Part A. Under crossed polarisers a Schlieren texture shows three point defects. When…

PHYSICS · Subjective · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Order, broken symmetry and collective behaviour · Soft condensed matter

Soft Matter at the Thermal Scale, Ch Frank elasticity, defects and smectic layers

Problem

Part A. Under crossed polarisers a Schlieren texture shows three point defects. When the crossed polarisers are slowly rotated together: Point

\mathrm A

shows

2

dark brushes, which rotate in the same sense as the polarisers. Point

\mathrm B

shows

4

dark brushes, which rotate in the same sense as the polarisers. Point

\mathrm C

shows

4

dark brushes, which rotate in the opposite sense to the polarisers. (a) Assign a signed strength

s

to each of

\mathrm A

\mathrm B

\mathrm C

. Part B. X-ray scattering from a second liquid-crystal sample shows a single sharp peak at

q_0 = 1.8\,\mathrm{nm^{-1}}

along one direction, with no other sharp peaks at any angle, and the sample flows under its own weight with no measurable shear yield stress. (b) Find the layer spacing

d

. (c) State, giving a reason from the pattern and the flow behaviour, whether this is a nematic or a smectic phase. (d) Explain why it is neither a crystal nor a nematic.

Browse the problem library