exist by calculating the new contact angle with both equations. By a minimization of free energy argument, the relation that predicted the smaller new contact angle is the state most to exist. Stated mathematically, for the Cassie-Baxter state to exist, the following inequality must be true. Cos θ < /A recent alternative criteria for the Cassie-Baxter state asserts that the Cassie-Baxter state exists when the following 2 criteria are met: 1) Contact line forces overcome body forces of unsupported droplet weight and 2) The microstructures are tall enough to prevent the liquid that bridges microstructures from touching the base of the microstructures.
Contact angle is a measure of static hydrophobicity, contact angle hysteresis and slide angle are dynamic measures. Contact angle hysteresis is a phenomenon; when a pipette injects a liquid onto a solid, the liquid will form some contact angle. As the pipette injects more liquid, the droplet will increase in volume, the contact angle will increase, but its three phase boundary will remain stationary until it advances outward; the contact angle the droplet had before advancing outward is termed the advancing contact angle. The receding contact angle is now measured by pumping the liquid back out of the droplet; the droplet will decrease in volume, the contact angle will decrease, but its three phase boundary will remain stationary until it recedes inward. The contact angle the droplet had before receding inward is termed the receding contact angle; the difference between advancing and receding contact angles is termed contact angle hysteresis and can be used to characterize surface heterogeneity and mobility.
Surfaces that are not homogeneous will have domains. The slide angle is another dynamic measure of hydrophobicity and is measured by depositing a droplet on a surface and tilting the surface until the droplet begins to slide. Liquids in the Cassie-Baxter state exhibit lower slide angles and contact angle hysteresis than those in the Wenzel state. A simple model can be used to predict the effectiveness of a manmade micro- or nano-fabricated surface for its conditional state, contact angle and contact angle hysteresis; the main factor of this model is the contact line density, Λ, the total perimeter of asperities over a given unit area. The critical contact line density Λc is a function of body and surface forces, as well as the projected area of the droplet. Λ C = − ρ g V 1 / 3 ( ( 3 + ( 1
A geomembrane is low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid migration in a human-made project, structure, or system. Geomembranes are made from thin continuous polymeric sheets, but they can be made from the impregnation of geotextiles with asphalt, elastomer or polymer sprays, or as multilayered bitumen geocomposites. Continuous polymer sheet geomembranes are, by far, the most common; the manufacturing of geomembranes begins with the production of the raw materials, which include the polymer resin, various additives such as antioxidants, fillers, carbon black, lubricants. These raw materials are processed into sheets of various widths and thickness by extrusion, and/or spread coating. Geomembranes dominate the sales of geosynthetic products, at 1.8 billion USD per year worldwide, 35% of the market. The US market is divided between HDPE, LLDPE, fPP, PVC, CSPE-R, EPDM-R and others, can be summarized as follows: high-density polyethylene ~ 35% or 105 M m2 linear low-density polyethylene ~ 25% or 75 M m2 polyvinyl chloride ~ 25% or 75 M m2 flexible polypropylene ~ 10% or 30 M m2 chlorosulfonated polyethylene ~ 2% or 6 M m2 ethylene propylene diene terpolymer ~ 3% or 9 M m2The above represents $1.8 billion in worldwide sales.
Projections for future geomembrane usage are dependent on the application and geographical location. Landfill liners and covers in North America and Europe will see modest growth, while in other parts of the world growth could be dramatic; the greatest increases will be seen in the containment of coal ash and heap leach mining for precious metal capture. The majority of generic geomembrane test methods that are referenced worldwide are by the ASTM International|American Society of Testing and Materials due to their long history in this activity. More recent are test method developed by the International Organization for Standardization. Lastly, the Geosynthetic Research Institute has developed test methods that are only for test methods not addressed by ASTM or ISO. Of course, individual countries and manufacturers have specific proprietary test methods; the main physical properties of geomembranes in the as-manufactured state are: Thickness Density Melt flow index Mass per unit area Vapor transmission.
