Phases of Liquid Crystals

Phases of Liquid Crystals Introduction to Liquid Crystals A liquid crystal is a thermodynamic stable phase characterized by anisotropy of properties Without the existence of a three-dimensional crystal lattice, generally lying in the temperature Range between the solid and isotropic liquid phase, hence the term mesophase. Liquid crystal materials are unique in their properties and uses. As research into this field Continues and as new applications are developed, liquid crystals will play an important role in Modern technology. This tutorial provides an introduction to the science and applications of these materials. The term liquid crystal signifies a state of aggregation that is intermediate between the crystalline solid and the amorphous liquid. As a rule a substance in this state is strongly anisotropic some of its properties and yet exhibits a certain degree of fluidity, which in some case may be comparable to that of an ordinary liquid. The first observations of liquid crystalline or mesomorphic behaviour were made towards the end of the last century by reinitzer and lehmann. What are Liquid Crystals? Liquid crystal materials generally have several common characteristics. Among these are a rodlike molecular structure, rigidness of the long axis, and strong dipole and/or easily polarizable substituents. A dipole is present when we have two equal electric or magnetic charges of opposite sign, separated by a small distance. In the electric case, the dipole moment is given by the product of one charge and the distance of separation. Applies to charge and current distributions as well. In the electric case, a displacement of charge distribution produces a dipole moment, as in a molecule. The distinguishing characteristic of the liquid crystalline state is the tendency of the molecules (mesogens) to point along a common axis, called the director (the molecular direction of preferred orientation in liquid crystalline mesophases). This is in contrast to molecules in the liquid phase, which have no intrinsic order. In the solid state, molecules are highly ordered and have little translational freedom. The characteristic orientational order of the liquid crystal state is between the traditional solid and liquid phases and this is the origin of the term mesogenic state, used synonymously with liquid crystal state. Note the average alignment of the molecules for each phase in the following diagram. A mesogen is rigid rodlike or disclike molecules which are components of liquid crystalline materials. It is sometimes difficult to determine whether a material is in a crystal or liquid crystal state. Crystalline materials demonstrate long range periodic order in three dimensions. By definition, an isotropic (Having properties that are the same regardless of the direction of measurement. In the isotropic state, all directions are indistinguishable from each other)liquid has no orientational order. Substances that arent as ordered as a solid, yet have some degree of alignment are properly called liquid crystals. Liquid Crystal Phases Liquid crystal phases are formed by a wide variety of molecules. They can be divided into two classes, thermo tropic and allotropic. Transitions to thermotropic phases are initiated by changes in temperature, while those to lyotropic phases can also be initiated by changes in concentration. Thermotropic Phases Thermotropic liquid crystals can generally be formed by prolate (calamitic) molecules or oblate (discotic) molecules. Liquid crystal phases formed by calamitic molecules fall into three different categories: nematic, chiral nematic, and smectic. Nematic Liquid Crystal Phase The simplest liquid crystal phase is called the nematic phase (N). It is characterized by a high degree of long range orientational order but no translational order. Molecules in a nematic phase spontaneously order with their (for calamitic molecules) long axes roughly parallel. Schematic diagram of a nematic liquid crystal A uniformly aligned nematic has a preferred direction, often described in terms of a unit vector called the director. More generally a bulk nematic will contain domains. The orientation of the director is constant in each domain but is different in different domains. Viewed under a polarizing microscope the defect regions linking these domains appear as dark threads Chiral Nematic Liquid Crystal Phase Chiral molecules can also form nematic phases called chiral nematic (or cholesteric) phases (N*). This phase shows nematic ordering but the preferred direction rotates throughout the sample. The axis of this rotation is normal to the director. An example of this is shown in Fig (b). The distance over which the director rotates by 360 is called the chiral pitch and is generally of the order of hundreds of nanometres, the wavelength of visible light. A non-chiral nematic phase can be thought of as a chiral nematic with an infinite pitch. Fig (b). Smectic Liquid Crystal Phases Smectic phases have further degrees of order compared to the nematic phase. In the simplest smectic phase, the smectic-A (SmA) phase, the molecules order into layers, with the layer normal parallel to the director. Within the layers, liquid like structure remains, as shown in Fig. 1.3. Closely related to the SmA phase is the smectic-C (SmC) phase. Here the molecules form a layer structure but the long axes of the molecules, and hence the director, lies at an angle to the layer