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Roll patterns normal

Figure 4. Snapshots of EC patterns slightly above onset for case A taken in a polarizing microscope with a single polarizer (shadowgraph images. Phase 5,d = 9/rm). a Ohliqne rolls, b normal roUs, c dielectric rolls (Note the difference in magnification.). The initial director orientation is horizontal. The contrast was enhanced by digital processing. Figure 4. Snapshots of EC patterns slightly above onset for case A taken in a polarizing microscope with a single polarizer (shadowgraph images. Phase 5,d = 9/rm). a Ohliqne rolls, b normal roUs, c dielectric rolls (Note the difference in magnification.). The initial director orientation is horizontal. The contrast was enhanced by digital processing.
Figure 9. Snapshots of electroconvection patterns superposed on the Freedericksz state in case C. a oblique rolls, b normal rolls. Figure 9. Snapshots of electroconvection patterns superposed on the Freedericksz state in case C. a oblique rolls, b normal rolls.
Another example is related to the motion of defects (dislocations in the roll pattern) which constitutes the basic mechanism of wavevector selection. In the normal-roll regime, the stationary structure is characterized by the condition q II H. However, when changing the field direction one can easily induce a temporary wavevector mismatch Aq = q ew — qoid which relaxes via a ghde (v II q) motion of defects. Experiments have confirmed the validity of detailed theoretical predictions, both with respect to the direction (v J. Aq) and the magnitude (consistent with logarithmic divergence at Aq —> 0) of the defect velocity v [42]. [Pg.72]

Before discussing these experiments we will present new measurements using the nematic Phase 5, which show a similar crossover at ft < 0.1 Hz. Thus for f < ft there are flexodomains as a first instability while for f > ft there are the usual EC roll patterns with conductive symmetry. Above the Lifshitz frequency //, 40 Hz there are normal rolls, which are replaced by... [Pg.120]

Figure 13.1 a) Cell geometry with section of a roll pattern for EHC (planar configuration). E = field, V = velocity b) Normal roll pattern for EHC with a dislocation. [Pg.261]

The large aspect-ratios, which can be achieved comparatively easily in EHC, make this system particularly well suited to test predictions of GLEs. Experimental results for the existence and stability of normal roll patterns in the q-e plane (Busse balloon) are shown in Fig. 13.7 [28, 36]. Similar experiments were reported by Ref [29]. The symbols give the experimental points and the curves represent parabolic fits for the neutral curve (N) and the... [Pg.279]

The situation is reminiscent of Rayleigh-Benard convection in isotropic fluids where stable roll attractors apparently compete with complex patterns, spiral defect turbulence [115-117], It has been shown very recently that if anisotropy is introduced into this system by inclining the convection cell, a normal roll pattern with dislocation defect turbulence occurs, which looks quite similar to patterns observed in EHC [118]. [Pg.285]

Fig. 6. (a) The flow pattern assumed in the physical model for the motion near the wall, (b) A more realistic flow pattern of the motion, (c) The roll cell pattern near the wall in a section normal to the main direction of flow. [Pg.58]

The over-all length of the chute, 58 inches, forces each fruit to rotate completely five to six times during its passage. Since the fruits are injected by a gating system normal to the irradiation chute, they already have some rotational energy in this direction. The effect is to add some gyrating motion to the fruits as they roll through the chute. Each element of the surface is then exposed to the variety of flux patterns as it rotates the five to six complete turns to the exit. [Pg.139]

Sheet textures may also be represented by inverse pole figures. Here three separate projections are needed to show the distribution of the sheet normal, rolling direction, and transverse direction. Figure 9-24(b) is such a projection for the normal direction of the steel sheet whose (110) pole figure was given in Fig. 9-20 it was calculated from the crystal orientation distribution mentioned in Sec. 9-8. The distribution of the normal direction is also shown in (c), for the same material. This distribution was measured directly in the following way. A powder pattern is made of the sheet in a diffractometer by the usual method, with the sheet equally... [Pg.319]

The original, so called 1-d formula of Carr and Helfrich, was later refined and generalized into a 3-d theory capable of calculating the wavevector and describing real, three dimensional patterns (like normal or oblique rolls), other geometries and the dielectric regime [16]. [Pg.63]

Measurements have shown that the prewavy pattern appears in a forward bifurcation [50]. Its threshold voltage Upw has a weak, nearly linear frequency dependence. It usually occurs at higher frequencies (see Fig. 1). Conductive normal rolls, dielectric rolls and the prewavy pattern may follow each other with increasing / (dielectric rolls may be skipped in compounds with higher conductivity). Near the crossover frequency /c, the conductive (or dielectric) rolls can coexist with the prewavy pattern resulting in the defect-free chevron structure [51]. [Pg.76]

Hy favours rolls with axes normal to y, but in the field-free case the rolls degenerate into a square pattern that may be regarded as a linear superposition of crossed convection rolls. When AT is increased well beyond A7 a complex hexagonal structure is found with a nematic-isotropic interface if the temperature of the upper plate is large enough. [Pg.205]

When a crystalline polymer is oriented, the random circular film pattern (random orientation) transforms to a collection of defined reflection arcs that are correlated with particular (hkl) planes that can be identified based on the crystal structure and Bragg relationship (see Fig. 12a-b). It follows that the magnitude of the azimuthal spread (x/2) of these reflections is indicative of the degree of orientation. (The breadth, k, of the reflection is related to crystal size and imperfection—see Ref. 32.) Also, the location of the reflection with respect to the sample axes indicates the orientation of the crystallographic planes. For example Fig. 5(a) and (b) show two X-ray photographs of polyethylene that had been cold rolled. From the (200) reflection in sample (a) one sees that the a-axis is aligned preferentially normal to Z whereas in (b) there are two distinct orientations of the a-axis—one along Z and one normal to this. [Pg.75]

Patterns in nematics are easily observed by optical means where the anisotropy of the refractive index is exploited. In this way the stripe patterns in electroconvection in the planar geometry are easily discriminated from flexodomains the angle a between the wave vector q of the EC stripes and the preferred direction no a is small (normal or oblique rolls) in contrast to a = 90° (longitudinal stripes) in flexodomains. [Pg.104]

It will be seen in the next and subsequent chapters that a wide variety of cell geometries (e.g. parallel plates, concentric cylinders, Swiss roll), types of electrode (e.g. plates, beds, porous, expanded metals and gauzes) and flow patterns are used in industrial electrochemistry. In most the flow is too complex to warrant a detailed fluid-mechanical calculation. Rather the normal approach to mass transport in electrolytic cells is to treat the cell as a unified whole and to seek expressions in terms of space-averaged quantities which permit some insight into the mass transport conditions within the cell. [Pg.25]


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See also in sourсe #XX -- [ Pg.441 , Pg.445 , Pg.450 ]




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