Lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. They are observed in cases where a phase transformation front moves leaving behind two solid products, as in rapid cooling of eutectic or eutectoid systems; such conditions force phases of different composition to form but allow little time for diffusion to produce those phases' equilibrium compositions. Fine lamellae solve this problem by shortening the diffusion distance between phases, but their high surface energy makes them unstable and prone to break up when annealing allows diffusion to progress. A deeper eutectic or more rapid cooling will result in finer lamellae. Two common cases of this include cooling a liquid to form an amorphous solid, cooling eutectoid austenite to form martensite. In biology, normal adult bones possess a lamellar structure which may be disrupted by some diseases
Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, liquid–gas interfaces. It includes the fields of surface surface physics; some related practical applications are classed as surface engineering. The science encompasses concepts such as heterogeneous catalysis, semiconductor device fabrication, fuel cells, self-assembled monolayers, adhesives. Surface science is related to interface and colloid science. Interfacial chemistry and physics are common subjects for both; the methods are different. In addition and colloid science studies macroscopic phenomena that occur in heterogeneous systems due to peculiarities of interfaces; the field of surface chemistry started with heterogeneous catalysis pioneered by Paul Sabatier on hydrogenation and Fritz Haber on the Haber process. Irving Langmuir was one of the founders of this field, the scientific journal on surface science, bears his name.
The Langmuir adsorption equation is used to model monolayer adsorption where all surface adsorption sites have the same affinity for the adsorbing species and do not interact with each other. Gerhard Ertl in 1974 described for the first time the adsorption of hydrogen on a palladium surface using a novel technique called LEED. Similar studies with platinum and iron followed. Most recent developments in surface sciences include the 2007 Nobel prize of Chemistry winner Gerhard Ertl's advancements in surface chemistry his investigation of the interaction between carbon monoxide molecules and platinum surfaces. Surface chemistry can be defined as the study of chemical reactions at interfaces, it is related to surface engineering, which aims at modifying the chemical composition of a surface by incorporation of selected elements or functional groups that produce various desired effects or improvements in the properties of the surface or interface. Surface science is of particular importance to the fields of heterogeneous catalysis and geochemistry.
The adhesion of gas or liquid molecules to the surface is known as adsorption. This can be due to either chemisorption or physisorption, the strength of molecular adsorption to a catalyst surface is critically important to the catalyst's performance. However, it is difficult to study these phenomena in real catalyst particles, which have complex structures. Instead, well-defined single crystal surfaces of catalytically active materials such as platinum are used as model catalysts. Multi-component materials systems are used to study interactions between catalytically active metal particles and supporting oxides. Relationships between the composition and chemical behavior of these surfaces are studied using ultra-high vacuum techniques, including adsorption and temperature-programmed desorption of molecules, scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy. Results can be used toward the rational design of new catalysts. Reaction mechanisms can be clarified due to the atomic-scale precision of surface science measurements.
Electrochemistry is the study of processes driven through an applied potential at a solid-liquid or liquid-liquid interface. The behavior of an electrode-electrolyte interface is affected by the distribution of ions in the liquid phase next to the interface forming the electrical double layer. Adsorption and desorption events can be studied at atomically flat single crystal surfaces as a function of applied potential and solution conditions using spectroscopy, scanning probe microscopy and surface X-ray scattering; these studies link traditional electrochemical techniques such as cyclic voltammetry to direct observations of interfacial processes. Geologic phenomena such as iron cycling and soil contamination are controlled by the interfaces between minerals and their environment; the atomic-scale structure and chemical properties of mineral-solution interfaces are studied using in situ synchrotron X-ray techniques such as X-ray reflectivity, X-ray standing waves, X-ray absorption spectroscopy as well as scanning probe microscopy.