normal, as shown in Fig. 1.4. There are many other smectic phases which have long range order within the layers Smectic phases can also be formed by chiral molecules, leading to chiral smectic phases. Discotic Liquid Crystal Phases Liquid crystal phases formed by discotic molecules fall into three different categories: discotic nematic, discotic chiral nematic, and columnar. The discotic nematic is similar in structure to the calamitic nematic, although in this case the short axes of the molecules tend to lie parallel. The same holds for the discotic chiral nematic phases. Columnar phases are the discotic equivalent of the smectic phase. Here the molecules form columns. In the simplest case the short axes of the molecules lie parallel to the axis of the column and the columns are randomly distributed in space. More complicated discotic phases exist, where the short molecular axes lie at an angle to the column and translational order exists between the columns, analogous to the more complicated smectic phases. Other Thermotropic Phases Most of the phases exhibited by low molecular mass liquid crystals are described above. Recently however there has been much interest in the so-called `banana phases formed by bent-core molecules Some of these phases are chiral although the molecules forming them are achiral. Some high molecular mass polymers, liquid crystalline polymers (LCP), can also form liquid crystal phases. These fall into two categories depending on where the mesogenic part of the molecule is located. If the mesogenic unit is contained within the main polymer chain then it is termed a main chain liquid crystal polymer (MCLCP). These fall into two categories depending on where the mesogenic part of the molecule is located. If the mesogenic unit is contained within the main polymer chain then it is termed a main chain liquid crystal polymer (MCLCP). Lyotropic Liquid Crystal Phases Lyotropic liquid crystal phases are formed by amphiphilic molecules. These often consist of a polar head group attached to one or more non-polar chains and are often known as surfactants (surface active agents). A schematic is shown in Fig. 1.5. When these are dissolved in an appropriate solvent they self-assemble so the polar (hydrophilic) heads protect the non-polar (hydrophobic) tails. These structures are known as micelles. At low surfactant concentrations these are roughly spherical, as shown in Fig. 1.6. As the surfactant Concentration increases then other phases are formed. These include the hexagonal phase where the amphiphiles form cylinders that pack in a hexagonal array and the lamellar phase where the amphiphiles form a bilayer structure. Structure-Property Relationships in Liquid Crystals Despite this there exists only a poor understanding of how changes in molecular structure affect material properties. For liquid crystals this is complicated by several factors. Firstly, liquid crystal phases are formed by materials from the whole spectrum of chemical classes: organic, organometallic, and biological molecules can all form liquid crystal phases. Secondly, mesogenic molecules are generally quite large. A typical low mass mesogen will have from about 40-100 atoms. For liquid crystalline polymers and dendrimers this can be closer to a few thousand atoms. Liquid crystals tend to be flexible. This inhibits crystallization, preventing the direct transition from an isotropic liquid to a crystalline solid. It also leads to a large degree of conformational freedom, so properties are generally determined by more than just the equilibrium structure. Finally liquid crystal molecules often contain disparate parts (such as alkyl and perfluoroalkyl chains) that can have a large effect on the phase behaviour. Investigation of these factors by experimental or theoretical means should hopefully lead to a better understanding of structure-property relationships in liquid crystals. In principle the relationship between molecular structure and macroscopic properties can be investigated through the synthesis of series of similar mesogenic compounds. This however can be time consuming and may involve many difficult and expensive syntheses. Thus, the ability to determine the properties of a molecular structure before synthesis would be desirable. It is here that simulations can play an important role. Simulations on general molecular models can be used to find features that can lead to a particular property or phase. Atomistic simulations can be used to determine material properties of a particular molecular structure. Chemical Properties of Liquid Crystals Liquid crystals can be classified into two main categories: thermotropic liquid crystals, And lyotropic liquid crystals. These two types of liquid crystals are distinguished by the mechanisms that drive their self-organization, but they are also similar in many ways. Thermotropic transactions occur in most liquid crystals, and they are defined by the fact that the transitions to the liquid crystalline state are induced thermally. That is, one can arrive at the Liquid crystalline state by raising the temperature of a solid and/or lowering the temperature of a Liquid. Thermotropic liquid crystals can be classified into two types: enantiotropic liquid crystals, Which can be changed into the liquid crystal state from either lowering the temperature of a Liquid or raising of the temperature of a solid, and monotropic liquid crystals, which can only be Changed into the