LANDSCAPE AND MEMORY LANDSCAPE AND MEMORY 1 Nonequilibrium

LANDSCAPE AND MEMORY

LANDSCAPE AND MEMORY 1. Nonequilibrium soft matter physics 2. Lecithin/bile-salt mixtures 3. Colloidal arrest in binary solvents 4. The vision thing! coworkers: W. Poon, S. Egelhaaf, J. Leng, K. Stratford, I. Pagonabarraga R. Adhikari funding: EPSRC Programme Grant + e. Science Testbed Project

0. FOR THE TEN-YEAR OLDS • Soft matter = gloop • Physics question: why does gloop? • How do the molecules or other bits inside interact to make it gloopy? • How do these interactions control when flow starts and stops? • Here are some ways: polymers: colloids: sand: molecular spaghetti molecular ping-pong balls just sand!

1. NONEQUILIBRIUM SOFT MATTER • Polymers, colloids, amphiphiles: chains, balls, bilayers nanoscale or mesoscale structure (5 nm – 50 mm) • Low free energy density: k. BT per soft degree of freedom perturbed by weak forcing (shear flow, T– quench etc. ) • Brownian motion strives to reach equilibrium state barriers exist: arrest, jamming, glassiness • Free energy landscape can be simple (one barrier) or complex (many) • Not one glass but an ever-growing zoo of different types

Surfactant Aggregation preferred local geometries sphere rod , small disc curvature tail/head size ratio flat bilayer

2. LECITHIN/BILE-SALT MIXTURES Physics: preferred curvature controlled by ratio lecithin: bile salt change this ratio by sudden dilution (“dialysis”) aggregates undergo shape + size transformation sphere disc with rim Physiology: digestion, lipid metabolism vesicle

2. LECITHIN/BILE-SALT MIXTURES Longstanding controversy: large end-state vesicles of pure lecithin observed. . trapped metastable states or equilibrium? Our answer: definitely metastable, despite indefinite lifetime How do we know? their size depends on bile salt properties!

Free energy landscape for disc closure V=r /4 E/Eves r: disc radius : rim tension : bilayer rigidity “reaction coordinate”

Kinetic pathway from disks to vesicles small discs grow by aggregation then drives closure of large discs end-state vesicle size R depends on bile salt, p. H, [Na. Cl] etc varying these away from “realistic” levels exposes the mechanism! experiments + quantitative model: J. Leng, S. Egelhaaf, MEC, Biophys J 2003

3. COLLOIDAL ARREST IN BINARY SOLVENTS Contact angle Neutral wetting: = π/2 solid-liquid tensions equal Energy gain πσa 2 k. BT colloids stick to interface π/2 ∟ COLLOIDS = IRREVERSIBLE SURFACTANTS!

DEMIXING OF SYMMETRIC BINARY FLUIDS local free energy landscape F( , ) = a 2 + b 4 + k( )2 a < 0 in immiscible region A/B repulsion F 2 -1 = proportion of species A = order parameter SPINODAL DECOMPOSITION

late crossover, Re =50 viscous hydrodynamic, Re = 0. 5

spinodal decomposition under shear © University of Edinburgh 2000

DEMIXING: GLOBAL FREE ENERGY LANDSCAPES without colloidal particles: complex but always downhill complete separation eventually “zero temperature fixed point” with colloidal particles: glassy landscape, many traps? arrested separation? strong memory effects?

LAB REALIZATIONS? P. Clegg, W. Poon, S. Egelhaaf & MEC WATCH THIS SPACE!! COMPUTER SIMULATIONS K. Stratford, I. Pagonabarraga, R. Adhikari & MEC Will be published soon. . .

4. SOFT MATTER: SCIENCE VISION • Beyond equilibrium self assembly: using controlled forcing (shear, quench) to steer through a landscape • Arrested end-states more robust than equilibrium: memory effects can ride out changes in the here and now • To control end-state “products” we must understand free energy landscapes: not just the free energy minima that define equilibrium • “Combinatorial physics”: glasses, plus phase separation, plus shear, plus fields, plus. . HOW DO WE AVOID GETTING LOST IN THIS IMMENSE SPACE?

4. THE B–WORD AND THE N–WORD • Soft matter physics can contribute to biology, and already does • Soft matter physics can contribute to nanotechnology, and already does • The central problems of soft matter involve nonequilibrium and forcing in systems with complicated free-energy landscapes • Soft matter will lose its relevance to biology, nanotechnology, and all other fields, if these central problems are not rigorously addressed THEREFORE • We should not all have to constantly invoke bio– and/or nano–relevance • We cannot apply this new physics without understanding it first