Condensed-Matter Physics (1986) / Chapter Skim
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9 Liquid-State Physics
9 Liquid-State Physics
Pages 190-205
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From page 190... ...
In a crystal the bonds or lines joining nearest neighbor atoms are oriented along specific directions in space. In a liquid, however, the lines joining pairs of nearest neighbor atoms will point with equal probability in all directions of space.
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From page 191... ...
In particular, the whole notion of experiment has broadened to include certain Monte Carlo and molecular-dynamical computer simulations noted below. The intent of the microscopic view of fluids is to try to understand the static and dynamic properties of fluids, typically in the classical regime (where quantum effects are unimportant)
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From page 192... ...
No longer do workers in the field seek one unique way of predicting liquid properties; instead there is a hierarchy of techniques to choose from in which increasing quantitative accuracy can be had for the price of decreasing analytic simplicity and increasing computational labor. These techniques include thermodynamic perturbation theory and its variants, as well as the use of integral equations for finding approximate pair-distribution functions.
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From page 193... ...
Dynamical Properties of Classical Liquids The determination of properties associated with molecular motion in condensed phases consists of three approaches: ~ I ~ direct experimental measurement of spectral lineshapes, transport coefficients, and relax
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From page 194... ...
Studies of vibrational lineshapes as functions of temperature and density have provided information that has stimulated the development of the first comprehensive theory grounded in a fundamental treatment of intermolecular forces and time scales. NMR studies of the pressure or density dependence of intramolecular rearrangement rates in small alkanes have provided the first experimental evidence that such rates decrease at low densities, a result in marked contrast to the predictions of transition rate theory but in accord with recent theoretical predictions based on the premise that reactions in fluids are friction controlled and require energy dissipation.
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From page 195... ...
Much of the current work using these techniques is aimed at studying collective motions on time scales where macroscopic hydrodynamics no longer applies; here the details of the intermolecular forces and collisional dynamics become more important. Such studies are the result of technical advances that have enabled measurements to be taken at higher frequencies or shorter times, and the extension of measurements to lower temperatures and higher viscosities, where the characteristic relaxation times are slower, bringing faster processes into experimentally accessible regions.
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From page 196... ...
The intramolecular rearrangements that take place in such molecules are primitive models for chemical reactions, and there has been renewed interest in determining the rates at which flexible molecules change shape and how such changes in shape affect properties involving overall rotation and translation. Historically, there has always been an interest in small-alkane dynamics, but earlier approaches dictated the motions by fiat, and thus provided few fundamental insights into molecular conformational dynamics.
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From page 197... ...
Computer simulations of fluids composed of nonspherical molecules have played a similar role by providing details of molecular dynamics inaccessible to experiment. For example, it has been observed that a characteristic feature of rotational dynamics in condensed phases is an oscillation in the angular-velocity timecorrelation function.
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From page 198... ...
Similarly, the experimental study of these systems is also more problematic than that of ordinary colloidal suspensions. During the past decade, much progress has been made in developing the theoretical concepts necessary for understanding micellization in aqueous soap solutions.
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From page 199... ...
The name liquid crystals covers a broad category of materials exhibiting molecular organization and macroscopic symmetry intermediate between the total disorder of an isotropic liquid and the order of perfect crystals: Nematic phases are ones in which the centers of the molecules making up the material are more or less randomly arranged as in an ordinary liquid, while the orientation of the molecules exhibits longrange order. For instance, rodlike molecules are oriented with their long axes parallel to one another or disklike molecules with their plane surfaces parallel.
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From page 200... ...
Biological sc~bcellul`'r structures such as cell membranes exhibit molecular organization and other properties similar to various liquidcrystal phases. In some cases these are really liquid crystals, while in other cases the structural complexity of the biological systems exceeds that of a liquid crystal, so using the terminology of liquid-crystal physics to describe the biological system is more an aid to thinking than a real physical description.
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From page 201... ...
These include changes between, an ordinary liquid or solution and a liquid crystal, as well as changes between various liquid-crystal phases. A number of these changes of state fall in the category of continuous phase transitions, which may exhibit critical phenomena owing to fluctuation effects.
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From page 202... ...
This achievement of research in liquid crystals has resulted from a combination of important contributions from various sources. First, the liquid-crystal displays now used are based on the twisted nematic polarization switch effect, an electric-field effect in which the internal orientational structure of the liquid crystal sample is changed in a way that rotates the polarization of light passing through it.
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From page 203... ...
There are numerous other outstanding achievements in this field, some of which would require detailed technical discussion to be described meaningfully. These include the development of ultra-highstrength fibers spun from liquid-crystal materials and the discovery of ferroelectric liquid crystals that have a spontaneous electrical polarization.
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From page 204... ...
In the area of liquid-crystal displays, which has served as a fundamental motivating force for much research, there is the potential for major new developments. The currently successful twisted nematic displays are capable of practical application only to situations requiring display of a relatively small amount of data.
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From page 205... ...
Some of the most promising research in this area now concerns the use of ferroelectric liquid crystals, one of the striking discoveries of the last decade that has not yet been fully developed. As with the twisted nematics, success in this area will depend on the cooperation of physicists and chemists and on the development of knowledge in a number of areas that are currently not well understood, such as the interaction of smectic liquid crystals with surfaces.
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