Wetting When It Isn’t Simple ! P.S. Pershan, Harvard Univ.

(T>Tboiling) Van der Waals D Wetting When It Isn’t Simple!P.S. Pershan, Harvard Univ. Simple Wetting

D Three Different Experiments • 1) Casimir Effect: Critical Binary Liquid • Fisher and de Gennes (1978), Krech and Dietrich (1991, 1992) • Correlation Length: M. Fukuto,Y. Yana

  2) Structured SurfaceC. Rascon and A. O. Parry, Nature 407, 986 (2000). O. Gang, B.Ocko,, K.Alvine, T. Russell,M. Fukuto, C. Black

Dry Monolayer  Adsorption (Wetting Liquid) 3) Reconstructing Surface D. Pontoni, K. Alvine, A. Checco, O. Gang, B. Ockio, F. Stellacci Nanoparticles & Controlled Solvation Thiol Stabilized Au Particles(~ 2 to 8 nm)

Outer cell: 0.03C Inner cell: 0.001C Wetting film on Si(100) at T = Trsv+DTm. Saturated vapor Bulk liquid reservoir: atT = Trsv. Control of Film Thickness Vapor Pressure Thickness Delicate Control: P ~ T Van der Waals

X-Ray Reflectivity: Film Thickness

Comparisons • Via gravity • For h < 100 mm, • Dm < 10-5J/cm3 • L > ~500 Å •  small Dm, large L • Via temperature offset L (2Weff /Dm)1/3 (DTm)-1/3 Thickness L [Å] • Via pressure under-saturation • For DP/Psat > 1%, • Dm > 0.2 J/cm3 • L < 20 Å •  large Dm, small L DTm [K] Dm [J/cm3] Example of 1/3 Power Law Methyl cyclohexane (MC) on Si at 46 °C

47.7 °C MC rich PFMC rich Temperature [C] 46.2 °C 45.6 °C x (PFMC mole fraction) Critical Casimir Effect in NanoThick Liquids: Binary Liquid Fisher and de Gennes (1978), Krech and Dietrich (1991, 1992) Methylcyclohexane (MC) Perfluoro- methylcyclohexane (PFMC) [Heady & Cahn, J. Chem. Phys. 58, 896 (1973)] Tc = 46.13  0.01 °C, xc = 0.361  0.002

Experimental Paths T=(T-Tc)/Tc wetting film on Si(100) T = Trsv+DTm. Liquid Phase Vapor Phase Outer cell: 0.03C Inner cell: 0.001C Film-TRes 2 Phase Coexistence Bulk MC + PFMC reservoir: (x ~ xc = 0.36) atT = Trsv. Thermodynamics Same Experiment: Thickness of Absorbed Film Critical Point

x = 0.36 ~ xc D vs........ T-Tc R/RF DTm 0.020 K Tc = 46.2 °C Film thickness L [Å] qz [Å-1] 0.10 K 0.50 K Tfilm [°C] X-ray reflectivity & Film thickness D Paths

d = 2 Ising (exact) [R. Evans & J. Stecki, PRB 1994] d = 4 Ising (mean field) [M. Krech, PRE 1997] Excess free energy/area of a wetting film:  Casimir term “Force” or “pressure” balance: (+,-) (+,-) +,(y) / +,-(0) +,(y) (+, +) (+, +) y = (L/x+)1/n = t (L/x0+)1/n y = (L/x+)1/n = t (L/x0+)1/n Theory

d = 4 (MFT) Q+,-(y) Q+,-(0) DTm 0.020 K 0.10 K d = 2 (exact) y = (L/x+)1/n = t (L/x0+)1/n Experiment vs. Theory There is prediction for for 3D. Theory for y-dependence in d=3 does not exist!

Universal “Casimir amplitudes” • At bulk Tc (t = 0), scaling functions reduce to: D q(0) = Q(0)/(d – 1) For recent experiments with superfluid He (XY systems), see: R. Garcia & M.H.W. Chan, PRL 1999, 2002; T. Ueno et al., PRL 2003

∞ Adsorbed Liquid Long Channels Variety of Shapes ( Adsorption vs..... Shape: Phase Diagram 1/ Sculpted Surfaces Theory:Rascon & Parry, Nature (2000) Crossover Geometry to Planar Planar Geometry Dominated

Self Alignment on Si PMMA removal by UV degradation & Chemical Rinse Reactive Ion Etching C. Black (IBM) Parabolic Pits: Tom Russell (UMA) Diblock Copolymer in Solvent ~40 nm Spacing ~20 nm Depth/Diameter

Diffraction Pattern of Dry PitsHexagonal Packing Cross over to other filling! Thickness D~ X-ray Grazing Incidence Diffraction (GID)]In-plane surface structure Liquid Fills Pore: Scattering Decreases:

Filling GID Reflectivity Electron Density vs..... T Filling X-ray Measurement of Filling

R-P Predictionc~3.4 Uncertainties? Results for Sculpted Surface Sculpted is Thinner than Flat Flat Sample c

Volume of Liquid Filling Pores: p Volume of Liquid above Pores: t Film only coats Flat Part Area_Flat/Area Total: Tasinkevych & Dietrich

Controlled Wetting:Dry Monolayer  Adsorption Formation LangmuirIsotherms Reconstructing Surface:Gold Nanoparticles & Controlled Solvation Stellacci et al (MIT) OT: MPA (2:1)OT=CH3(CH2)7SHMPA=HOOC(CH2)2SH Liftoff Area Of Monolayer Bimodal Size Distribution of Particles

Adsorption GID Qz Qxy Qxy Qxy GID: X-ray vs. Liquid Adsorption(small particles) Return to Dry

Three FeaturesThat Can Be Understood! 1-Minimum at low qz 2-Principal Peak Reduces and Shifts Solid lines are just guides for the eye! 3-2nd Minima Moves to Lower qz Temperature Dependence of Reflectivity:

Construction of Model: Dry Sample Model Fit: Based on Particle Size Distribution Vertical electron density profile Core size distribution

3-Second Minima Moves to Lower qz 2-Principal Peak Reduces and Shifts 1-Minimum at low qz Fits of Physical Model

Thick wetting film regime Beginning of bilayer transition Thin wetting film regime Evolution of Model with Adsorption

Summary of Nano-particle experiments Bimodal Au nanocrystals in equilibrium with undersaturated vapor Poor vs..... Good Solvent Good Solvent Aggregation in Poor Solvent Reversible Dissolution in Good Solvent Self Assembly

Summary • Delicate Control of Wetting:  • Wetting of Critical Liquid (Casimir) M. Fukuto,Y. Yana • Wetting of Structured Surface (Rascon/Parry & Tasikevych/Dietrich)O. Gang, B.Ocko,, K.Alvine, T. Russell,M. Fukuto, C. Black • Nano-Particles: Self AssemblyD. Pontoni, K. Alvine, A. Checco, O. Gang, B. Ockio, F. Stellacci

Wetting When It Isn’t Simple ! P.S. Pershan, Harvard Univ.