 |
| Pair of point defects--"Boojums" in a thin, hybrid aligned nematic film. |
Polarizing microscope texture of a thin, liquid crystalline film. Two centers with emerging dark brushes represent "boojum," point defects in the molecular orientation of the liquid crystal. The defects are formed at the surface of a thin film of a nematic fluid, the simplest form of a liquid crystal. The thin film (one- to two-micrometers thick) is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but they tend to be parallel to each other. The average direction of orientation is called the director. Since the direction of alignment in the plane of the film is not fixed, the film exhibits distortions, with the director changing from point to point. The changing interference colors of the film result from different director tilt near the "cores" of the defects. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer of the optical microscope. Since both defects have a radial director orientation near the core, each defect emanates four dark brushes. The term "boojum" entered the world of science from Lewis Carroll's "The Hunting of the Snark," thanks to Professor David Mermin, who was the first to introduce the word in his Physical Review Letters article in 1977. At the time, Mermin was studying topological defects similar to those presented here, in a superfluid helium 3 that, amazingly, is also a liquid crystal in many respects.
IMAGE 2:
 |
| Thin, nematic film on isotropic surface--one-dimensional periodicity. |
This periodic stripe structure, seen under a polarizing microscope, occurs in a nematic fluid--the simplest form of a liquid crystal--when a thin film of the material is spread over the surface of an isotropic fluid (glycerine). The upper surface of the nematic film is free (in contact with air). In the nematic, the rod-like elongated molecules are free to move around, but they tend to remain parallel to each other; the average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favor normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. When the film is very thin, less than one micrometer, these distortions use too much energy and the system relaxes through periodic, in-plane director variations. The effect is similar to buckling instability of an elastic rod stressed at two ends; at some critical stress, the rod bulges. The periodic pattern illustrates a fine balance of elastic and surface anchoring forces. The picture shows a one-dimensional periodic pattern; the period varies from 5 to 150 micrometers, depending on the thickness of the film; more complex two-dimensional patterns can also be observed.
IMAGE 3:
 |
| Hybrid aligned nematic film. More about this Image Polarizing microscope texture of a thin, liquid crystalline film. This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin, nematic film is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but tend to remain parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. |
IMAGE 4:
 |
| Polarizing microscope texture of a smectic A liquid crystal. More about this Image In smectic A, rod-like, elongated molecules are arranged parallel to each other, forming layers of monomolecular length. The layers are stacked on top of each other and are flexible. When the smectic layers bend, they tend to preserve their equidistance, as it is fixed by the molecular length. The restriction of constant layer thickness leads to a peculiar geometry of deformations, so-called focal conic domains, in which the smectic layers are wrapped around line defects in the form of ellipses (seen in the figure) and hyperbolae (most of them are oriented normally to the plane of fiew). The ellipses form a fractal-type of structure, with smaller ones filling the gaps between the larger ones. The smectic order was discovered and correctly identified from optical observations of textures similar to the one shown here, on the basis of geometrical properties of ellipses and hyperbolae, before X-ray techniques were invented. |
IMAGE 5:
 |
| Mathematically created model of smectic layers in three neighboring focal, conic domains. |
In smectic A, rod-like, elongated molecules are arranged parallel to each other, forming layers of monomolecular length, shown as surfaces in the model. The layers are stacked on top of each other and are flexible. When the smectic layers bend, they tend to preserve their equidistance, as it is fixed by the molecular length. The restriction of constant layer thickness leads to a peculiar geometry of deformations, so-called focal conic domains, in which the smectic layers are wrapped around line defects in the form of ellipses and hyperbolae. The smectic order was discovered and correctly identified from an optical observation of textures similar to the one shown here, on the basis of geometrical properties of ellipses and hyperbolae, before X-ray techniques were invented.
IMAGE 6:
 |
| Hybrid aligned nematic film. More about this Image Polarizing microscope texture of a thin, liquid crystalline film. This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin, nematic film (one- to two-micrometers thick) is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but tend to be parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. Very often, the director distortions collapse into "strings" of practically constant width, seen in the texture as parallel dark extinction bands. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer. |
IMAGE 7:
 |
| Thin, nematic film (0.4 micrometer) placed onto an isotropic substrate. More about this Image Polarizing microscope texture of a thin, liquid crystalline film. This periodic stripe structure with two different periodicities and different directions occurs in a nematic fluid, the simplest form of a liquid crystal. The nematic film is spread over the surface of an isotropic fluid (glycerine). The upper surface is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but they try to remain parallel to each other. The average direction of orientation is called the director. The director is distorted in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation and the glycerine-nematic interface favors tangential (parallel) orientation. When the film is very thin--less than one micrometer--these distortions use too much energy and the system relaxes through the periodic pattern of in-plane director variations. The effect is similar to buckling instability of an elastic rod stressed at two ends; at some critical stress, the rod bulges. |
IMAGE 8:
 |
| Hybrid aligned nematic film. |
Polarizing microscope texture of a thin, liquid crystalline film. This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin, nematic film (one- to two-micrometers thick) is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but tend to be parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer of the optical microscope.
IMAGE 9:
 |
| Polarizing microscope texture of a thin, liquid crystalline film. |
This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin, nematic film (one- to two-micrometers thick) is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like elongated molecules are free to move around, but tend to be parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer of the optical microscope. The sharp point with four dark brushes emerging from it is a topological point defect called boojum. The term "boojum" entered the world of science from Lewis Carroll's "The Hunting of the Snark," thanks to Professor David Mermin, who was the first to introduce the word in his Physical Review Letters article in 1977. At the time, Mermin was studying topological defects similar to those presented here, in a superfluid helium 3 that, amazingly, is also a liquid crystal in many respects.
IMAGE 10:
 |
| Polarizing microscope texture of a thin, liquid crystalline film. |
This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin, nematic film (one- to two-micrometers thick) is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but tend to be parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer of the optical microscope.
IMAGE 11:
 |
| Periodic square lattice, as seen under a polarizing microscope. |
This periodic square lattice, seen under a polarizing microscope, occurs in a nematic fluid--the simplest form of a liquid crystal--when a thin film of the material is spread over the surface of an isotropic fluid (glycerine). The upper surface of the nematic film is free (in contact with air). In the nematic, the rod-like, elongated molecules are free to move around, but tend to remain parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. When the film is very thin--less than one micrometer--these distortions use too much energy and the system relaxes through periodic, in-plane director variations. The effect is similar to buckling instability of an elastic rod stressed at two ends; at some critical stress, the rod bulges. The periodic pattern illustrates a fine balance of elastic and surface anchoring forces. In this particular, rarely observed case, the director distortions adopt the form of square lattice; more often, one observes a periodic, one-dimensional pattern of stripes.
IMAGE 12:
 |
| Hybrid aligned nematic film. |
Polarizing microscope texture of a thin, liquid crystalline film. This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin nematic film (one- to two-micrometers thick) is spread over the surface of an isotropic fluid (glycerine). The upper surface of the film is free (in contact with air). In the nematic, the rod-like elongated molecules are free to move around, but they tend to be parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. The dark bands mark the regions where the orientation of liquid-crystal molecules is parallel to either the polarizer or analyzer of the optical microscope.
-------------------------------------------------------------------------------------------------------------------------------
Nice example of plagiarism.
ReplyDelete