ILCS

Ingo Dierking is Senior Lecturer (Associate Professor) at the School of Physics of the University of Manchester in the United Kingdom. His interest of work lies in the physics of soft matter systems based on liquid crystalline materials, such as defect formation and annihilation, liquid crystalline nano- and micro-particle composites, and polymer modified liquid crystals. He is a member on the editorial board of several journals, the editor of Liquid Crystals Today, the chair of the British Liquid Crystal Society and the author of the book “Textures of Liquid Crystals”.

A cholesteric Grandjean texture (greenish) with oily streaks, subjected to an electric field along the helix axis. The sandwich cell has planar boundary conditions and the helix axis is thus oriented perpendicular to the substrates. The liquid crystal exhibits a positive dielectric anisotropy, and the molecular long axis will therefore turn into the direction of the applied electric field, breaking up the field free undisturbed helical superstructure. The director field reorientation causes the formation of periodic patterns which nucleate at the oily streaks defects (bright yellow). The short image size is approximately 600 micrometers in length.


Jury comment: The oily streak texture, with it's beautifully uniform background punctuated by complex "stringy" features, leading to an "oily" appearance, has been known essentially since the beginning of liquid crystal science, and has been featured in the ILCS picture gallery (see November 2009, September 2010, March 2011). This texture occurs in "lamellar" systems such as thermotropic smectics and cholesterics.

The oily streak texture, beautiful in its seeming simplicity, but structurally complex at the defects leading to the "strings," is seen in cholesteric LCs when the substrate surfaces favor planar alignment, leading to a system where the helix is perpendicular to the substrates. In this LC picture of the month, the LC material possesses positive dielectric anisotropy. Application of an electric field causes the system to convert to a "uniform-lying helix" alignment showing a strongly birefringent texture with complex patterns identical in some places with a typical lamellar focal conic texture, and also including unusual, highly periodic features. As pointed out by this month's photomicroscopic artist, in this image the reorientation nucleates at the oily streaks.