Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between cohesive forces. Wetting deals with the three phases of materials: gas and solid, it is now a center of attention in nanotechnology and nanoscience studies due to the advent of many nanomaterials in the past two decades. Wetting is important in the adherence of two materials. Wetting and the surface forces that control wetting are responsible for other related effects, including capillary effects. There are two types of wetting: active wetting. Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to avoid contact with the surface; the contact angle, as seen in Figure 1, is the angle at which the liquid–vapor interface meets the solid-liquid interface. The contact angle is determined by the balance between cohesive forces.
As the tendency of a drop to spread out over a flat, solid surface increases, the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability. A contact angle less than 90° indicates that wetting of the surface is favorable, the fluid will spread over a large area of the surface. Contact angles greater than 90° means that wetting of the surface is unfavorable, so the fluid will minimize contact with the surface and form a compact liquid droplet. For water, a wettable surface may be termed hydrophilic and a nonwettable surface hydrophobic. Superhydrophobic surfaces have contact angles greater than 150°, showing no contact between the liquid drop and the surface; this is sometimes referred to as the "Lotus effect". The table describes varying contact angles and their corresponding solid/liquid and liquid/liquid interactions. For nonwater liquids, the term lyophilic is used for low contact angle conditions and lyophobic is used when higher contact angles result; the terms omniphobic and omniphilic apply to both polar and apolar liquids.
Liquids can interact with two main types of solid surfaces. Traditionally, solid surfaces have been divided into low-energy types; the relative energy of a solid has to do with the bulk nature of the solid itself. Solids such as metals and ceramics are known as'hard solids' because the chemical bonds that hold them together are strong. Thus, it takes a large input of energy to break these solids, so they are termed "high energy". Most molecular liquids achieve complete wetting with high-energy surfaces; the other type of solids is weak molecular crystals where the molecules are held together by physical forces. Since these solids are held together by weak forces, a low input of energy is required to break them, thus they are termed "low energy". Depending on the type of liquid chosen, low-energy surfaces can permit either complete or partial wetting. Dynamic surfaces have been reported that undergo changes in surface energy upon the application of an appropriate stimuli. For example, a surface presenting photon-driven molecular motors was shown to undergo changes in water contact angle when switched between bistable conformations of differing surface energies.
Low-energy surfaces interact with liquids through dispersion forces. William Zisman produced several key findings:Zisman observed that cos θ increases linearly as the surface tension of the liquid decreased. Thus, he was able to establish a linear function between cos θ and the surface tension for various organic liquids. A surface is more wettable when θ is low. Zisman termed the intercept of these lines when cos θ = 1, as the critical surface tension of that surface; this critical surface tension is an important parameter because it is a characteristic of only the solid. Knowing the critical surface tension of a solid, it is possible to predict the wettability of the surface; the wettability of a surface is determined by the outermost chemical groups of the solid. Differences in wettability between surfaces that are similar in structure are due to differences in the packing of the atoms. For instance, if a surface has branched chains, it will have poorer packing than a surface with straight chains.
Lower critical surface tension means a less wettable material surface. An ideal surface is flat, rigid smooth, chemically homogeneous, has zero contact angle hysteresis. Zero hysteresis implies the receding contact angles are equal. In other words, only one thermodynamically stable contact angle exists; when a drop of liquid is placed on such a surface, the characteristic contact angle is formed as depicted in Fig. 1. Furthermore, on an ideal surface, the drop will return to its original shape; the following derivations apply only to ideal solid surfaces. Figure 3 shows the line of contact. In equilibrium, the net force per unit length acting along the boundary line between the three phases must be zero; the components of net force in the direction along each of the interfaces are given by: γ α θ
A composite material is a material made from two or more constituent materials with different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure, differentiating composites from mixtures and solid solutions; the new material may be preferred for many reasons: common examples include materials which are stronger, lighter, or less expensive when compared to traditional materials. More researchers have begun to include sensing, actuation and communication into composites, which are known as Robotic Materials. Typical engineered composite materials include: Reinforced concrete and masonry Composite wood such as plywood Reinforced plastics, such as fibre-reinforced polymer or fiberglass Ceramic matrix composites Metal matrix composites and other Advanced composite materialsComposite materials are used for buildings and structures such as boat hulls, swimming pool panels, racing car bodies, shower stalls, storage tanks, imitation granite and cultured marble sinks and countertops.
