Point defects boojums

An especially clear demonstration of twist relaxation is given by tangentially anchored spherical nematic droplets suspended in an isotropic matrix (glycerin). Figure 5.3. The director lines join two point defects—boojums at... 

Figure 5.15. Cholesteric textures in spherical droplets with tangential director anchoring at the boundary. Top A monopole configuration with a point defect N =l in the field / of normals to the cholesteric layers and an attached nonsingular line k — 1, stable when R/p 1 (microphotograph in crossed polarizers). Bottom A boojum configuration with an isolated k — 2 surface point defect at R/p 1 (no crossed polarizers). The insert shows the director field at the surface of the droplet.

The point defect at a surface of an ordered medium can represent either the end of a line that is topologically stable in the bulk or a true surface point defect with no bulk singularity attached [61]. In cholesteric liquid crystals, all points with A = i 1 are the ends of bulk disclinations. Only when k = 2 An rotations of the director field), the point defect might be an isolated surface singularity. However, even in this case one should take care of the requirement of the layers equidistance. For example, the classical boojum configuration cannot be observed in a cholesteric vessel when 1. 

The boojum has been introduced by Mermin [62] for superfluid 3He-A as a way of reducing the energy of the monopole. In the 3He-A spherical volume, the energy of a monopole decreases when the line shrinks into a point at the surface this point is the boojum. However, in the cholesteric phase, such a transformation violates the equidistance between the layers. As a result, the monopole structure remains stable, at least when R/p 1. Only when R/p I, can the isolated point defect with An rotations of the director field... 

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