Papers

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Shaping Boundaries to Control and Transport Topological Defects in Colloidal Nematic Liquid Crystals

by G. Campos-Villalobos*, A. F. V. Matias*, E. I. L. Jull, L. Tran, and M. Dijkstra

Anisotropic rod-like particles form liquid crystalline phases with varying degrees of orientational and translational order. When confined geometrically, these phases can give rise to topological defects, which can be selected and controlled by tuning how the rods align near boundaries, known as anchoring. While anchoring in molecular liquid crystals can be controlled through surface functionalization, this approach is not easily applicable to microscale colloidal systems, which have so far been limited to planar anchoring. Here, using particle-based simulations, Landau-de Gennes theory, and experiments on colloidal rods, we demonstrate that topographical patterning of the boundary can effectively control the anchoring type and, in turn, the defect state in two-dimensional confined nematics. Building on this, we show that dynamically shape-shifting the boundaries can transform and transport topological defects, enabling the design of liquid crystal analogs for binary information storage.
*denotes equal contribution

Curvature-directed anchoring and defect structure of colloidal smectic liquid crystals in confinement

now published in PNAS Nexus

by E. I. L. Jull, G. Campos-Villalobos, Q. Tang, M. Dijkstra, and L. Tran

Rod-like objects at high packing fractions can exhibit liquid crystalline ordering. By controlling how the rods align near a boundary, i.e. the anchoring, the defects of a liquid crystal can be selected and tuned. For smectic phases, the rods break rotational and translational symmetry by forming lamellae. Smectic defects thereby include both discontinuities in the rod orientational order (disclinations), as well as in the positional order (dislocations). In this work, we use experiments and simulations to uncover the geometrical conditions necessary for a boundary to set the anchoring of a confined, particle-resolved, smectic liquid crystal. We confine a colloidal smectic within elliptical wells of varying size and shape for a smooth variation of the boundary curvature. We find that the anchoring depends upon the boundary curvature, with an anchoring transition observed at a critical radius of curvature approximately twice the rod length. Surprisingly, the critical radius of curvature for an anchoring transition holds across a wide range of rod lengths and packing fractions. The anchoring controls the defect structure. By analyzing topological charges and networks composed of maximum density (rod centers) and minimum density (rod ends), we quantify disclinations and dislocations formed with varying confinement geometry. Circular confinements, characterized by planar anchoring, promote disclinations, whereas elliptical confinements, featuring antipodal regions of homeotropic anchoring, promote long-range smectic order and dislocations. Our findings demonstrate how geometrical constraints can control the anchoring and defect structures of liquid crystals - a principle that is applicable from molecular to colloidal length scales.

Flexible, photonic films of surfactant-functionalized cellulose nanocrystals for pressure and humidity sensing

now published in Small Structures

by D. V. Saraiva*, S. N. Remiëns*, E. I. L. Jull, I. R. Vermaire, and L. Tran

Most paints contain pigments that absorb light and fade over time. A robust alternative can be found in nature, where structural coloration arises from the interference of light with submicron features. Plant‐derived, cellulose nanocrystals (CNCs) mimic these features by self‐assembling into a cholesteric liquid crystal that exhibits structural coloration when dried. While much research has been done on CNCs in aqueous solutions, less is known about transferring CNCs to apolar solvents that are widely employed in paints. This study uses a common surfactant in agricultural and industrial products to suspend CNCs in toluene . Surprisingly, a stable liquid crystal phase is formed within hours, even with concentrations of up to 50 wt%. Evaporating the apolar CNC suspensions results in photonic films with peak wavelengths ranging from 660 to 920 nm. The resulting flexible films have variable mechanical properties with surfactant content, allowing for an optical response with applied force. The films also act as humidity sensors, with increasing relative humidity swelling the films, yielding a redshift in the reflected wavelength. With the addition of a single surfactant, CNCs can be made compatible with existing production methods of industrial coatings, while improving the strength and responsiveness of structurally colored films to external stimuli.
*denotes equal contribution

Published work

*denotes corresponding authorship

For the most up-to-date list, see Google Scholar.

G. Campos-Villalobos, A.F.V. Matias, E.I.L. Jull, L. Tran*, and M. Dijkstra*, "Shaping Boundaries to Control and Transport Topological Defects in Colloidal Nematic Liquid Crystals," arXiv preprint (2025) arXiv:2506.18664.
E.I.L. Jull, G. Campos-Villalobos, Q. Tang, M. Dijkstra, and L. Tran*, "Curvature-directed anchoring and defect structure of colloidal smectic liquid crystals in confinement," PNAS Nexus 3:11 (2024) pgae470. DOI:10.1093/pnasnexus/pgae470.
D.V. Saraiva, S.N. Remiëns, E.I.L. Jull, I.R. Vermaire, and L. Tran*, "Flexible, Photonic Films of Surfactant-Functionalized Cellulose Nanocrystals for Pressure and Humidity Sensing", Small Structures 5:7 (2024) 2400104. DOI:10.1002/sstr.202400104.
C. Blanc, G. Durey, R.D. Kamien, T. Lopez-Leon, M.O. Lavrentovich*, and L. Tran*, "Helfrich-Hurault elastic instabilities driven by geometrical frustration", Reviews of Modern Physics 95:1 (2023) 015004. DOI:10.1103/RevModPhys.95.015004.
L. Tran*, "Topological defects with a half twist", Nature Physics (2023) - News & Views.
DOI:10.1038/s41567-022-01909-0.
Y. Gu, L. Tran, S. Lee, J. Zhang, K.J.M. Bishop*, "Convection Confounds Measurements of Osmophoresis for Lipid Vesicles in Solute Gradients", Langmuir (2023). DOI:10.1021/acs.langmuir.2c02040.
L. Tran* and K.J.M. Bishop, "Swelling cholesteric liquid crystal shells to direct particles at the interface'', ACS Nano 14:5 (2020) 5459–5467. DOI:10.1021/acsnano.9b09441.
M.O. Lavrentovich* and L. Tran*, "Undulation instabilities in cholesteric liquid crystals induced by anchoring transitions'', Physical Review Research 2:2 (2020) 023128.
L. Tran, M.F. Haase*, "Templating interfacial nanoparticle assemblies via in-situ techniques", Langmuir 35:26 (2019) 8584-8602.
L. Tran*, H.-N. Kim, N. Li, S. Yang, K.J. Stebe, R.D. Kamien, and M.F. Haase*, "Shaping nanoparticle fingerprints at the interface of cholesteric droplets", Science Advances 4:10 (2018) eaat8597.
L. Tran, M.O. Lavrentovich, G. Durey, A. Darmon, M.F. Haase, N. Li, D. Lee, K.J. Stebe, R.D. Kamien*, and T. Lopez-Leon*, "Change in Stripes for Cholesteric Shells via Anchoring in Moderation", Physical Review X 7 (2017) 041029.
L. Tran, M.O. Lavrentovich, D.A. Beller, N. Li, K.J. Stebe, and R.D. Kamien*, "Lassoing saddle splay and the geometrical control of topological defects", Proceedings of the National Academy of Sciences 113 (2016) 7106. front cover article

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