The document summarizes initial experiments on producing a ferromagnetic nematic suspension using iron oxide platelets dispersed in the liquid crystal 5CB. Oleic acid was added to stabilize the platelets and induce homeotropic anchoring of 5CB on the platelet surfaces. High oleic acid concentrations decreased the melting temperature of 5CB. Effects of the magnetic platelets on 5CB were only seen at very strong fields, suggesting the anchoring was too strong or the field too weak. Future work will optimize concentrations and investigate the Faraday effect.

1. Introduction
Discussion
Experimental Approach
Future Directions
Literature Cited
Results
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Recently it has been shown that suspending ferromagnetic barium hexaferrite platelets
(~70 nm) in nematic liquid crystals, such as 5CB, can produce a ferromagnetic fluid by
enhancing the liquid crystal’s susceptibility to magnetic fields. 1 When an external
magnetic field is applied, the magnetic moments of the platelets rotate, causing a
reorientation of the nematic director as well. Here we report the initial steps we have
taken for producing a ferromagnetic nematic suspension using iron oxide platelets
dispersed in 5CB.
While previous studies have proven that a ferromagnetic nematic system can be
experimentally produced and exhibits optical switching and magnetic hysteresis, we
intend to examine other properties, such as the Faraday effect. Since nematic liquid
crystals are optically anisotropic, ferromagnetic nematic suspensions should exhibit the
Faraday effect and be a very promising material for magneto-optical devices, such as
optical isolators. Additionally, since the nematic director is intrinsically sensitive to
electric fields, a sort of switch could potentially be produced to turn the optical isolation
effect on and off.
Oleic acid/5CB mixtures were prepared by putting varying amounts of oleic acid into
100 mg of 5CB, heating the liquid crystal in the isotropic phase (~35°C), and then
quenching it back to the nematic phase. Small amounts of the suspensions were drawn
into 0.10 mm x 1.00 mm rectangular glass capillaries. Melting temperatures were
determined using an Instec STC200 Temperature Controller, HS1 heating stage, and a
Type K thermocouple with the sample under a polarized optical microscope.
Suspensions were prepared by mixing ferromagnetic iron oxide platelets (~100 nm) in
hexanes (initial concentration: 10 mg/mL) with oleic acid in varying ratios.
Approximately 20 μl of hexanes was then added to 100 mg of 5CB in the isotropic
phase immediately before the platelets were dispersed in 5CB. Mixtures were kept in
the isotropic and the hexanes left to evaporate for at least one hour before first use. To
achieve planar anchoring of the 5CB, glass cells were treated using rubbed poly(vinyl
alchohol).
High concentrations of oleic acid, which is used to stabilize the platelets and ensure
homeotropic anchoring of 5CB to the platelet surface, can significantly decrease the
melting temperature of 5CB and must be carefully considered.
The addition of oleic acid does not seem to affect platelet aggregation. Aggregation does
appear to be lessened by filling cells with suspensions in the nematic instead of isotropic
phase, however.
We have noticed an effect of the iron oxide platelets on the sensitivity of 5CB to a
magnetic field only when the suspensions are in untreated glass capillaries with a very
strong field applied (estimated 300-400 mT). This might suggest that the anchoring of the
rubbed PVA layer is too strong for reorientation or that the field generated by the iron
cores on the stage is too weak.
Figure 3. a) POM images of 5CB containing specified concentrations of oleic acid.
b) POM images of 5CB with 258 mM of oleic acid over time when a warm object
approaches.. c) Melting of same sample from (b) using body heat. All images taken
around 24.5°C. POM images taken under crossed polarizers.
Work in the immediate future will include continuing to investigate the effects of iron
oxide and oleic acid concentration on the susceptibility of 5CB to the magnetic field. Once
this ratio has been optimized for maximum sensitivity and minimal platelet aggregation,
other studies such as the Faraday effect will be investigated.
1. Mertelj, A., Lisjak, D., Drofenik, M., Čopič, M. Ferromagnetism in suspensions of
magnetic platelets in liquid crystal. Nature. 2013. 504. 237-241.
Madeline Van Winkle, Dr. Alexis de la Cotte, Professor Arjun G. Yodh
Doping of a Thermotropic Liquid Crystal with Ferromagnetic Platelets
University of Pennsylvania, Department of Physics, Philadelphia, PA 19104
Figure 2. Relationship between oleic acid concentration and 5CB melting temperature.
258 mM 301 mM 330 mM 354 mM
10 s 20 s 30 s0 s
a)
b)
c)
9 s6 s3 s0 s
50 mT – 50 mT
Figure 4. POM images of cells containing 5CB, 0.1 wt% Fe3O4,, and 200 mM oleic acid.
Sample on top was filled into the cell in the isotropic phase. Sample on bottom was filled
into the cell in the nematic phase.
Figure 5. a) POM Images of 5CB with 0.1 wt% Fe3O4 and 200 mM oleic acid in rubbed
PVA cells placed in controlled magnetic field. b) POM images of 0.05 wt% Fe3O4 and
100 mM oleic acid in 5CB in untreated glass capillary and with cylindrical magnets
manually placed adjacent to the capillary.
0 mT ~350 mT
Results
Figure 1. a) Schematic of capillary (left) and rubbed PVA cell (right). b) Stage design
for generating controlled magnetic field (left) and schematic of magnets (right).
a)
b)
Acknowledgements
I would like to thank Professor Arjun G. Yodh and Dr. Alexis de la Cotte for their
mentorship and guidance. I would also like to thank Dr. Angel Martinez for designing and
building the magnetic field stage as well as the Yodh Group for their support. Funding for
this research was provided by the National Science Foundation through the Research
Experience for Undergraduates at the Laboratory for Research on the Structure of Matter.
Field direction
Hinges for repositioning
Sample viewing area
Copper solenoids
Wire connectors
Iron cores
20 μm spacing
SpacersRubbing direction
5CB5CB
0.10mm
1.00 mm
15
20
25
30
35
40
45
0 100 200 300 400 500 600
Temperature(°C)
Oleic Acid Concentration (mM)
Start Melting (STC)
End Melting (STC)
Start Melting (Thermo)
End Melting (Thermo)