liquid crystal state from either an increase in the temperature of a solid or a Decrease in the temperature of a liquid, but not both. In general, thermotropic mesophases occur Because of anisotropic dispersion forces between the molecules and because of packing Interactions. In contrast to thermotropic mesophases, lyotropic liquid crystal transitions occur with the Influence of solvents, not by a change in temperature. Lyotropic mesophases occur as a result of Solvent-induced aggregation of the constituent mesogens into micellar structures. Lyotropic mesogens are typically amphiphilic, meaning that they are composed of both lyophilic (solventattracting) And lyophobic (solvent-repelling) parts. This causes them to form into micellar structures in the presence of a solvent, since the lyophobic ends will stay together as the lyophilic ends extend outward toward the solution. As the concentration of the solution is increased and The solution is cooled, the micelles increase in size and eventually coalesce. This separates the newly formed liquid crystalline state from the solvent. A very large number of chemical compounds are known to exhibit one or several liquid crystalline phases. Despite significant differences in chemical composition, these molecules have some common features in chemical and physical properties. There are two types of thermotropic liquid crystals: discotics and rod-shaped molecules. Discotics are flat disc-like molecules consisting of a core of adjacent aromatic rings. This allows for two dimensional columnar ordering. Rod-shaped molecules have an elongated, anisotropic geometry which allows for preferential alignment along one spatial direction. The rod-like low molar mass (LMM) liquid crystals, such as 5CB shown in the following Diagram: require an extended conformation of the molecule which must be maintained through the rigidityand linearity of its constituents. That is, in order for a molecule to display the characteristics of a liquid crystal, it must be rigid and rod-shaped. This is accomplished by the interconnection of two rigid cyclic units. The interconnecting group should cause the resulting compound to have a linear planar conformation. Linking units containing multiple bonds such as -(CH=N)-, -N=N-, -(CH=CH)n-, -CH=N-N=CH-, etc. are used since they restrict the freedom of rotation. These groups can conjugate with phenylene rings, enhancing the anisotropic polarizability. This increases the molecular length and maintains the rigidity. Applications of Liquid Crystals Liquid crystal technology has had a major effect many areas of science and engineering, as well as device technology. Applications for this special kind of material are still being discovered and continue to provide effective solutions to many different problems. Liquid Crystal Displays The most common application of liquid crystal technology is liquid crystal displays (LCDs.) This Field has grown into a multi-billion dollar industry, and many significant scientific and Engineering discoveries have been made. Liquid Crystal Thermometers As demonstrated earlier, chiral nematic (cholesteric) liquid crystals reflect light with a wavelength equal to the pitch. Because the pitch is dependent upon temperature, the color reflected also is dependent upon temperature. Liquid crystals make it possible to accurately gauge temperature just by looking at the color of the thermometer. By mixing different compounds, a device for practically any temperature range can be built. The mood ring, a popular novelty a few years ago, took advantage of the unique ability of the chiral nematic liquid crystal. More important and practical applications have been developed in such diverse areas as medicine and electronics. Special liquid crystal devices can be attached to the skin to show a map of temperatures. This is useful because often physical problems, such as tumors, have a different temperature than the surrounding tissue. Liquid crystal temperature sensors can also be used to find bad connections on a circuit board by detecting the cha racteristic higher temperature Optical Imaging An application of liquid crystals that is only now being explored is optical imaging and recording. In this technology, a liquid crystal cell is placed between two layers of photo conductor. Light is applied to the photoconductor, which increases the materials conductivity. This causes an electric field to develop in the liquid crystal corresponding to the Intensity of the light. The electric pattern can be transmitted by an electrode, which enables the Image to be recorded. This technology is still being developed and is one of the most promising Areas of liquid crystal research. Other Liquid Crystal Applications Liquid crystals have a multitude of other uses. They are used for nondestructive mechanical Testing of materials under stress. This technique is also used for the visualization of RF (radio frequency) waves in waveguides. They are used in medical applications where, for example, transient pressure transmitted by a walking foot on the ground is measured. Low molar mass (LMM) liquid crystals have applications including erasable optical disks, full color electronic slides for computer-aided drawing (CAD), and light modulators for color electronic imaging. As new properties and types of liquid crystals are investigated and researched, these materials are sure to gain increasing importance in industrial and scientific applications.