There are a number of mechanical tests that have been developed to determine the strength of polymeric sheet materials. Many have been adopted for use in evaluating geomembranes, they represent i.e. index versus performance tests. Tensile strength and elongation tear resistance impact resistance puncture resistance interface shear strength anchorage strength stress cracking. Any phenomenon that causes polymeric chain scission, bond breaking, additive depletion, or extraction within the geomembrane must be considered as compromising to its long-term performance. There are a number of potential concerns in this regard. While each is material-specific, the general behavior trend is to cause the geomembrane to become brittle in its stress-strain behavior over time. There are several mechanical properties to track in monitoring such long term degradation: the decrease in elongation at failure, the increase in modulus of elasticity, the increase in stress at failure, the general loss of ductility. Many of the physical and mechanical properties could be used to monitor the polymeric degradation process.
Ultraviolet light exposure radioactive degradation biological degradation chemical degradation thermal behavior oxidative degradation. Geomembranes degrade enough that their lifetime behavior is as yet uncharted. Thus, accelerated testing, either by high stress, elevated temperatures and/or aggressive liquids, is the only way to determine how the material will behave long-term. Lifetime prediction methods use the following means of interpreting the data: Stress limit testing: A method by the HDPE pipe industry in the United States for determining the value of hydrostatic design basis stress. Rate process method: Used in Europe for pipes and geomembranes, the method yields similar results as stress limit testing. Hoechst multiparameter approach: A method that utilizes biaxial stresses and stress relaxation for lifetime prediction and can include seams as well. Arrhenius modeling: A method for testing geomembranes described in Koerner for both buried and exposed conditions; the fundamental mechanism of seaming polymeric geomembrane sheets together is to temporarily reorganize the polymer structure of the two opposing surfaces to be joined in a controlled manner that, after the application of pressure, results in the two sheets being bonded together.
This reorganization results from an input of energy that originates from either thermal or chemical processes. These processes may involve the addition of additional polymer in the area to be bonded. Ideally, seaming two geomembrane sheets should result in no net loss of tensile strength across the two sheets, the joined sheets should perform as one single geomembrane sheet. However, due to stress concentrations resulting from the seam geometry, current seaming techniques may result in minor tensile strength and/or elongation lo
A flat roof is a roof, level in contrast to the many types of sloped roofs. The slope of a roof is properly known as its pitch and flat roofs have up to 10°. Flat roofs are an ancient form used in arid climates and allow the roof space to be used as a living space or a living roof. Flat roofs, or "low-slope" roofs, are commonly found on commercial buildings throughout the world; the National Roofing Contractors Association defines a low-slope roof as having a slope of 3-in-12 or less. Flat roofs exist all over the world and each area has its own tradition or preference for materials used. In warmer climates, where there is less rainfall and freezing is unlikely to occur, many flat roofs are built of masonry or concrete and this is good at keeping out the heat of the sun and cheap and easy to build where timber is not available. In areas where the roof could become saturated by rain and leak, or where water soaked into the brickwork could freeze to ice and thus lead to'blowing' these roofs are not suitable.
Flat roofs are characteristic of the Egyptian and Arabian styles of architecture. Any sheet of material used to cover a flat or low-pitched roof is known as a membrane and the primary purpose of these membranes is to waterproof the roof area. Materials that cover flat roofs allow the water to run off from a slight inclination or camber into a gutter system. Water from some flat roofs such as on garden sheds sometimes flows off the edge of a roof, though gutter systems are of advantage in keeping both walls and foundations dry. Gutters on smaller roofs lead water directly onto the ground, or better, into a specially made soakaway. Gutters on larger roofs lead water into the rainwater drainage system of any built up area. However, flat roofs are designed to collect water in a pool for aesthetic purposes, or for rainwater buffering. Traditionally most flat roofs in the western world make use of tar or asphalt more felt paper applied over roof decking to keep a building watertight; the felt paper is in turn covered with a flood coat of bitumen and gravel to keep the sun's heat, UV rays and weather off it and helps protect it from cracking or blistering and degradation.
Roof decking is of plywood, chipboard or OSB boards of around 18mm thickness, steel or concrete. The mopping of bitumen is applied in two or more coats as a hot liquid, heated in a kettle. A flooded coat of bitumen is applied over the felts and gravel is embedded in the hot bitumen. A main reason for failure of these traditional roofs is ignorance or lack of maintenance where people or events cause the gravel to be moved or removed from the roof membrane called a built-up roof, thus exposing it to weather and sun. Cracking and blistering occurs and water gets in. Roofing felts are a'paper' or fiber material impregnated in bitumen; as gravel cannot protect tarpaper surfaces where they rise vertically from the roof such as on parapet walls or upstands, the felts are coated with bitumen and protected by sheet metal flashings. In some microclimates or shaded areas these rather'basic' felt roofs can last well in relation to the cost of materials purchase and cost of laying them, however the cost of modern membranes such as EPDM has come down over recent years to make them more and more affordable.
