State of stick-slip motion in confined films “A Liquid or not a liquid” Present student: I-Cheng Chen Date: Dec. 2, 2011 Course: CHEN 633

Motivation  Stick-slip motion is everyday phenomena: creaking door, brake sound, earthquakes…  In class, confined structures shows density distribution and viscosity elevation…. What’s happening with stick-slip motion in confined area?  Apply to computer hardware, miniature moter, and aerospace components, lubricants, low- friction surfaces…..

Friction  Normal friction occur with surface being damaged  Boundary friction (boundary lubrication) Occur restricted to thin region. Damage also occur during sliding.  Interfacial friction(interfacial sliding),1990 Molecularly thin region, uniform gap thickness and well defined contact area. No presence of damage

Surface Friction Theory  Amontons’ law ( presence of damage on surface ) F= u * L u: friction coefficient L: normal loads  Bowden and Taber ( adhesive contacts dominate ) F= Sc * A ; Sc: critical shear stress A: molecular contact area Dynamic condition Static condition Hertz and JKR theory(1971) Adhesive force between surfaces

Apparatus  Surface force apparatus (SFA) Taber, Winterton, Israelachvili developed 1969,1972 SCIENCE, VOL 240,1988

Material  Surface: Mica (discussed here)  Material between gap: dry air, vapor, organic compound, electrolyte solution…etc.

Shear properties of thin film  Experimentally a)effective viscosity rise 5-7 order than bulk value. b)Newtonian viscosity breaks down. c) Some Properties are quantized with the number of layers(layers ~<10)  Theoretical simulation a)Monte Carlo simulation with Lennard-Jones liquids in 6-10 molecular diameter filmsd

Sliding traces of liquid or solid For liquid(Couette flow) when sliding stopped F=K x ; S=F/A Main feature between Solid and liquid 1) Slope of solid curve 2) Decay after the sliding stopped 3) Magnitude of the shear stress (Solid>liqud)

Types of surface deformation Spherical molecules used as lubricant oil like cylcohexane and octamethylcyclotetrasiloxane(OMCTS) Octane and tetradecane as linear chain molecules Easy to observe layer transition Longer time needed to reach steady state liquid-like (d)  solid like(f) Spherical molecule did not exist liquid-like behavior in comparable thin films

Traces of friction force 1.∆F increase as number of layers falls; quantization property (for spherical) 2. With increasing velocity frequency increases ∆F falls until critical velocity reach ∆F*frequency=velocity Inset:

No stick-slip regime  Above critical velocity F is independent of v Spherical molecules: S is constant at different layers; S=F/A Chain molecules: S is proportional to loads S=C*L ; C is different from the well-known frictioncoefficient.C is related to the surface and liquid molecule property. Smoother surface with smaller C

Stick-slip regime Sensitive to sliding velocity and immediate previous sliding spherical molecule persisted to high velocity chain molecule is not ; shear induced Molecular ordering is sensitive to shear rate

Relaxation phenomena For chain molecule like tetradecane or 2-methyloctadecane, when the surface stop sliding, molecules in the gap starts to relax and change the configuration. Tetradecane C14H30 2-methyloctadecane C19H40

Regimes for relaxation conformation  Sliding regime v>vc; chain molecules most shear aligned  Resting regime molecules relaxes to a more solid-like state. Latency time: time needed for molecules to fully frozen.  Sticking regime When exceed latency time, solidlike structure reformed  Slipping regime reorder back to liquidlike

Latency time:1. branched chain molecule >chain > spherical 2. increased with applied load 3. no spikes when stop time is shorter than latency time Slip time vs. latency time (spring (film property) constant)

Relaxation behavior  Higher velocity lower the subsequent spike height (same pressure)   Latency time increases with pressure.

Effect of water Friction is low when covered with water on mica surfaces. Repulsion hydration forces between them. Humidity control is important in this kind of experiment

Effect of contaminants and impurity

Brief conclusion  Properties of molecularly thin film a) structure of the molecule b) structure and commensurability of the surface c) surface-liquid interaction potential d) pressure(load) between surfaces e) direction of shear f) shear rate g)history  Two confining surfaces are needed for a liquid to become solid-like

Cobblestone model Proposed by Taber 1978 To explain the two solid hydrocarbon surfaces sliding each in the absence of wear. Pushing a cart over a road of cobblestone. Cart wheel: liquid molecule Cobblestone: atoms of the surface So wheels have to roll before cart move. At rest(static), wheels find grooves to sit in potential energy minimum. Dynamic: Lateral force to raise the wheel and then move (adhesion)

Cobblestone model ∆d= move distance ; ∆ D=normal distance; F ad =adhesion force F= Shear force 2r s A= surface energy Typically 2r s is 5x10 -2 Jm -2, and if ∆d, ∆D ~1A. ɛ ~0.1 (10% surface energy needs to overcome) S=F/A=5X10 7 Nm -2  experimental value 2x10 7 Nm -2 for cyclohexane

Stick-Slip models  Surface topology model Surface topography and system mechanical property dominate in this model. Irregular stick-slip spike  Distance-dependent model  Rate and state model( velocity dependent model) mainly for lubricated surfaces

Distance dependent model  Propose at 1950s  Stuck surfaces creep over a characteristic distance,D 0,before friction drops  Often used in dry surfaces. May also apply to polymer surfaces

Velocity dependent model  Mainly for lubricated surfaces  Solid-like to liquid-like transition

Velocity dependent model  Underdamped regime( inertia-dominated) High spring constant and low mass, where mechanical response time is longer than slip time

Velocity dependent model  Overdamped regime( friction-dominated) low spring constant slip response time is longer than mechnical time, so we can observe double exponential decay

Starting spikes and stopping spikes  Two surfaces did not stop instantly There is some time v<vc, even very short, so the molecule starts to relax. (not seen in branched molecule because of long relaxation time.)

Take home conclusions  Stick-slip motion in confined area is like freezing- melting transition.  Molecular thin films can be in a state of neither solid nor liquid. Properties can be changed continuously from liquid-like to solid-like, unlike the bulk first order transition.( Two confining surfaces are needed)