The personal :: essays research papers

Yahoo! My Yahoo! Mail Welcome, demetriasmith2002 [Sign Out, My Account] Mail Home - Mail Tutorials - Help Mail | Addresses | Calendar | Notepad What's New - Mail Upgrades - Mail Options Check MailCompose Search MailSearch the Web Get unlimited calls to U.S./Canada Folders[Add - Edit] Inbox Draft Sent Bulk (26)[Empty] Trash[Empty] My Folders[Hide] junk What's your Credit Score? See it FREE! Netflix DVD Rentals Delivered! Credit poor? Try us! FREE year's supply of groceries worth $1,800! Previous | Next | Back to Messages Printable View - Full Headers DeleteReplyForwardNot SpamMove... This message is not flagged. [ Flag Message - Mark as Unread ] To: "Donald" From: "Washer Dryer Combos" Add to Address Book Date: Sun, 17 Jul 2005 15:32:28 -0700 Subject: It’s stylish, it’s compact and it’s on us Get a Washer/Dryer Combination for nothing!! See here. You are getting this excellent cyber correspondence you inquired about it or you are a past official member of one of our several online services. If you want to be disconnected from this prominent email service any more. Go ahead and Go Here. The HTML graphics in this message have been blocked. [Show HTML Graphics - Edit Preferences] DeleteReplyForwardNot SpamMove... Previous | Next | Back to Messages Save Message Text Check MailCompose Search MailSearch the Web Move Options [New Folder] Inbox junk Forward Options As Inline Text As Attachment Reply Options Reply To Sender Reply To Everyone Mail Shortcuts Check Mail Ctrl++C Compose Ctrl++P Folders Ctrl++F Advanced Search Ctrl++S Options Help Ctrl++H Address Book Shortcuts Add Contact Add Category Add List View Contacts View Lists Quickbuilder Import Contacts Synchronize Addresses Options Addresses Help Calendar Shortcuts Add Event Add Task Add Birthday

Problem of Young Generation Essay

Although some generational differences have existed throughout history, modern generational gaps have often been attributed to rapid cultural change in the postmodern period  ·Culture is not merely an accumulation of works and knowledge which an à ©lite produces, collates and conserves in order to place it within reach of all, or that a people rich in its  ·past and its heritage offers to others as a model which their  ·own history has failed to provide for them †¦ Culture is not  ·limited to access to works of art and the humanities ; it is at  ·one and the same time the acquisition of knowledge, the  Ã‚ ·Culture is the name for what people are interested in, their thoughts, their models, the books they read and the speeches they hear, their table-talk, gossip, controversies, historical sense and scientific training, the values they appreciate, the quality of life they admire. All communities have a culture. It is the climate of their civilization.  ·Today, teenagers spend more time in front of media than they do sleeping. Between Internet, facebook, cell phones, television and music, the teens are almost always plugged into the media. Media has significant influence over the formation of youth culture. The media culture is dangerous because it provides adults with a way to communicate to youth without permission slips or chaperones. The media culture is not interested in raising young people in virtue and character. Rather, the media exploits youth in order to make money at the expense of their moral character. To see an excellent documentary on how the media exploits youth culture, see the PBS Frontline documentary, Merchant’s of Cool.