The most advanced examples perform on spacecraft and aircraft in demanding environments. The earliest man-made composite materials were straw and mud combined to form bricks for building construction. Ancient brick-making was documented by Egyptian tomb paintings. Wattle and daub is one of the oldest man-made composite materials, at over 6000 years old. Concrete is a composite material, is used more than any other man-made material in the world; as of 2006, about 7.5 billion cubic metres of concrete are made each year—more than one cubic metre for every person on Earth. Woody plants, both true wood from trees and such plants as palms and bamboo, yield natural composites that were used prehistorically by mankind and are still used in construction and scaffolding. Plywood 3400 BC by the Ancient Mesopotamians. Cartonnage layers of linen or papyrus soaked in plaster dates to the First Intermediate Period of Egypt c. 2181–2055 BC and was used for death masks. Cob Mud Bricks, or Mud Walls, have been used for thousands of years.
Concrete was described by Vitruvius, writing around 25 BC in his Ten Books on Architecture, distinguished types of aggregate appropriate for the preparation of lime mortars. For structural mortars, he recommended pozzolana, which were volcanic sands from the sandlike beds of Pozzuoli brownish-yellow-gray in colour near Naples and reddish-brown at Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for cements used in buildings and a 1:2 ratio of lime to pulvis Puteolanus for underwater work the same ratio mixed today for concrete used at sea. Natural cement-stones, after burning, produced cements used in concretes from post-Roman times into the 20th century, with some properties superior to manufactured Portland cement. Papier-mâché, a composite of paper and glue, has been used for hundreds of years; the first artificial fibre reinforced plastic was bakelite which dates to 1907, although natural polymers such as shellac predate it. One of the most common and familiar composite is fibreglass, in which small glass fibre are embedded within a polymeric material.
The glass fibre is strong and stiff, whereas the polymer is ductile. Thus the resulting fibreglass is stiff, strong and ductile. Concrete is the most common artificial composite material of all and consists of loose stones held with a matrix of cement. Concrete is an inexpensive material, will not compress or shatter under quite a large compressive force. However, concrete cannot survive tensile loading. Therefore, to give concrete the ability to resist being stretched, steel bars, which can resist high stretching forces, are added to concrete to form reinforced concrete. Fibre-reinforced polymers s include glass-reinforced plastic. If classified by matrix there are thermoplastic composites, short fibre thermoplastics, long fibre thermoplastics or long fibre-reinforced thermoplastics. There are numerous thermoset composites, including paper composite panels. Many advanced thermoset polymer matrix systems incorporate aramid fibre and carbon fibre in an epoxy resin matrix. Shape memory polymer composites are high-performance composites, formulated using fibre or fabric reinforcement and shape memory polymer resin as the matrix.
Since a shape memory polymer resin is used as the matrix, these composites have the ability to be manipulated into various configurations when they are heated above their activation temperatures and will exhibit high strength and stiffness at lower temperatures. They can be reheated and reshaped without losing their material properties; these composites are ideal for applications such as lightweight, deployable structures. High strain composites are another type of high-performance composites that are designed to perform in a high deformation setting and are used in deployable systems where structural flexing is advantageous. Although high strain composites exhibit many similarities to shape memory polymers, their performance is dependent on the fibre layout as opposed to the resin content of the matrix. Comp
Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth. In the system of Greek numerals it has the value 9. In Ancient Greek, θ represented the aspirated voiceless dental plosive /t̪ʰ/, but in Modern Greek it represents the voiceless dental fricative /θ/. In its archaic form, θ was written as a cross within a circle, as a line or point in circle. Archaic crossed forms of theta are seen in the wheel letters of Linear A and Linear B; the cursive form ϑ was retained by Unicode as U+03D1 ϑ "GREEK THETA SYMBOL", separate from U+03B8 θ "GREEK SMALL LETTER THETA". For the purpose of writing Greek text, the two can be font variants of a single character, but θ and ϑ are used as distinct symbols in technical and mathematical contexts. In Latin script used for the Gaulish language, theta developed into the tau gallicum, conventionally transliterated as Ð, although the bar extends across the centre of the letter; the phonetic value of the tau gallicum is thought to have been.
