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Barbara Ruzicka

SAPIENZA UNIVERSIT À DI ROMA. Relaxation dynamics in a colloidal glass. Outline Laponite Aging and Phase Diagram. Microscopic and structural relaxations across the glass transition. Conclusions. Barbara Ruzicka. 101° Congresso Nazionale della Società Italiana di Fisica

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Barbara Ruzicka

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  1. SAPIENZA UNIVERSITÀ DI ROMA Relaxation dynamics in a colloidal glass Outline • Laponite • Aging and Phase Diagram. • Microscopic and structural relaxations across the glass transition. • Conclusions. Barbara Ruzicka 101° Congresso Nazionale della Società Italiana di Fisica Rome, 21 - 25 September 2015

  2. Laponite Sinthetic clay: Na+0.7 [(Si8Mg5.5Li0.3)O20(OH)4]-0.7 Idealised structural formula: Dispersed in water Laponite originates a charged colloidal suspension of disks of nanometric size with inhomogeneous charge distribution

  3. Laponite Aging Initially LIQUID. Viscosity increases with waiting time up to when Laponite reaches an ARRESTED state

  4. Phase diagram and Arrested States Small Angle X-ray Scattering tw Cw=2.8 % a) C=0.3% b) C=1.5% c) C=3.0% Photographs & Representation Phase Separation Equilibrium Gel Wigner Glass Dynamic Light Scattering Fluid Nature Materials 10, 56 (2011). Nature Commun. 5, 4049(2014). Next talk by Roberta Angelini

  5. Aging in colloidal systems The dynamics is not stationary but changes with waiting time (tw) as the sample evolves towards an arrested state. t1: related to the interactions between a particle and the cage of its nearest neighbors. Microscopic relaxation: t1 Structural relaxation: t2 t2: related to a structural rearrangement of the particles.

  6. Aging of Laponite samples G. Grubel et al. J Alloys Comp. 362, 3, 2004.

  7. Multiangle Dynamic Light Scattering (DLS) Setup

  8. MultiAngle Dynamic Light Scattering (DLS) DLS on a Cw=3.0 % in D2O in the ERGODIC (Early aging) regime 6.2 x 10-4 < Q < 2.1 x 10-3Å-1 10-6< t < 1s τ1microscopic relaxation time t1 ≈ Q-2 τ2structural relaxation time t2 ≈ Q-2 b < 1 Diffusive dynamics for both microscopic and structural relaxation times

  9. Neutron Spin Echo (NSE) NSE at IN15 (ILL, Grenoble) on a Cw=3.0 % in D2O ERGODIC and NON ERGODIC regime (glass transition at tw ≈600 min) 1.3 x 10-2 < Q < 1.3 x 10-1 Å-1 10-9< t < 2 x 10-7s τ1microscopic relaxation time t1 ≈ Q-2

  10. Microscopic Dynamics across the glass transition Earlyaging regime Full-aging regime t1≈ Q-2 Microscopic dynamics remains DIFFUSIVE across the glass transition

  11. X-rays Photon Correlation Spectroscopy (XPCS) XPCS at ID10 (ESRF, Grenoble) on a Cw=3.0 %in D2O. NON ERGODIC regime (glass transition at tw = 560 min) 3.1 x 10-3 < Q < 2.2 x 10-1 Å-1 1 < t < 4 x 103s Kohlrausch-Williams-Watts (KWW) τ2structural relaxation time t2 ≈ Q-1 b < 1 Structural relaxation time NON DIFFUSIVE across the glass transition

  12. Structural relaxation across the glass transition Full-aging regime probed by XPCS Earlyaging regime probed by DLS t2 ≈ Q-2→ Q-1 The structural relaxation changes across the glass transition

  13. Molecular Dynamics Simulations Simple model of low density glass former: 50 - 50 non-crystallizing binary mixture of N=1000 Yukawa particles of equal screening length and different repulsion strength. b < 1 Increasing tw gradual transition t2 ≈ Q-2→ Q-1