Free Essays on Japanese Internment

Right after the Japan’s sneak attack on our naval base at Pearl Harbor fear spread throughout the country. Many feared for their loved ones now joining the Second World War, many feared for the economy, their lives and safety at the home front. The biggest concern with safety at the home front was another attack from Japan. Since we were now a war with Japan many believed that the Japanese living in the U.S were a potential danger. They feared that the Japanese here could help Japan attack the U.S. As the days progressed the fear of the U.S decided to put all Japanese born here or not in relocation camps. Men women and children were stripped of their rights as U.S citizens, lost all their belongings and put in relocation camps where military officials guarded them closely. More than 120,000 Japanese American and Japanese immigrants participated in the Japanese Internment. May 16th, 1942, Gordon Kiyoshi Hirabayashi was at the FBI office in Seattle Washington A few days earlier Gordon had ignored the military order stating that â€Å"all persons of Japanese ancestry† to register for evacuation to the sate fairground at Puyallup, south Seattle. From there they would be shipped to relocation camps in California and Arkansas. He was reminded he could face a year in prison for not cooperation Gordon still refused to register for evacuation, after they offered him one last chance to do so. After being placed in the King County jail a police agent discovered in his briefcase a diary in which Gordon wrote how he had violated curfew orders that kept Japanese Americans off the street in the weeks before evacuation. The find was reported to the U.S attorney general, who quickly filed an additional criminal charge against Gordon for curfew violation. Gordon said he disobeyed the curfew because he â€Å"†¦ received a lift- perhaps it is a release- when I consciously break the silly old curfew.†(70) He also said, â€Å"If I were to register and cooper... Free Essays on Japanese Internment Free Essays on Japanese Internment Right after the Japan’s sneak attack on our naval base at Pearl Harbor fear spread throughout the country. Many feared for their loved ones now joining the Second World War, many feared for the economy, their lives and safety at the home front. The biggest concern with safety at the home front was another attack from Japan. Since we were now a war with Japan many believed that the Japanese living in the U.S were a potential danger. They feared that the Japanese here could help Japan attack the U.S. As the days progressed the fear of the U.S decided to put all Japanese born here or not in relocation camps. Men women and children were stripped of their rights as U.S citizens, lost all their belongings and put in relocation camps where military officials guarded them closely. More than 120,000 Japanese American and Japanese immigrants participated in the Japanese Internment. May 16th, 1942, Gordon Kiyoshi Hirabayashi was at the FBI office in Seattle Washington A few days earlier Gordon had ignored the military order stating that â€Å"all persons of Japanese ancestry† to register for evacuation to the sate fairground at Puyallup, south Seattle. From there they would be shipped to relocation camps in California and Arkansas. He was reminded he could face a year in prison for not cooperation Gordon still refused to register for evacuation, after they offered him one last chance to do so. After being placed in the King County jail a police agent discovered in his briefcase a diary in which Gordon wrote how he had violated curfew orders that kept Japanese Americans off the street in the weeks before evacuation. The find was reported to the U.S attorney general, who quickly filed an additional criminal charge against Gordon for curfew violation. Gordon said he disobeyed the curfew because he â€Å"†¦ received a lift- perhaps it is a release- when I consciously break the silly old curfew.†(70) He also said, â€Å"If I were to register and cooper...

Respiratory Virus essays

Respiratory Virus essays Respiratory syncytial virus (RSV) is a very common virus that causes mild cold-like symptoms in adults and older healthy children. It can cause serious respiratory problems in young babies, especially those born prematurely, who have heart or lung disease, or who are immunocompromised. # RSV was first founded in an outbreak of copious rhinorrhea in a colony of chimpanzees; the chimpanzee coryza agent was soon isolated from human infants with similar illnesses.# Respiratory Synctial Virus (RSV) is an infection that is contagious that resembles the common cold a lot of times. RSV is the most common viral pathogen that causes lower respiratory tract infections in infants.# RSV infection in infancy cause severe bronchiolitis and pneumonia and may incline children to the following development of asthma, the most common chronic illness of childhood. RSV is the most single related cause of pneumonia and bronchiolitis. Epidemics of the virus are seen each winter, 80% of infections typically occur during a three-month period.# The virus is not typically severe during infancy but it is rare if it happens. (As, in my case I was born with the RSV virus and was hospitalized for eighteen days and was put on assisted ventilation.) In infants RSV infection can spread to the bronchial tubes and lungs. The virus can also infect adults, in where the infection can cause viral pneumonia, which is sometimes followed by a bacterial infection of the lower respira tory system.# RSV is the largest single cause of childhood hospitalization and is therefore a major drain on public health resources. Each year more than 90,000 infants are hospitalized with the RSV infection, and nearly 75% of bronchiolitis cases and more than 50% of pneumonia cases are hospitalized also because of the infection.#The infection usually lasts from four to twenty-one days. If the infection is severe it could last a ...