There are now firms supplying modern alternatives. If a leak does occur on a flat roof, damage goes unnoticed for considerable time as water penetrates and soaks the decking and any insulation and/or structure beneath; this can lead to expensive damage from the rot which develops and if left can weaken the roof structure. There are health risks to people and animals breathing the mould spores: the severity of this health risk remains a debated point. While the insulation is wet, the “R” value is destroyed. If dealing with an organic insulation, the most common solution is removing and replacing the damaged area. If the problem is detected early enough, the insulation may be saved by repairing the leak, but if it has progressed to creating a sunken area, it may be too late. One problem with maintaining flat roofs is that if water does penetrate the barrier covering, it can travel a long way before causing visible damage or leaking into a building where it can be seen. Thus, it is not easy to find the source of the leak.
Once underlying roof decking is soaked, it sags, creating more room for water to accumulate and further worsening the problem. Another common reason for failure of flat roofs is lack of drain maintenance where gravel and debris block water outlets; this causes a pressure head of water which can crack. In colder climates, puddling water can freeze, it is therefore important to maintain your flat roof to avoid excessive repair. An important consideration in tarred flat roof quality is knowing that the common term'tar' applies to rather different products: tar or pitch, coal tar and bitumen; some of these products appear to have been interchanged in their use and are sometimes used inappropriately, as each has different characteristics, for example whether or not the product can soak into wood, its anti-fungal properties and its reaction to exposure to sun and varying temperatures. Modern flat roofs can use single large
A membrane is a selective barrier. Such things may be ions, or other small particles. Biological membranes include cell membranes. Synthetic membranes are made by humans for use in laboratories and industry; this concept of a membrane has been known since the eighteenth century, but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not exploited; the first use of membranes on a large scale was with micro-filtration and ultra-filtration technologies. Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, a number of experienced companies serve the market; the degree of selectivity of a membrane depends on the membrane pore size. Depending on the pore size, they can be classified as microfiltration, ultrafiltration and reverse osmosis membranes.
Membranes can be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, particle transport can be active or passive; the latter can be facilitated by pressure, chemical or electrical gradients of the membrane process. Membranes can be classified into synthetic membranes and biological membranes. Microfiltration operates within a range of 7-100 kPa. Microfiltration is used to remove residual suspended solids, to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment step for reverse osmosis. Recent developments are membrane bioreactors which combine microfiltration and a bioreactor for biological treatment. Ultrafiltration operates within a range of 70-700kPa. Ultrafiltration is used for many of the same applications as microfiltration; some ultrafiltration membranes have been used to remove dissolved compounds with high molecular weight, such as proteins and carbohydrates. In addition, they are able to remove some endotoxins.
Nanofiltration is known as “loose” RO and can reject particles smaller than 0,002 µm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is developed as a membrane softening process which offers an alternative to chemical softening. Nanofiltration can be used as a pre-treatment before directed reverse osmosis; the main objectives of NF pre-treatment are:. Minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, prevent scaling by removal of the hardness ions, lower the operating pressure of the RO process by reducing the feed-water total dissolved solids concentration. Reverse osmosis is used for desalination; as well, RO is used for the removal of dissolved constituents from wastewater remaining after advanced treatment with microfiltration. RO requires high pressures to produce deionized water. In the membrane field, the term module is used to describe a complete unit composed of the membranes, the pressure support structure, the feed inlet, the outlet permeate and retentate streams, an overall support structure.
The principal types of membrane modules are: Tubular, where membranes are placed inside a support porous tubes, these tubes are placed together in a cylindrical shell to form the unit module. Tubular devices are used in micro and ultra filtration applications because of their ability to handle process streams with high solids and high viscosity properties, as well as for their relative ease of cleaning. Hollow fiber consists of a bundle of hundreds to thousands of hollow fibers; the entire assembly is inserted into a pressure vessel. The feed can be applied to the outside of the fiber. Spiral Wound, where a flexible permeate spacer is placed between two flat membranes sheet. A flexible feed spacer is added and the flat sheets are rolled into a circular configuration. Plate and frame consist of a series of flat membrane sheets and support plates; the water to be treated passes between the membranes of two adjacent membrane assemblies. The plate supports the membranes and provides a channel for the permeate to flow out of the unit module.