  14. T decreasing Dynamics across the glass transition

  15. Conclusions DLS & NSE Early-aging regime Diffusive nature of particles motion Full-aging regime DLS & XPCS Full-aging regime Early-aging regime Diffusive nature of the structural relaxation Discontinuous hopping of caged particles MD Intrinsic generality, occurring also in numerical and theoretical studies on different glass-formers. Gradual transition from Q-2 to Q-1 F.A. Marques, R. Angelini, E. Zaccarelli, B. Farago, B. Ruta, G. Ruocco and B. Ruzicka Soft Matter 11, 466 (2015).

  16. Coworkers M. Sztucki THEORY and SIMULATIONS SAXS E. Zaccarelli EXPERIMENTS R. Angelini T. Narayanan A. Fluerasu F. A. de Melo Marques XPCS NSE G. Ruocco B. Farago Thank you for your attention! B. Ruta A. Madsen

  17. Aging of Laponite samples Two different non ergodic statesbelowandaboveCw =2.0 % Above Cw =2.0 %: ~37 nm ~14 nm • Structure does not change much with waiting time. • Fast aggregation. • First peak at distances larger than contact. • WIGNER GLASS. • Short-time REPULSIVE dominated tw tw Below Cw =2.0 %: • Structure changes considerably with waiting time. • Slow aggregation. • First peak compatible with rim-face bonding. • HOUSE of CARDS structure GEL • Long time ATTRACTIVE dominated 0.45 0.17 Phys Rev Lett93, 258301 (2004); JPCM 16, S4993 (2004); Langmuir 22, 1106 (2006); Phys Rev E 77, 020402 (2008); Phys Rev Lett104, 085701 (2010); Soft Matter 7, 1268 (2011); Nat Mat 10, 56 (2011),

  18. Aging Dynamics XPCS measurements at ID10-ESRF Spontaneously Aged sample Long waiting time – full aging regime – Dichotomic Aging Behaviour b <1 Stretched b >1 Compressed Anomalous behaviour Same tQ≈Q-1 behaviour Rejuvenated sample (tR≈3.5 days) R. Angelini, L. Zulian, A. Fluerasu, A. Madsen, G. Ruocco and B. Ruzicka Soft Matter 9, 10955 (2013).

  19. Waiting time dependence of t2 Early aging regime Full-aging regime

  20. Gold/polystyrene nanocomposite thin films (70 nm)

  21. T decreasing At low T tl seems to cross from a Q-2 to a Q-1 behaviour

  22. Fitting Expressions b: distribution width of the slow relaxation time. t1: Fast or microscopic relaxation time. t2: Slow or structural relaxation time. B. Ruzicka, L. Zulian G. Ruocco Phys. Rev. Lett. 93, 258301 (2004).

  23. Waiting Time Dependence tw = 0 defined at sample filtration

  24. Waiting Time Dependence of the structural relaxation time b parameter has been found to be lower than 1 (stretched behaviour) for all measured waiting times and Q-values differently from the anomalous dynamics normally found in different glass-formers with b > 1, i.e. compressed behaviour.

  25. Q-Dependence of the structural relaxation time NON ERGODIC regime probed by XPCS Structural relaxation times NON DIFFUSIVE across the glass transition (at tw ≈600 min). Correlation functions stretched.

  26. Q-1 tw0.9 =1.5 Compressed Behaviour exp tw Anomalous dynamics polystyrene Ballistic motion vs Q Power law behaviour Exponential behaviour

  27. AGING (Stretched) FULL AGING (Compressed) Anomalous dynamics in Laponite Glass β >1 β <1

  28. XPCS and DLS on Laponite Large tw (XPCS region) b<1 tQ ≈ Q-1 Small tw (DLS region) b<1 tQ ≈ Q-2 Increasing twtQ cross from a Q-2 to a Q-1 behaviour.

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