Human Trafficking

Human Trafficking Free Online Research Papers By definition human trafficking is the commerce and trade in the movement or migration of people, legal and illegal, including both legitimate labor activities as well as forced labor. Human trafficking is now one of the fastest growing criminal industries in the world, with the total annual revenue for trafficking in persons estimated to be between $5 billion and $9 billion. The Council of Europe states that people who are being trafficked has reached epidemic proportions over the past decade, with a global annual market of about $42.5 billion. Trafficking victims typically are recruited using deception, fraud, the abuse of power, or outright abduction. Threats, violence, and economic leverage such as debt forgiveness and the promise of a â€Å"good† life can persuade a victim into being exploited. When I first started to research the issue of human trafficking I really did not know as much as I thought I knew about the issue. Soon I started to understand the economics of such a business, how easy it is to make lots of money, how universal the demand, how difficult it is to prosecute. Human trafficking is a low-risk, high-profit enterprise, and because it looks to the casual observer and even to cops like garden variety prostitution, it is tolerated. And worse, it is growing. At the beginning of my research I learned about a girl named Shauna. In Shaunas case, she made friends with a new girl at school. Shauna described the girls so-called father as a guy who hired Shauna and her new friend to clean condos. He always gave them money for the mall and she remembers that he was always very attentive to her. She says she could never have predicted that the man would hurt her. The father turned out to be a trafficker and the new friend was his recruiter. How could a high school girlfriend be part of such a terrible plot? The father or one of his cohorts slipped Shauna a date rape drug in a glass of water. She was beaten and raped repeatedly by a group of men. Shauna says that she remembers talk of money changing hands and a conversation about going to Texas. When she was finally dropped off after her captors were threatened by investigators, she was overdosing on six different drugs and had to be revived three times on the way to the hospital. The trafficking operations are described by law enforcement as being mob-like networks; some are mom and pop, still others are local city-wide networks. The kids are either brought to a prostitution district or they are moved around to large work sites like New Orleans, and the Mississippi coast or they are even moved around to convention cities. They are brought to where there is demand, something which, experts will tell you, exists everywhere across the United States. Research Papers on Human TraffickingThe Relationship Between Delinquency and Drug UseThe Effects of Illegal ImmigrationNever Been Kicked Out of a Place This NiceDefinition of Export Quotas19 Century Society: A Deeply Divided EraHip-Hop is ArtGenetic EngineeringMarketing of Lifeboy Soap A Unilever ProductAssess the importance of Nationalism 1815-1850 EuropeTwilight of the UAW

Yeo soft drinks division SBU strategic market plan Case Study

Yeo soft drinks division SBU strategic market plan - Case Study Example Yeo Hiap Seng soft drinks division was chosen as the strategic business unit in the marketing plan. It is a division of the company in Singapore and it is the unit charged with the soft drinks marketing in the company. Being a large company with different products, YHS (Yeo Hiap Seng) strives to optimize the performance of the company through the individual contribution of the subdivisions.Soft drinks appear to be slowly but steadily overtaking hot drinks as the largest beverage sector in the world (Deloitte, 2008). A report by Zenith international shows that the consumption of the soft drinks is rising by 5% each year. As an industry, soft drinks market is rapidly expanding (Aqua-Trace, 2008). The industry is faced by several market trends and factors that either push or pull them to lead them in the direction that we view them to proceed.The companies operating in this industry are therefore generally faced by several challenges that include ever-changing consumer tastes, in crease d emphasis on products safety and the rising power of the retailers at the global level.Due to the several factors that are considered while having a picture into what happens with this industry, it is important to explore the opportunities for process improvement which could ultimately lead to better quality products and in the process guarantee safety of the product for the consumer while at the same time optimizing resources to provide best profits. A view of all the important industry factors with an eye into the future gives a positive prospect that ultimately secures a company's business or operations in the future. Although United States remains the biggest market currently, Asia likely to be the major driver of the sales in the near future (Deloitte, 2008). The soft drinks industry is pictured by opening into the entire beverage industry. The beverage industry has had its opportunities and challenges. Constant dynamism in consumer demands and preferences, call for new ways of attracting new customers while maintaining the old ones. Since this is the only way to increase sales and consequently achieve growth. The beverage companies strive to court customers, offer high quality products, efficiently distribute them, ensure safety and keep prices low. These must all happen with the eyes still open so as to able to exploit new opportunities by launching new products. The company therefore succeeds by quickly exploiting new opportunities. As a subsection of the beverage industry; the soft drinks market has been growing quite significantly, particularly in the emerging markets which include Asia as a key player