1. Abstract
The rich variety of defects in ordered media has been interesting for a long time. Group theory [1] as well as analytical calculations [2] have been used to describe defects. The numerical algorithm applied here provides a third method, using elements of both of the above approaches and thereby closing the gap between them. As a bonus, the simulation of sophisticated structures such as disclinations elucidates the capabilities and also the weaknesses of the present theory.
But also apart from the field of basic research, the dynamics of nematic defects is of growing interest. In the past, defects did not seem to be too important for device modeling, if one only knew how to avoid them1. Presently, a new generation of devices -the PDLCD and PDCLCD2—is being developed. For them to operate, the presence of disclinations is essential. A future prospect is the application of disclinations to store and process information, which appears possible due to their considerable amount of entropy.
Here, transient defects (mainly the well known shrinking loops [6]) are investigated experimentally and also theoretically with the aid of a previously developed numerical algorithm [7,8]. It is based on a dynamic equation for the alignment tensor aμν and takes into account the rotational diffusion, the influence of an orienting external field, and the Frank elasticity (in the one-coefficient approximation), but flow processes are neglected
The numerical method proved capable to reproduce the complicated structures of a network of disclination lines that simplifies to a shrinking loop. Moreover, the theoretically obtained shrinking behavior agrees quite well with the experimental one, which provides a simple method to estimate the rotational viscosity for a substance whose elastic constants are known. In performing computer simulations of defects subject to an external field3, I found that a shrinking loop should stop shrinking and grow again at a specific field strength. I confirmed this result experimentally.