Ceramic and polymeric Flat Sheet modules. Flat sheet membranes are built-into a submerged vacuum driven filtration systems which consist of stacks of modules each with a number of sheets. Filtration mode is outside-in where the water passes through the membrane and is collected in permeate channels. Cleaning can be performed by aeration, back wash and CIP; the key elements of any membrane process relate to the influence of the following parameters on the overall permeate flux are: The membrane permeability The operational driving force per unit membrane area The fouling and subsequent cleaning of the membrane surface. The total permeate flow from a membrane system is given by following equation: Q p = F w ⋅ A Where Qp is the permeate stream flowrate, Fw is the water flux rate and A is the membrane area The permeability of a membrane is giv
A raincoat or slicker is a waterproof or water-resistant coat worn to protect the body from rain. The term rain jacket is sometimes used to refer to raincoats. A rain jacket may be combined with a pair of rain pants to make a rain suit. Modern raincoats are constructed of breathable, waterproof fabrics such as Gore-Tex or Tyvek and coated nylons; these fabrics allow some air to pass through, allowing the garment to'breathe' so that sweat vapour can escape. The amount of pouring rain a raincoat can handle is sometimes measured in the unit millimeters, water gauge; the first modern waterproof raincoat was created following the patent by Scottish chemist Charles Macintosh in 1824 of new tarpaulin fabric, described by him as "India rubber cloth," and made by sandwiching a core of rubber softened by naphtha in two pieces of fabric. Anorak, derived from traditional Inuit designs Cagoule Cagoul, Kagool Driza-Bone, Australian oiled cotton Gannex Inverness cape Mackintosh, rubberised cloth Mino, traditional Japanese raincoat made out of straw Oilskin Poncho Sou'wester Trench coat, derived from traditional raincoat Waxed jacket The dictionary definition of raincoat at Wiktionary Media related to Raincoats at Wikimedia Commons
Damp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Dampness problems are among the most frequent problems encountered in residences. Damp proofing is defined by the American Society for Testing and Materials as a material that resists the passage of water with no hydrostatic pressure and waterproof as a treatment that resists the passage of water under pressure. Damp proofing keeps moisture out of a building where vapor barriers keep interior moisture from getting into walls. Moisture resistance is not absolute. Damp proofing is accomplished several ways including: A damp-proof course is a barrier through the structure designed to prevent moisture rising by capillary action such as through a phenomenon known as rising damp. Rising damp is the effect of water rising from the ground into property; the damp proof course may be vertical. A DPC layer is laid below all masonry walls, regardless if the wall is a load bearing wall or a partition wall.
A damp-proof membrane is a membrane material applied to prevent moisture transmission. A common example is polyethylene sheeting laid under a concrete slab to prevent the concrete from gaining moisture through capillary action. A DPM may be used for the DPC. Integral damp proofing in concrete involves adding materials to the concrete mix to make the concrete itself impermeable. Surface coating with thin water proof materials for resistance to non-pressurized moisture such as rain water or a coating of cement sprayed on such as shotcrete which can resist water under pressure. Cavity wall construction, such as rainscreen construction, is where the interior walls are separated from the exterior walls by a cavity. Pressure grouting cracks and joints in masonry materials. Materials used for damp proofing include: Flexible materials like butyl rubber, hot bitumen, plastic sheets, bituminous felts, sheets of lead, etc. Semi-rigid materials like mastic asphalt Rigid materials, like impervious brick, slate, cement mortar, or cement concrete painted with bitumen, etc.