For example, studies of heavy metal or actinide adsorption onto mineral surfaces reveal molecular-scale details of adsorption, enabling more accurate predictions of how these contaminants travel through soils or disrupt natural dissolution-precipitation cycles. Surface physics can be defined as the study of physical interactions that occur at interfaces, it overlaps with surface chemistry. Some of the topics investigated in surface physics include friction, surface states, surface diffusion, surface reconstruction, surface phonons and plasmons, the emission and tunneling of electrons and the self-assembly of nanostructures on surfaces. Techniques to investigate processes at surfaces include Surface X-Ray Scattering, Scanning Probe Microscopy, surface enhanced Raman Spectroscopy and X-ray Photoelectron Spectroscopy; the study and analysis of surfaces involves both chemical analysis techniques. Several modern methods probe the topmost 1–10 nm of surfaces exposed to vacuum; these include X-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, electron energy loss spectroscopy, thermal desorption spectroscopy, ion scattering spectroscopy, secondary ion mass spectrometry, dual polarization interferometry, other surface analysis methods included in the list of materials analysis methods.
Many of these techniques require vacuum as they rely on the detection of ele
Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms and evolution. Despite the complexity of the science, there are certain unifying concepts that consolidate it into a single, coherent field. Biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, evolution as the engine that propels the creation and extinction of species. Living organisms are open systems that survive by transforming energy and decreasing their local entropy to maintain a stable and vital condition defined as homeostasis. Sub-disciplines of biology are defined by the research methods employed and the kind of system studied: theoretical biology uses mathematical methods to formulate quantitative models while experimental biology performs empirical experiments to test the validity of proposed theories and understand the mechanisms underlying life and how it appeared and evolved from non-living matter about 4 billion years ago through a gradual increase in the complexity of the system.
See branches of biology. The term biology is derived from the Greek word βίος, bios, "life" and the suffix -λογία, -logia, "study of." The Latin-language form of the term first appeared in 1736 when Swedish scientist Carl Linnaeus used biologi in his Bibliotheca botanica. It was used again in 1766 in a work entitled Philosophiae naturalis sive physicae: tomus III, continens geologian, phytologian generalis, by Michael Christoph Hanov, a disciple of Christian Wolff; the first German use, was in a 1771 translation of Linnaeus' work. In 1797, Theodor Georg August Roose used the term in the preface of a book, Grundzüge der Lehre van der Lebenskraft. Karl Friedrich Burdach used the term in 1800 in a more restricted sense of the study of human beings from a morphological and psychological perspective; the term came into its modern usage with the six-volume treatise Biologie, oder Philosophie der lebenden Natur by Gottfried Reinhold Treviranus, who announced: The objects of our research will be the different forms and manifestations of life, the conditions and laws under which these phenomena occur, the causes through which they have been effected.
The science that concerns itself with these objects we will indicate by the name biology or the doctrine of life. Although modern biology is a recent development, sciences related to and included within it have been studied since ancient times. Natural philosophy was studied as early as the ancient civilizations of Mesopotamia, the Indian subcontinent, China. However, the origins of modern biology and its approach to the study of nature are most traced back to ancient Greece. While the formal study of medicine dates back to Hippocrates, it was Aristotle who contributed most extensively to the development of biology. Important are his History of Animals and other works where he showed naturalist leanings, more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the Lyceum, wrote a series of books on botany that survived as the most important contribution of antiquity to the plant sciences into the Middle Ages. Scholars of the medieval Islamic world who wrote on biology included al-Jahiz, Al-Dīnawarī, who wrote on botany, Rhazes who wrote on anatomy and physiology.
Medicine was well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew on Aristotelian thought in upholding a fixed hierarchy of life. Biology began to develop and grow with Anton van Leeuwenhoek's dramatic improvement of the microscope, it was that scholars discovered spermatozoa, bacteria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop the basic techniques of microscopic dissection and staining. Advances in microscopy had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the cell. In 1838, Schleiden and Schwann began promoting the now universal ideas that the basic unit of organisms is the cell and that individual cells have all the characteristics of life, although they opposed the idea that all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory.
Meanwhile and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735, in the 1750s introduced scientific names for all his species. Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Although he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought. Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck, the first to present a coherent theory of evolution, he posited that evolution was the result of environmental stress on properties of animals, meaning that the more and rigorously an organ was used, the more complex and efficient it would become, thus adapting the animal to its environment. Lamarck believed that these acquired traits could be passed on to the animal's offspring, who would
A product recall is a request from a manufacturer to return a product after the discovery of safety issues or product defects that might endanger the consumer or put the maker/seller at risk of legal action. The recall is an effort to limit ruination of the corporate image and limit liability for corporate negligence, which can cause significant legal costs, it can be difficult, if not impossible, to determine how costly can be releasing to the consumer a product that could endanger someone's life and the economic loss resulting from unwanted publicity. Recalls are costly. Costs include having to handle the recalled product, replacing it and being held financially responsible for the consequences of the recalled product. A country's consumer protection laws will have specific requirements in regard to product recalls; such regulations may include how much of the cost the maker will have to bear, situations in which a recall is compulsory, or penalties for failure to recall. The firm may initiate a recall voluntarily subject to the same regulations as if the recall were compulsory.
