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LIQUID-CRYSTALLINE PHASES IN COLLOIDAL SUSPENSIONS OF DISC-SHAPED PARTICLES

LIQUID-CRYSTALLINE PHASES IN COLLOIDAL SUSPENSIONS OF DISC-SHAPED PARTICLES. Y. Martínez (UC3M) E. Velasco (UAM) D. Sun, H.-J. Sue, Z. Cheng (Texas A&M). Aqueous suspensions of disc-like colloidal particles (diameter m m) Same thickness (nm) Polydisperse in diameter.

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LIQUID-CRYSTALLINE PHASES IN COLLOIDAL SUSPENSIONS OF DISC-SHAPED PARTICLES

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  1. LIQUID-CRYSTALLINE PHASES IN COLLOIDAL SUSPENSIONS OF DISC-SHAPED PARTICLES Y. Martínez (UC3M) E. Velasco (UAM) D. Sun, H.-J. Sue, Z. Cheng (Texas A&M) • Aqueous suspensions of disc-like • colloidal particles (diameter mm) • Same thickness (nm) • Polydisperse in diameter

  2. Anisotropic colloids discotic colloids Non-spherical colloidal particles (at least in one dimension) Give rise to mesophases rod-like (prolate) disc-like (oblate) • ORIENTED PHASES • PARTIAL SPATIAL ORDER rods prefer smectic discs prefer columnar But there is another factor: POLYDISPERSITY

  3. But all synthetic colloids are to some extent polydisperse in size parent phase coexisting phases FRACTIONATION polydispersity parameter Polydispersity could destabilise non-uniform phases, since it is difficult to accommodate range of diameters in an ordered arrangement

  4. Effect of polydispersity in discotics thickness polydispersity: destabilization of smectic diameter polydispersity: destabilization of columnar columnar phase smectic phase

  5. Gibbsite platelets in toluene: a hard-disc colloidal suspension van der Kooij et al., Nature (2000) Platelets made of gibbsite a-Al(OH)3 200nm "hard" platelet steric stabilisation with polyisobutylene (PIB) (C4H8)n Suspensions between crossed polarisers f=0.19 0.28 0.41 0.47 0.45 I+N N N+C C C before fractionation dR=25% after fractionation dR=17% (without polarisers)

  6. GEL 14% 18% SMECTIC? dR=17% dR=25% phase sequence: I-N-C f platelet volume fraction of monodisperse discs with <L> and <R> • But what happens at higher/lower diameter polydispersity? • Can the smectic phase be stable? • Role of thickness polydispersity?

  7. Zirconium phosphate platelets a-Zr(HPO4)2· H2O TEM of pristine a-ZrP platelets TEM of a-ZrP platelet coated with TBA

  8. PROCESS OF EXFOLIATION OF LAYEREDa-Zr(HPO4)2·H2O aspect ratio • diameter optical lengths • COLUMNAR • thickness X rays • SMECTIC

  9. Optical images: white light and crossed polarisers I I+N N N+S f = platelet volume fraction volume occupied by platelets = total volume

  10. ISOTROPIC-NEMATIC phase transition I I + N N non-linearity in the two-phase region: some fractionation extremely large volume-fraction gap: dR In gibbsite

  11. Small Angle X-ray scattering smectic order, with weak N to S transition sharp peaks with higher-order reflections (well-defined layers) large variation in smectic period with f (almost factor 3) long-range forces? SMECTIC NEMATIC

  12. Isotropic-nematic Restricted-orientation approximation: Distribution projected on Cartesian axes: where is a Schultz distribution characterised by dR Hard interactions treated at the excluded-volume level (Onsager or second-virial theory) minimum

  13. f dR dR CHARACTERISTICS OF SMECTIC PHASE FROM EXPERIMENT

  14. Nematic-smectic-columnar Fundamental-measure theory for polydisperse parallel cylinders Second-virial theory not expected to perform well : complicated distribution function Simplifying assumption: perfect order SMECTIC COLUMNAR number of particles at r in a volume d3r with diameter between D and D+dD

  15. dR=0.52 dR fS=0.452 fS=0.452

  16. Attractive polydisperse platelets free-energy functional: L

  17. Phase diagrams (Gaussian tail distribution) l = 2 l = 1 dR = 0.294

  18. Microfractionation in the coexisting smectic phase dR = 0.294, l = 2, be = 1.665

  19. Future work • Improve and extend experiments • larger range of polydispersities (in particular lower) • overcome relaxation problems • Improve and extend theory. Include polydispersity in both diameter and thickness • Terminal polydispersities in diameter (columnar) • and thickness (smectic)? • Better understanding of platelet interactions • better modelling of interactions

  20. THE END

  21. CHARACTERISTICS OF SMECTIC PHASE FROM EXPERIMENT

  22. pair potential Theory: some ideas Potential energy: will contain short-range repulsive contributions + soft interactions (vdW, electrostatic, solvent-mediated forces,...?) We treat soft interactions via an effective thicknessLeff (f) of hard discs • Criteria: • fIN in correct range • in smectic phase • approximate theory of screened • Coulomb interactions?

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