Soft Matter Research at UPenn

At the Materials Research Science & Engineering Center (MRSEC) at the University of Pennsylvania, three research groups are hard at work figuring out some of the fundamen-tal physics behind liquid crystal ma-terials of soft matter materials. Pro-fessors Kate Stebe, Randy Kamien, and Shu Yang each head up a re-search group, and collaborate at the university. The goal of the MRSEC is to preemptively obtain knowledge that will be useful to future applica-tions of soft matter materials. Other senior investigators involved in re-lated work in the MRSEC group on geometric routes to soft assembly include Professors Tom Lubensky, Arjun Yodh, Peter Collings, Denis Discher, Ravi Radhakrishnan and To-bias Baumgart.

The coalition has published sev-eral papers together. They have worked on demonstrating how Smectic-A liquid crystals respond to underlying substrate geometry, allowing grapho-epitaxial assem-bly of toric focal conic domains of smectic-A liquid crystals (1). They furthered this line of thought by showing how liquid crystal film thickness and pillar height affect for-mation of hierarchical assembly of such domains (2). The results from these studies will be useful to future attempts to create complex 3D de-signs, allowing further development of LC-based electronics and optical devices.

The groups have demonstrated that electric fields can be used to

organize anisotropic bullet shaped particles into 1D chains 2D hexag-onal structures (3). They have also shown that controlling boundary conditions and particle shape can allow the creation of hybrid washers which combine homeotropic and degenerate planar anchoring (4). The washers change their angle with respect to the far-field direction and are capable of trapping spherical colloids, allowing for the construc-tion of multi-shape colloidal assem-blages. These methods can be used to create complex shapes, further in-creasing the number of techniques available to those involved in creat-ing complex structures.

The team is actively contributing to the goals of the MRSEC by ex-panding the techniques available for use in the construction of com-plex assemblies. Their work also provides a firm footing for planning future projects, giving researchers a broader array of methods and tech-niques.

Learn more about their collabora-tion at the group website.

-Michael Lane

1. “Pillar-Assisted Epitaxial Assembly of Toric Focal Conic Domains of Smec-tic-A Liquid Crystals”, A.Honglawan , D.A. Beller , M.Cavallaro , R.D. Kamien , K.J. Stebe , and S.Yang, Advanced Materials, 2011, 23, 5519–5523

2. “Topographically induced hierarchical assembly and geometrical transfor-mation of focal conic domain arrays in smectic liquid crystals” Apiradee Honglawana, D.A. Bellerb,1, M.Cavallaro, Jr.a, Randall D. Kamienb, K. J. Ste-be, and Shu Yanga,c,2 PNAS, 2013, 110 (1)

3. “Microbullet assembly: interactions of oriented dipoles in confined nematic liquid crystal.” M.A.Gharbia,b,c , M.Cavallaro Jr.b , G.Wuc , D.A. Bellera , R.D. Kamiena ,S.Yang, and K.J. Stebeb. Liquid Crystals, 2013

4. “Ring Around the Colloid”, Marcello Cavallaro Jr.,M.A. Gharbi,D. A. Beller,S Copar, Z.Shi, R.D. Kamien,Shu Yang,Tobias Baumgart, and Kathleen J. Stebe.

Background image: An illustration of edge-pinning effect of focal conic domains (FCDs) in smectic-A liquid crystals with nonzero eccentricity on short, circular pillars in a square array, together with surface topography and cross-polarized optical image of four FCDs on each pillar. The arrangement of FCDs is strongly influenced by the height and shape of the pillars. Image courtesy of Felice Macera, Daniel Beller, Apiradee Honglawan, and Simon Copar. Advanced Materials Cover: This paper demonstrated an epitaxial approach to tailor the size and symmetry of toric focal conic domain (TFCD) arrays over large areas by exploiting 3D confinement and directed growth of Smectic-A liquid crystals with variable dimensions much like the FCDs.

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September | 2013 | Issue #56

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Morphology transition in lipid vesicles due to in-plane order and topological defectsLinda S. Hirst, Adam Ossowski, Matther Fraser, Jun Geng, Jonathan Selinger, and Robin Selinger. PNAS (2013) vol. 110 no. 9. 3242-3247

Figure 1 A-B: Two flourescence microscopy images of the DPPC GUVs in their crumpled state after cooling to room temperature (Scale bars, 10 μm).Figure 1C: confoncal microscopy image of a GUV which had fractured whilst cooling into its gel phase (Lβ’). Figure 1D: a less topologically deformed sample, remained intact throughout its mem-brane wall keeping the red dye from leaking in Figure 2: A coarse grained model of a GUV used to investigate the two competing mecha-nisms responsible for surface morphology. The parameters support the hypothesis as GUV B is markedly more deformed than A. The numbers above the images represent angle of view.