A product recall involves the following steps, which may differ according to local laws: Maker or dealer notifies the authorities responsible of their intention to recall a product. In some cases the government can request a recall of a product. Consumer hotlines or other communication channels are established; the scope of the recall, that is, which serial numbers or batch numbers etc. are recalled, is specified. Product recall announcements are released on the respective government agency's website, as well as in paid notices in the metropolitan daily newspapers. In some circumstances, heightened publicity will result in news television reports advising of the recall; when a consumer group learns of a recall it will notify the public by various means. The consumer is advised to return the goods, regardless of condition, to the seller for a full refund or modification. Avenues for possible consumer compensation will vary depending on the specific laws governing consumer trade protection and the cause of recall.
USA 1959-60 Cadillacs. UK: The Triumph Toledo, Triumph 1500 and Triumph Dolomite were the subject of the UK's largest vehicle recall to date; the recall affected 103,000 cars and involved the replacement of a front radius strut in the front suspension assembly, addressing a risk that the component might break and render the car impossible to steer. The manufacturers stated they had replicated the alleged defect by driving the car into a solid kerb at between 10 and 15 mph. Despite undertaking the recall, they insisted that the condition could only "arise through misuse". USA: The Little Wonder TV antenna was recalled by the CPSC, it was one of the earliest recalls of an electronic device. The product connected the antenna terminals on the back of the TV directly to the AC mains. USA The Ford Motor Company recalled 1.5 million Ford Pintos, the largest recall in automotive history at that time, to install a modification to reduce the risk of fire. USA: Tylenol Scare of 1982 Late 1982-McDonald's recalled 10 million Playmobil Happy Meal toys due to the risk of choking hazards.
USA: 1986 Excedrin Tampering A few bottles of Excedrin were poisoned with cyanide. 2 people died, 1 recovered in the hospital. A woman named Stella Nickell was charged with attempted murder and murder, she was sentenced to 90 years in prison. Worldwide: Intel recalled the original Pentium processors due to the Pentium FDIV bug. Worldwide Audi recalled the original Audi TT Mk1 both Coupé and Roadster due to crashes and related fatalities that occurred at speeds in excess of 180 kilometres per hour, during abrupt lane changes or sharp turns. USA: Burger King organized a recall of 25 million plastic container toys resembling Poké Balls as they presented a suffocation hazard. Taco shells manufactured for Taco Bell recalled USA: Ford Motor Company's handling of the recall of the 6.5 million 15-inch Firestone tires fitted to the Ford Explorer SUV soon culminated in the resignation of Ford's CEO at the time, Jacques Nasser. November 2000-February 2001: McDonald's recalled 234,000 of the Scooter Bug Happy Meal toys due to the risk of choking hazards.
Australia: The recall of a variety of goods manufactured by Pan Pharmaceuticals as a result of failures in quality assurance and standards. The company was soon put under receivership. United Kingdom and Canada: Potentially carcinogenic Sudan I food colouring was found in over 400 products containing Worcester sauce and had to be recalled. Worldwide:June 2005: Engine stalls linked to faulty wirings on 6.0L Powerstroke Diesel engines have caused hundreds of thousands of 2004-2005 Ford Super Duty and Econoline models to be recalled. Ireland and United Kingdom: Cadbury-Schweppes announced that there had been a salmonella scare surrounding its products, causing millions of chocolate bars from stores across Ireland and the UK to be recalled. 2006 Sony notebook computer batteries recall: Worldwide: August 2006: Dell recalls over four million notebook computer batteries, after a number of instances where the batteries, made by Sony, overheated or caught fire. Most of the defective notebooks were sold in the US, however some one million faulty batteries could be found elsewhere in the world.