Stones Mortar with waterproofing compounds Coarse sand layers under floors Continuous plastic sheets under floors A DPC is a durable, impermeable material such as slate, felt paper, plastic or special engineered bricks bedded into the mortar between two courses of bricks or blocks. It can be seen as a thin line in the mortar near ground level. To create a continuous barrier, pieces of DPC or DPM may be sealed together. In addition, the DPC may be sealed to the DPM around the outside edges of the ground floor sealing the inside of the building from the damp ground around it. In a masonry cavity wall, there is a DPC in both the outer and inner wall. In the outer wall it is 150 millimetres to 200 mm above ground level; this allows rain to form puddles and splash up off the ground, without saturating the wall above DPC level. The wall below the DPC may become saturated in rainy weather; the DPC in the inner wall is below floor level, or, with a solid concrete floor, it is found above the floor slab so that it can be linked to the DPM under the floor slab.
This enables installation of skirting boards above floor level without fear of puncturing it. Alternatively, instead of fitting separate inner and outer DPCs, it is common in commercial housebuilding to use a one-piece length of rigid plastic, which fits neatly across the cavity and slots into both walls; this method requires the need for weep vents to enable rainwater ingress to drain from the cavities otherwise rising dampness could occur from above the DPC. Concrete allows moisture to pass through so a vertical damp proof barrier is needed. Barriers may be a membrane applied to the exterior of the concrete; the coating may be asphalt, asphalt emulsion, a thinned asphalt called cutback asphalt, or an elastomer. Membranes are rubberized asphalt or EPDM rubber. Rubberized products perform better because concrete sometimes develops cracks and the barrier does not crack with the concrete; until the 20th century, masonry buildings in Europe and North America were constructed from permeable materials such as stone and lime-based mortars and renders covered with soft water-based paints which all allowed any damp to diffuse into the air without damage.
The application of impermeable materials which prevent the natural dispersion of damp, such as tile, linoleum and gypsum-based materials and synthetic paints is thought by some to be the most significant cause of damp problems in older buildings. There are many solutions for dealing with dampness in existing buildings, the choice of which will be determined by the types of dampness that are affecting the building, e.g. rising damp, hygroscopic damp, penetrating damp, etc. Some DPC materials may contain asbestos fibres; this was more found in the older, grey sealants as well as flexible tar boards. Not a trusted source
Tar paper is a heavy-duty paper used in construction. Tar paper is made by impregnating paper or fiberglass mat with tar, producing a waterproof material useful for roof construction. Tar paper is distinguished from roofing felt, impregnated with asphalt instead of tar. Tar paper has been in use for centuries. Felt was made from recycled rags but today felts are made of recycled paper products and sawdust; the most common product is #15 felt. Before the oil crisis, felt weighed about 15 pounds per square and hence the asphalt-impregnated felt was called "15#" and "15-pound felt". Modern, inorganic mats no longer weigh 0.73 kg/m2, to reflect this fact the new felts are called "#15". In fact, #15 mats can weigh from 7.5 to 12.5 pounds/sq depending on the manufacturer and the standard to which felt is made. Thirty-pound felt is now #30 felt, weighs 16 to 27 pounds per square. Tar paper is more a Grade D building paper, used in the West. Building paper is manufactured from virgin kraft paper, unlike felts, impregnated with asphalt.
The longer fibres in the kraft paper allow for a lighter weight product with similar and better mechanical properties than felt. Grade papers are rated in minutes—the amount of time it takes for a moisture sensitive chemical indicator to change colour when a small boat-like sample is floated on water. Common grades include 10, 20, 30, 60 minute; the higher the rating the more moisture resistant and the heavier. A typical 20 minute paper will weigh about 3.3 lbs per square, a 30-minute paper 3.75, a 60-minute paper about six. The smaller volume of material however does tend to make these papers less resistant to moisture than heavier felts. Tar paper is far less common than asphalt felt paper and is used, among other things, for waterproofing roofs to prevent ingress of moisture, it is used in a small percentage of built-up roofs, as underlayment with asphalt, wood and other shingles, or gravel, since tar paper itself isn't wind- or sun-resistant. It is sold in rolls of various widths and thicknesses – 3-foot-wide rolls, 50 or 100 feet long and "15 lb" and "30 lb" weights are common in the U.
S. – marked with chalk lines at certain intervals to aid in laying it out straight on roofs with the proper overlap. It can be installed in several ways, such as staples or roofing nails, but it is sometimes applied in several layers with hot asphalt, cold asphalt, or non-asphaltic adhesives. Older construction sometimes used a lighter-weight tar paper, stapled up with some overlap, as a water- and wind-proofing material on walls, but modern carpenters more uses 8-or-10-foot widths of housewrap