Single-Molecule Probing of Dynamical Heterogeneity in Molecular Glass Formers T. Xia, R. Zondervan, F. Kulzer, M. Orrit Molecular Nano-Optics and Spins C. Storm, W. van Saarloos Instituut Lorentz M. Möbius, M. van Hecke Kamerlingh Onnes Laboratory LION, Leiden University (NL)

Dr. Rob Zondervan Ted Xia Dr. Florian Kulzer

Introduction - single-molecule microscopy - rotational diffusion, previous work Local viscosity measurements - supercooled glycerol - heterogeneity Rheology - home-built Couette cell - plate-plate geometry Discussion and Outlook Outline

Spatial selection Collection Excitation Confocal microscope

Color scale: 1500 to counts/s with 750 W/cm 2 exc nm (R. Zondervan, 2003) Room Temperature DiI in Zeonex ®

Probing viscosity with fluorescence Fluorescence Anisotropy (during emission) Polarization fluctuations (small ensembles) Single-molecule orientation Rotational diffusion time:

Previous Work

T g +10 K T g +5 K T g +2 K Deschenes et al. JPC 106 (2002) Environmental exchanges Spread of diffusion rates

Schob et al., Eur. Polym. J. 40 (2004) 1019 Similar results in polymers, with rare environmental exchanges.

- Very small samples (1 fL) quick heating and cooling (1  s). - Many repeated cycles possible Zondervan et al., Biophys. J. 90 (2006) Thermal Cycles for Single Protein Dynamics

Rotational diffusion of single fluorescent dyes Perylene-di-imide (a) in glycerol (b) R. Zondervan et al., P. N. A. S. 104 (2007) 12628

Viscosity of supercooled glycerol from: Schröter & Donth, J. Chem. Phys. 113 (2000) 9101

Agreement with rheology

Polarized single-molecule fluorescence

Anticorrelation of polarization channels

Single-molecule tumbling at variable T

T-dep. of tumbling rates for 69 molecules

Long memory time of local tumbling rate

Seen already for colloidal suspensions in the glass phase: E. R. Weeks et al., Science 287(2000) 627 A solid matrix should be elastic : rheology? Extremely long memory of diffusion rate: evidence for solid walls? (foam)

Rheology of Super-Cooled Glycerol R. Edgeworth et al., Eur. J. Phys. (1984) 198 The Pitch-Drop Experiment (Ig-Nobel 2005)

Optical probing of shear deformation R. Zondervan et al., P. N. A. S. 105 (2008) 4993

Bulk viscosity values: Glycerol: Schröter & Donth JCP 113 (2000) 9101 o-Terphenyl Laughlin & Uhlman JPC 76 (1972) 2317 Symbols: data from Zondervan et al., PNAS 105 (2008) 4993

Shear strain traces upon sudden load

Thermal histories yielding solid-like rheology

Model of mixed response M

Exponential jump and viscous drift : liquid’s viscosity : network’s spring constant : effective viscosity (creep)

Shear modulus and viscosities from fits

Better fit with a stretched exponential (measurement with commercial rheometer)

Aging and hardening (over 2 weeks)

Ortho-Terphenyl

Shear rejuvenation

Viscosity with Couette cell (1mm gap) in rheometer (by M. M ö bius)

Thermal History Slow cooling from 260 K to 195 K at 5 K/hour, anneal at 195 K for 2 hour, warm up to aging temperature at 1 K/min Monitor G’,G’’ at f =0.1 Hz at low strain γ= (dips are dewar changes)

Aging at T=220K Solid response sets in after 25.8 hour of aging (growth)

Aging at T=240K

“Slushy” appearance

- Importance of free volume; captured in liquid lakes, fenced in by solid walls? - Mobility as a function of effective pressure (Reiser et al. Europhys. Lett. 76 (2006) 1137) - Convergence of temperature dependences to about 230 K, Tg + 40 K; MCT? Discussion

- Relation to Fischer clusters? - Crystallization? Characterize at various stages… - Relation to glacial phase?

Images of structure growth in triphenylphosphite: from H. Tanaka, R. Kurita, H. Mataki, PRL 92 (2004) T g + 15 K T g + 8 K Glacial phase

- Influence of thermal history on microstructure? - Size and shape of lakes? To do: Relevance to glass transition? - General for several molecular glass formers - Seen also in colloidal models

Conclusions Inhomogeneity and spread of local viscosity Soft Glassy Rheology of supercooled glycerol and o-TP. General for glass formers? Relation to glacial phases and microscopic picture? Outlook: SM’s and nano-probing for soft matter studies

Rob Zondervan Meindert van Dijk Florian Kulzer Clemens Hofman n Aurélien Nicolet

Kramers theory for bond breaking Closed Broken F : applied force Effective viscosity (plastic deformation) K : spring constant of bond

according to model, the force on the spring is the effective viscosity decreases much slower at high forces (as 1/ F only) Applied force is reduced by liquid shear

Shear thinning

«… glassy dynamics is a natural consequence of two properties shared by all [ Foams, emulsions, pastes, slurries, quicksand, dense suspensions, …] soft materials: structural disorder and metastability. » P. Sollich et al., Phys. Rev. Lett. 78 (1997) 2020 Also supercooled liquids! Soft Glassy Rheology

Crossing the Melting Temperature

Slow warming (12K/hr). Melting around 291K – melting point of glycerol

-Friction: local pressure and temperature, third body,... - Adhesion: role of a soft layer in between two solids, probed with different dyes Solid-solid interaction with SM’s