August 2006: Following Dell's battery recall Apple Computer recalls 1.8 million Sony notebook computer batteries. Similar to Dell, most of the notebooks were sold in the United States; however some 700,000 units could be found o
A heat exchanger is a device used to transfer heat between two or more fluids. Heat exchangers are used in both heating processes; the fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are used in space heating, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, sewage treatment; the classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium air or a liquid coolant. There are three primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, travel in parallel to one another to the other side.
In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat medium per unit mass due to the fact that the average temperature difference along any unit length is higher. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel perpendicular to one another through the exchanger. For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger; the exchanger's performance can be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence. The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference". Sometimes direct knowledge of the LMTD is not available and the NTU method is used.
Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. On the other hand, their low efficiency coupled with the high space occupied in large scales, has led modern industries to use more efficient heat exchangers like shell and tube or plate. However, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basics to students as the fundamental rules for all heat exchangers are the same. Shell and tube heat exchangers consist of a series of tubes which contain fluid that must be either heated or cooled. A second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are used for high-pressure applications.
This is because the tube heat exchangers are robust due to their shape. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: There can be many variations on the shell and tube design; the ends of each tube are connected to plenums through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes. Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used, thus to determine the tube diameter, the available space and fouling nature of the fluids must be considered. Tube thickness: The thickness of the wall of the tubes is determined to ensure: There is enough room for corrosion That flow-induced vibration has resistance Axial strength Availability of spare parts Hoop strength Buckling strength Tube length: heat exchangers are cheaper when they have a smaller shell diameter and a long tube length.
Thus there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length. Long, thin tubes are difficult to take out and replace. Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger. Tube corrugation: this type of tubes used for the inner tubes, increases the turbulence of the fluids and the effect is important in the heat transfer giving a better performance. Tube Layout: refers to. There are four main types of tube layout, which are, rotated triangular and rotated square; the triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping.
Square patterns are employed where high fouling is experienced and cleaning is more regular. B
Ancient Greece was a civilization belonging to a period of Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of antiquity. Following this period was the beginning of the Early Middle Ages and the Byzantine era. Three centuries after the Late Bronze Age collapse of Mycenaean Greece, Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and colonization of the Mediterranean Basin; this was followed by the period of Classical Greece, an era that began with the Greco-Persian Wars, lasting from the 5th to 4th centuries BC. Due to the conquests by Alexander the Great of Macedon, Hellenistic civilization flourished from Central Asia to the western end of the Mediterranean Sea; the Hellenistic period came to an end with the conquests and annexations of the eastern Mediterranean world by the Roman Republic, which established the Roman province of Macedonia in Roman Greece, the province of Achaea during the Roman Empire. Classical Greek culture philosophy, had a powerful influence on ancient Rome, which carried a version of it to many parts of the Mediterranean Basin and Europe.
For this reason, Classical Greece is considered to be the seminal culture which provided the foundation of modern Western culture and is considered the cradle of Western civilization. Classical Greek culture gave great importance to knowledge. Science and religion were not separate and getting closer to the truth meant getting closer to the gods. In this context, they understood the importance of mathematics as an instrument for obtaining more reliable knowledge. Greek culture, in a few centuries and with a limited population, managed to explore and make progress in many fields of science, mathematics and knowledge in general. Classical antiquity in the Mediterranean region is considered to have begun in the 8th century BC and ended in the 6th century AD. Classical antiquity in Greece was preceded by the Greek Dark Ages, archaeologically characterised by the protogeometric and geometric styles of designs on pottery. Following the Dark Ages was the Archaic Period, beginning around the 8th century BC.
The Archaic Period saw early developments in Greek culture and society which formed the basis for the Classical Period. After the Archaic Period, the Classical Period in Greece is conventionally considered to have lasted from the Persian invasion of Greece in 480 until the death of Alexander the Great in 323; the period is characterized by a style, considered by observers to be exemplary, i.e. "classical", as shown in the Parthenon, for instance. Politically, the Classical Period was dominated by Athens and the Delian League during the 5th century, but displaced by Spartan hegemony during the early 4th century BC, before power shifted to Thebes and the Boeotian League and to the League of Corinth led by Macedon; this period saw the Greco-Persian Wars and the Rise of Macedon. Following the Classical period was the Hellenistic period, during which Greek culture and power expanded into the Near and Middle East; this period ends with the Roman conquest. Roman Greece is considered to be the period between Roman victory over the Corinthians at the Battle of Corinth in 146 BC and the establishment of Byzantium by Constantine as the capital of the Roman Empire in AD 330.