2A: A system with high defect mobility and slower membrane motion. 2B: A system with low defect mobility and faster membrane motion.

The relationship between complex shapes and topo-logical defects has been identified and studied in soft matter systems for some time. Liquid crystals, block co-polymer vesicles, liquid-crystalline elastomers, colloidal crystals and superfluids are some interesting examples. Typically, such phenomena arise from the chemically heterogeneous nature of these systems. In this paper, researchers present a new study on chemically ho-mogenous giant unilamellar vesicles (GUVs) which dis-play significant morphological changes (referred to as crumpled vesicles as seen in Figure 1a-b) as they cool through the liquid crystalline (Lα) to gel (Lβ’) phase tran-sition.

The authors propose that the complex crumpling is driven by topological defects in the lipid tilt orientation. In order to properly test their hypothesis they employed a dual approach of computational (Robin Selinger and Johnathan Selinger Kent State) coarse grained simula-tions and experimental fluorescence microscopy tech-niques (Hirst Group UC Merced).

The experimental portion of the study focused of

dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid GUVs formed in water via the electro-formation meth-od. To provide evidence for topological defects in the vesicles, DPPC GUVs were labeled with a polarization sensitive molecular probe (Laurdan) which aligned parallel to the tilt orientation of the surrounding lipids. The probe fluoresced with varying intensity in different domains dependent on the angle of polarizer rotation. This provided evidence that the lipid tilt orientation var-ied as a function of position around the vesicle.

An additional experiment was designed to observe whether the crumpled vesicles remained intact across the liquid crystalline (Lα) to gel (Lβ’) phase transition by dispersing vesicles in an aqueous solution containing fluorescent dextran. The vesicles were dispersed while in the gel phase then cooled to room temperature into the liquid crystalline phase and observed using con-focal microscopy (Figure 1c-d). The images showed a distinct contrast between the exterior red dye solutions against the dark interior of the crumpled vesicles (Fig-ure 3d). Despite a number of instances where crumpled GUVs fractured (Figure 3a typically in vesicles with larg-er radii or highly deformed samples) a majority of the crumpled membranes remained intact.

The coarse-grained model was designed to provide insight into the competing mechanisms responsible for inducing defects and the resulting crumpled morphol-ogy. The parameters assumed a fluid, non-hexatic bond orientation. The simulation provided evidence that the extent of crumpling is determined via kinetic competi-tion between defect mobility and changing membrane curvature. If defect mobility is fast compared with mem-brane motion, or if the vesicle size is relatively small (Figure 2a), then all extra defect pairs quickly annihi-late before the overall membrane shape has changed significantly and a vesicle reaches the prolate ground state with only two defects. However, if defect mobility is relatively slow compared with membrane translation, or if the vesicle size is larger, defects become trapped in deeply metastable states and the vesicle forms a crum-pled morphology with non-uniform Gaussian curvature (Figure 2b). Visit the SMW website to watch a video of this.

Visit PNAS to read more.

-Adam Ossowski

September | 2013 | Issue #56

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I-CAMP liquid crystal summer school lectures online

This summer a large group of soft matter students and academics got to-gether at Cambridge University in the UK for the I-CAMP Summer school on liquid crystals. The school was chaired by Ivan Smalyukh (Boulder, Colorado), Mark Warner and Tim Wilkinson (Both Cambridge) and organized to coincide with the Isaac Newton Institute’s “Math-ematics of Liquid crystals” program. Students from both groups attend-ed a week-long series of tutorials and

research lectures by Professors from around the world and a large number of these lectures have now been post-ed online for anybody to watch along with full pdfs of the lecture slides.

The lectures range across all aspects of liquid crystal science, with topics as diverse as topology and liquid crystal defects, device physics, new hybrid materials and biological liquid crystals.

We hope you enjoy broWsing and come back soon

Linda S. HirSt & adam P. OSSOwSki

softmatterworld.org presents

2014 CAlendAR CoMPetItIon CAll foR IMAgeS Submission of images for the 2nd Annual SoftMatter-

World Calendar Competition is now open. This year we have improved the submission system by adding a form to the Gallery section of the website where you can directly upload your images, captions and author imformation. The deadline is November 30th, 2013. This will be strictly followed. The calendars must be printed before the holidays to allow ample time for shipping in-ternationally. Should you have any issues with the form online, please email:

gallery@softmatterworld.org

We would like to thank the 12 users who submitted their images last year. We extend our appreciation not only for their contribution, but their patience and en-thusiasm as well. This year’s flyers and posters are com-posed with images from last years calendar and can be downloaded in a press package on the website.

Most importantly, remember that here are only two criteria which have to be fulfilled in order for an image to be considered: Soft Matter and Art Visit the Soft-MatterWorld.org gallery for some great inspiration.