We conduct research on the properties and applications of polymer cholesteric liquid crystal (pCLC) flakes. Developed and characterized in our laboratory in the mid '90's, pCLC's retain the optical property of wavelength-selective reflection found in low molecular weight mesogens, but in a robust form that is environmentally stable. Thin films of pCLC material are freeze-fractured into small, flat particles of arbitrary size (20-80 µm) with a thickness of ~5 µm.

Flakes may be added to an adhesive and coated onto a surface, or they may be suspended in a host liquid medium and confined within a cell with transparent, conductive coatings. Particles that are fixed in a binder find applications as decorative paints, or as polarizing pigments for document security. In a low viscosity fluid confined between substrates, flake rotation with an external electric field may some day form the basis for electronic paper color displays.

Light normally incident on a flake surface is circularly polarized and selectively reflected. The application of a small electric field causes the particles to tilt away from normal, causing the intensity of reflected light of a given wavelength to change. Under moderate dc and ac fields, pCLC flakes exhibit motion that results in large intensity changes with response times under a second. Our work has shown that, by using host fluids with moderately large dielectric constants, controlled flake motion from 0 to 90 degrees of tilt may be observed with applied ac voltages less than 50 mv rms/µm and response times less than 500 ms. There are significant opportunities for these flakes in particle display technology and electronic paper, where they would offer both polarization for 3-D and variable color. However, significant materials and process issues need to be addressed.

Our models for flake motion indicate that there are several variables that might be exploited to reduce response time and improve control of particle tilt. Among these are particle dielectric properties, shape anisotropy, host viscosity, host dielectric properties, and confinement geometry. These are subjects for our research.

Colored and shaped flakes manufactured with soft lithography; on the cover of Advanced Functional Materials, Feb. 2004

We recently succeeded in manufacturing shaped flakes by soft lithography. When driven with 50 Hz AC fields in dielectric fluid-filled, ITO coated cells, these flakes exhibited very uniform reorientation behavior with response times below 300 ms.

Randomly shaped PCLC flakes encapsulated in a PVA binder. The fluid host was doped with a red dye to distinguish microcapsules from air voids.

We also recently succeeded in encapsulating flakes in flexible binders. This work may open up the possibility for bistable, flexible, electronic display applications. Earlier papers are listed here. More recent papers are listed elsewhere on this site.

T. Z. Kosc, K. L.Marshall, A. Trajkovska-Petkoska, R. Varshneya and S. D. Jacobs, "Development of Polymer Cholesteric Liquid Crystal Flakes for Electro- Optic Applications," Optics & Photonics News, p. 33 (Dec., 2004).

T. Z. Kosc, K. L. Marshall, and S. D.Jacobs, "Electrically Addressable Optical Devices Using a System of Composite Layered Flakes Suspended in a Fluid Host to Obtain Angularly Dependent Optical Effects," U.S. Patent No. 6,829,075 (7 Dec., 2004).

K. L. Marshall, E. Kimball, S. McNamara, T. Z. Kosc, A. Trajkovska-Petkoska, S. D. Jacobs, "Electro-optical behavior of polymer cholesteric liquid crystal flake/fluid suspensions in a microencapsulation matrix," Liquid Crystals VIII, edited by I.-C. Khoo, (SPIE, Bellingham, WA, 2004), Vol. 5518, pp. 170-180.

K. Marshall, T. Kosc, S. Jacobs, S. Faris, L. Li. “Electrically Switchable Polymer Liquid Crystal and Polymer Birefringent Flake/Fluid Host Systems and Optical Devices Utilizing Same.” U.S. Patent 6,650,042 issued on December 16, 2003.

T. Z. Kosc, K. L. Marshall, and S. D. Jacobs“ Polymer Cholesteric Liquid Crystal Flake Particle Displays Utilizing Maxwell-Wagner Polarization Effects for Switching,” , presented at the SID 23rd Annual International Display Research Conference, Phoenix, AZ, 8 September 2003, pp. 237–239.

T. Z. Kosc, K. L. Marshall, and S. D. Jacobs ,“Polymer Cholesteric Liquid Crystal Flakes for Particle Displays,” in 2003 SID International Symposium, Digest of Technical Papers , 1st ed., edited by J. Morreale (Society for Information Display, San Jose, CA, 2003), Vol. 34, Book 1, pp. 581 - 583.

T. Z. Kosc, K. L. Marshall, S. D. Jacobs, J. C. Lambropoulos, and S. Faris, "Electric-Field-Induced Motion of Polymer Cholesteric Liquid Crystal Flakes in a Moderately Conductive Fluid," Appl. Opt. 41, 5362-5366 (2002).

T. Z. Kosc, K. L. Marshall, and S. D. Jacobs, "Electric Field Induced Rotation of Polymer Cholesteric Liquid Crystal Flakes: Mechanisms and Applications," in Liquid Crystals IV, edited by I. C. Khoo (SPIE, Bellingham, WA, 2002), Vol. 4799, pp. 96-101 (invited).

T. Z. Kosc, S. D. Jacobs, K. L. Marshall, and B. Klehn, "Polymer Cholesteric Liquid Crystal Flakes for Display and Other Electro-Optic Applications," presented at the Novel Optical Materials and Applications Meeting (NOMA), Cetraro, Italy, 20-27 May 2001.