The early Cyrillic letter fita developed from θ. This letter existed in the Russian alphabet until the 1918 Russian orthography reform. In the International Phonetic Alphabet, represents the voiceless dental fricative, as in thick or thin, it does not represent the consonant in the, the voiced dental fricative. A similar-looking symbol, described as a lowercase barred o, indicates in the IPA a close-mid central rounded vowel; the lowercase letter θ is used as a symbol for: A plane angle in geometry An unknown variable in trigonometry A special function of several complex variables One of the Chebyshev functions in prime number theory The potential temperature in meteorology The score of a test taker in item response theory Theta Type Replication: a type of bacterial DNA replication specific to circular chromosomes Threshold value of an artificial neuron A Bayer designation letter applied to a star in a constellation. According to Porphyry of Tyros, the Egyptians used an X within a circle as a symbol of the soul.
Johannes Lydus says that the Egyptians used a symbol for Kosmos in the form of theta, with a fiery circle representing the world, a snake spanning the middle representing Agathos Daimon. The Egyptians used the symbol of a point within a circle to represent the sun, which might be a possible origin of its use as the Sun's astrological glyph, it is worthwhile to note that θῆτα has the same numerical value in isopsephy as Ηλιος: 318. In classical Athens, it was used as an abbreviation for the Greek θάνατος and as it vaguely resembles a human skull, theta was used as a warning symbol of death, in the same way that skull and crossbones are used in modern times, it survives on potsherds used by Athenians. Petrus de Dacia in a document from 1291 relates the idea that theta was used to brand criminals as empty ciphers, the branding rod was affixed to the crossbar spanning the circle. For this reason, use of the number theta was sometimes avoided where the connotation was felt to be unlucky—the mint marks of some Late Imperial Roman coins famously have the sum ΔΕ or ΕΔ substituted as a euphemism where a Θ would otherwise be expected.
Greek ThetaCoptic ThetheCyrillic FitaMathematical ThetaThese characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style. Ѳ, ѳ—Fita, a letter of the early Cyrillic alphabet derived from the Greek theta ʘ—Bilabial click Voiceless dental fricative Theta nigrum
The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of Nelumbo or "lotus flower". Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Ultrahydrophobicity and self-cleaning properties are found in other plants, such as Tropaeolum, Alchemilla, on the wings of certain insects; the phenomenon of ultrahydrophobicity was first studied by Dettre and Johnson in 1964 using rough hydrophobic surfaces. Their work developed a theoretical model based on experiments with glass beads coated with paraffin or PTFE telomer; the self-cleaning property of ultrahydrophobic micro-nanostructured surfaces was studied by Wilhelm Barthlott and Ehler in 1977, who described such self-cleaning and ultrahydrophobic properties for the first time as the "lotus effect". Other biotechnical applications have emerged since the 1990s; the high surface tension of water causes droplets to assume a nearly spherical shape, since a sphere has minimal surface area, this shape therefore demands least solid-liquid surface energy.
On contact with a surface, adhesion forces result in wetting of the surface. Either complete or incomplete wetting may occur depending on the structure of the surface and the fluid tension of the droplet; the cause of self-cleaning properties is the hydrophobic water-repellent double structure of the surface. This enables the contact area and the adhesion force between surface and droplet to be reduced resulting in a self-cleaning process; this hierarchical double structure is formed out of a characteristic epidermis and the covering waxes. The epidermis of the lotus plant possesses papillae with 10 µm to 20 µm in height and 10 µm to 15 µm in width on which the so-called epicuticular waxes are imposed; these superimposed waxes form the second layer of the double structure. This system regenerates; this bio-chemical property is responsible for the functioning of the water repellency of the surface. The hydrophobicity of a surface can be measured by its contact angle; the higher the contact angle the higher the hydrophobicity of a surface.