Late Antiquity refers to the period of Christianization during the 4th to early 6th centuries AD, sometimes taken to be complete with the closure of the Academy of Athens by Justinian I in 529. The historical period of ancient Greece is unique in world history as the first period attested directly in proper historiography, while earlier ancient history or proto-history is known by much more circumstantial evidence, such as annals or king lists, pragmatic epigraphy. Herodotus is known as the "father of history": his Histories are eponymous of the entire field. Written between the 450s and 420s BC, Herodotus' work reaches about a century into the past, discussing 6th century historical figures such as Darius I of Persia, Cambyses II and Psamtik III, alluding to some 8th century ones such as Candaules. Herodotus was succeeded by authors such as Thucydides, Demosthenes and Aristotle. Most of these authors were either Athenian or pro-Athenian, why far more is known about the history and politics of Athens than those of many other cities.
Their scope is further limited by a focus on political and diplomatic history, ignoring economic and social history. In the 8th century BC, Greece began to emerge from the Dark Ages which followed the fall of the Mycenaean civilization. Literacy had been lost and Mycenaean script forgotten, but the Greeks adopted the Phoenician alphabet, modifying it to create the Greek alphabet. Objects with Phoenician writing on them may have been available in Greece from the 9th century BC, but the earliest evidence of Greek writing comes from graffiti on Greek pottery from the mid-8th century. Greece was divided into many small self-governing communities, a pattern dictated by Greek geography: every island and plain is cut off from its neighbors by the sea or mountain ranges; the Lelantine War is the earliest documented war of the ancient Greek period. It was fought between the important poleis of Chalcis and Eretria over the fertile Lelantine plain of Euboea. Both cities seem to have suffered a decline as result of the long war, though Chalcis was the nominal victor.
A mercantile class arose in the first half of the 7th century BC, shown by the introduction of coinage in about 680 BC. This
Nacre known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer. It is strong and iridescent. Nacre is found in some of the most ancient lineages of bivalves and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, this results in a non-iridescent shine, or more in non-nacreous iridescence such as flame structure as is found in conch pearls; the outer layer of pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae. Nacre is composed of hexagonal platelets of aragonite 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Depending on the species, the shape of the tablets differ. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules.
These layers are separated by sheets of organic matrix composed of elastic biopolymers. This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa. Strength and resilience are likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation; this structure, at multiple length sizes increases its toughness, making it as strong as silicon. The statistical variation of the platelets has a negative effect on the mechanical performance because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates weak regions which allows the crack to propagate without much resistance causing the fracture toughness decreases.
Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours; the crystallographic c-axis points perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have different c-axis orientation randomly oriented within ~20° of vertical. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis, this way inclined; the interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers. Nacre formation is not understood; the initial onset assembly, as observed in Pinna nobilis, is driven by the aggregation of nanoparticles within an organic matrix that arrange in fibre-like polycrystalline configurations.
The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. Nacre growth is mediated by organics, controlling the onset and form of crystal growth. Individual aragonite "bricks" are believed to grow to the full height of the nacreous layer, expand until they abut adjacent bricks; this produces the hexagonal close-packing characteristic of nacre. Bricks may nucleate on randomly dispersed elements within the organic layer, well-defined arrangements of proteins, or may grow epitaxially from mineral bridges extending from the underlying tablet. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis is slow in nacre, fast in fibrous aragonite. Nacre is secreted by the epithelial cells of the mantle tissue of various molluscs; the nacre is continuously deposited onto the inner surface of the shell, the iridescent nacreous layer known as mother of pearl. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues.
The process is called encystation and it continues as long as the mollusc lives. The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, in cephalopods, they are pseudohexagonal monocrystals, which are twinned; the main commercial sources of mother of pearl have been the pearl oyster, freshwater pearl mussels, to a lesser extent the abalone, popular for their sturdiness and beauty in the latter half of the 19th century. Used for pearl buttons during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus; the international trade in mother of pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries. Nacre has been used for centuries for a variety o