Surfaces with a contact angle < 90° are referred to as hydrophilic and those with an angle >90° as hydrophobic. Some plants show contact angles up to 160° and are called ultrahydrophobic, meaning that only 2–3% of the surface of a droplet is in contact. Plants with a double structured surface like the lotus can reach a contact angle of 170°, whereby the droplet's contact area is only 0.6%. All this leads to a self-cleaning effect. Dirt particles with an reduced contact area are picked up by water droplets and are thus cleaned off the surface. If a water droplet rolls across such a contaminated surface the adhesion between the dirt particle, irrespective of its chemistry, the droplet is higher than between the particle and the surface; as this self-cleaning effect is based on the high surface tension of water it does not work with organic solvents. Therefore, the hydrophobicity of a surface is no protection against graffiti; this effect is of a great importance for plants as a protection against pathogens like fungi or algae growth, for animals like butterflies and other insects not able to cleanse all their body parts.
Another positive effect of self-cleaning is the prevention of contamination of the area of a plant surface exposed to light resulting in reduced photosynthesis. When it was discovered that the self-cleaning qualities of ultrahydrophobic surfaces come from physical-chemical properties at the microscopic to nanoscopic scale rather than from the specific chemical properties of the leaf surface, the discovery opened up the possibility of using this effect in manmade surfaces, by mimicking nature in a general way rather than a specific one; some nanotechnologists have developed treatments, paints, roof tiles and other surfaces that can stay dry and clean themselves by replicating in a technical manner the self-cleaning properties of plants, such as the lotus plant. This can be achieved using special fluorochemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates. In addition to chemical surface treatments, which can be removed over time, metals have been sculpted with femtosecond pulse lasers to produce the lotus effect.
The materials are uniformly black at any angle, which combined with the self-cleaning properties might produce low maintenance solar thermal energy collectors, while the high durability of the metals could be used for self-cleaning latrines to reduce disease transmission. Further applications have been marketed, such as self-cleaning glasses installed in the sensors of traffic control units on German autobahns developed by a cooperation partner; the Swiss companies HeiQ and Schoeller Textil have developed stain-resistant textiles under the brand names "HeiQ Eco Dry" and "nanosphere" respectively. In October 2005, tests of the Hohenstein Research Institute showed that clothes treated with NanoSphere technology allowed tomato sauce and red wine to be washed away after a few washes. Another possible application is thus with self-cleaning awnings and sails, which otherwise become dirty and difficult to clean. Superhydrophobic coatings applied to microwave antennas can reduce rain fade and the buildup of ice and snow.
"Easy to clean" products in ads are mistaken in the name of the self
Surface roughness shortened to roughness, is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough. In surface metrology, roughness is considered to be the high-frequency, short-wavelength component of a measured surface. However, in practice it is necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose. Roughness plays an important role in determining how a real object will interact with its environment. In tribology, rough surfaces wear more and have higher friction coefficients than smooth surfaces. Roughness is a good predictor of the performance of a mechanical component, since irregularities on the surface may form nucleation sites for cracks or corrosion. On the other hand, roughness may promote adhesion. Speaking, rather than scale specific descriptors, cross-scale descriptors such as surface fractality provide more meaningful predictions of mechanical interactions at surfaces including contact stiffness and static friction.
Although a high roughness value is undesirable, it can be difficult and expensive to control in manufacturing. For example, it is difficult and expensive to control surface roughness of fused deposition modelling manufactured parts. Decreasing the roughness of a surface increases its manufacturing cost; this results in a trade-off between the manufacturing cost of a component and its performance in application. Roughness can be measured by manual comparison against a "surface roughness comparator", but more a surface profile measurement is made with a profilometer; these can be of optical. However, controlled roughness can be desirable. For example, a gloss surface can be too shiny to the eye and too slippery to the finger so a controlled roughness is required; this is a case where both amplitude and frequency are important. A roughness value can either be calculated on a surface; the profile roughness parameter are more common. The area roughness parameters give more significant values; each of the roughness parameters are calculated using a formula for describing the surface.
Standard references that describe each in detail are their Measurement. The profile roughness parameters are included in BS EN ISO 4287:2000 British standard, identical with the ISO 4287:1997 standard; the standard is based on the ″M″ system. There are many different roughness parameters in use, but R a is by far the most common, though this is for historical reasons and not for particular merit, as the early roughness meters could only measure R a. Other common parameters include R z, R q, R s k; some parameters are used only within certain countries. For example, the R k family of parameters is used for cylinder bore linings, the Motif parameters are used in the French automotive industry; the MOTIF method provides a graphical evaluation of a surface profile without filtering waviness from roughness. A motif consists of the portion of a profile between two peaks and the final combinations of these motifs eliminate ″insignificant″ peaks and retains ″significant″ ones. Please note that R a is a dimensional unit that can be micrometer or microinch.
Since these parameters reduce all of the information in a profile to a single number, great care must be taken in applying and interpreting them. Small changes in how the raw profile data is filtered, how the mean line is calculated, the physics of the measurement can affect the calculated parameter. With modern digital equipment, the scan can be evaluated to make sure there are no obvious glitches that skew the values; because it may not be obvious to many users what each of the measurements mean, a simulation tool allows a user to adjust key parameters, visualizing how surfaces which are different to the human eye are differentiated by the measurements. For example, R a fails to distinguish between two surfaces where one is composed of peaks on an otherwise smooth surface and the other is composed of troughs of the same amplitude; such tools can be found in app format. By convention every 2D roughness parameter is a capital R followed by additional characters in the subscript; the subscript identifies the formula, used, the R means that the formula was applied to a 2D roughness profile.
Different capital letters imply. For example, Ra is the arithmetic average of the roughness profile, Pa is the arithmetic average of the unfiltered raw profile, Sa is the arithmetic average of the 3D roughness; each of the formulas listed in the tables assume that the roughness profile has been filtered from the raw profile data and the mean line has been calculated. The roughness profile contains n ordered spaced points along the trace, y i is the vertical distance from the mean line to the i th data point. Hei
OCLC Online Computer Library Center, Incorporated d/b/a OCLC is an American nonprofit cooperative organization "dedicated to the public purposes of furthering access to the world's information and reducing information costs". It was founded in 1967 as the Ohio College Library Center. OCLC and its member libraries cooperatively produce and maintain WorldCat, the largest online public access catalog in the world. OCLC is funded by the fees that libraries have to pay for its services. OCLC maintains the Dewey Decimal Classification system. OCLC began in 1967, as the Ohio College Library Center, through a collaboration of university presidents, vice presidents, library directors who wanted to create a cooperative computerized network for libraries in the state of Ohio; the group first met on July 5, 1967 on the campus of the Ohio State University to sign the articles of incorporation for the nonprofit organization, hired Frederick G. Kilgour, a former Yale University medical school librarian, to design the shared cataloging system.
Kilgour wished to merge the latest information storage and retrieval system of the time, the computer, with the oldest, the library. The plan was to merge the catalogs of Ohio libraries electronically through a computer network and database to streamline operations, control costs, increase efficiency in library management, bringing libraries together to cooperatively keep track of the world's information in order to best serve researchers and scholars; the first library to do online cataloging through OCLC was the Alden Library at Ohio University on August 26, 1971. This was the first online cataloging by any library worldwide. Membership in OCLC is based on use of services and contribution of data. Between 1967 and 1977, OCLC membership was limited to institutions in Ohio, but in 1978, a new governance structure was established that allowed institutions from other states to join. In 2002, the governance structure was again modified to accommodate participation from outside the United States.
As OCLC expanded services in the United States outside Ohio, it relied on establishing strategic partnerships with "networks", organizations that provided training and marketing services. By 2008, there were 15 independent United States regional service providers. OCLC networks played a key role in OCLC governance, with networks electing delegates to serve on the OCLC Members Council. During 2008, OCLC commissioned two studies to look at distribution channels. In early 2009, OCLC negotiated new contracts with the former networks and opened a centralized support center. OCLC provides bibliographic and full-text information to anyone. OCLC and its member libraries cooperatively produce and maintain WorldCat—the OCLC Online Union Catalog, the largest online public access catalog in the world. WorldCat has holding records from private libraries worldwide; the Open WorldCat program, launched in late 2003, exposed a subset of WorldCat records to Web users via popular Internet search and bookselling sites.
In October 2005, the OCLC technical staff began a wiki project, WikiD, allowing readers to add commentary and structured-field information associated with any WorldCat record. WikiD was phased out; the Online Computer Library Center acquired the trademark and copyrights associated with the Dewey Decimal Classification System when it bought Forest Press in 1988. A browser for books with their Dewey Decimal Classifications was available until July 2013; until August 2009, when it was sold to Backstage Library Works, OCLC owned a preservation microfilm and digitization operation called the OCLC Preservation Service Center, with its principal office in Bethlehem, Pennsylvania. The reference management service QuestionPoint provides libraries with tools to communicate with users; this around-the-clock reference service is provided by a cooperative of participating global libraries. Starting in 1971, OCLC produced catalog cards for members alongside its shared online catalog. OCLC commercially sells software, such as CONTENTdm for managing digital collections.
It offers the bibliographic discovery system WorldCat Discovery, which allows for library patrons to use a single search interface to access an institution's catalog, database subscriptions and more. OCLC has been conducting research for the library community for more than 30 years. In accordance with its mission, OCLC makes its research outcomes known through various publications; these publications, including journal articles, reports and presentations, are available through the organization's website. OCLC Publications – Research articles from various journals including Code4Lib Journal, OCLC Research, Reference & User Services Quarterly, College & Research Libraries News, Art Libraries Journal, National Education Association Newsletter; the most recent publications are displayed first, all archived resources, starting in 1970, are available. Membership Reports – A number of significant reports on topics ranging from virtual reference in libraries to perceptions about library funding. Newsletters – Current and archived newsletters for the library and archive community.
Presentations – Presentations from both guest speakers and OCLC research from conferences and other events. The presentations are organized into five categories: Conference presentations, Dewey presentations, Distinguished Seminar Series, Guest presentations, Research staff
A porous medium or a porous material is a material containing pores. The skeletal portion of the material is called the "matrix" or "frame"; the pores are filled with a fluid. The skeletal material is a solid, but structures like foams are also usefully analyzed using concept of porous media. A porous medium is most characterised by its porosity. Other properties of the medium can sometimes be derived from the respective properties of its constituents and the media porosity and pores structure, but such a derivation is complex; the concept of porosity is only straightforward for a poroelastic medium. Both the solid matrix and the pore network are continuous, so as to form two interpenetrating continua such as in a sponge. However, there is a concept of closed porosity and effective porosity, i.e. the pore space accessible to flow. Many natural substances such as rocks and soil, biological tissues, man made materials such as cements and ceramics can be considered as porous media. Many of their important properties can only be rationalized by considering them to be porous media.
The concept of porous media is used in many areas of applied science and engineering: filtration, engineering, geosciences and biophysics, material science. Fluid flow through porous media is a subject of common interest and has emerged a separate field of study; the study of more general behaviour of porous media involving deformation of the solid frame is called poromechanics. The theory of porous flows has applications in inkjet printing and nuclear waste disposal technologies, among others. There are many idealized models of pore structures, they can be broadly divided into three categories: networks of capillaries arrays of solid particles trimodalPorous materials have a fractal-like structure, having a pore surface area that seems to grow indefinitely when viewed with progressively increasing resolution. Mathematically, this is described by assigning the pore surface a Hausdorff dimension greater than 2. Experimental methods for the investigation of pore structures include confocal microscopy and x-ray tomography.
One of the Laws for porous materials is the generalized Murray's law. The generalized Murray’s law is based on optimizing mass transfer by minimizing transport resistance in pores with a given volume, can be applicable for optimizing mass transfer involving mass variations and chemical reactions involving flow proceses, molecule or ion diffusion. For connecting a parent pipe with radius of r0 to many children pipes with radius of ri, the formula of generalized Murray's law is: r o a = 1 1 − X ∑ i = 1 N r i a, where the X is the ratio of mass variation during mass transfer in the parent pore, the exponent α is dependent on the type of the transfer. For laminar flow α =3. Cenocell Nanoporous materials NMR in porous media Percolation theory Reticulated foam