liquid flows on surfaces:
DESCRIPTION
Liquid flows on surfaces:. the boundary condition. Nanoscale Interfacial Phenomena in Complex Fluids - May 19 - June 20 2008. The Kavli Institute of Theoretical Physics China. 500 nm. Microchannels…. …nanochannels. Downsizing flow devices raises new problems. - PowerPoint PPT PresentationTRANSCRIPT
Liquid flows on surfaces:the boundary condition
Nanoscale Interfacial Phenomena in Complex Fluids - May 19 - June 20 2008
The Kavli Institute of Theoretical Physics China
Pressure driving becomes insufficient
L
h
VPo
Po +P
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V = 1 mm/s, L=1cm, = 10-3 Pa.s
h = 0.1 m P = 100 bar
Downsizing flow devices
raises new problems500 nm
Microchannels… …nanochannels
New solutions are needed
Miniaturization increases surface to volume ratio:
importance of surface phenomena
The description of flows requires constitutive equation (bulk property of fluid) + boundary condition (surface property)
We saw that N.S. equation for simple liquids is very robust constitutive equation down to (some) molecular scale.
What about boundary condition ?
The no-slip boundary condition (bc): a long lasting empiricism regularly questionned
Theory of the h.b.c. for simple liquids
Some examples of importance of the b.c. in nanofluidics
Pressure drop in nanochannelsElektrokinetics effectsDispersion & mixing
Usual b.c. : the fluid velocity vanishes at wall
z
VS = 0
Hydrodynamic boundary condition (h.b.c.) at a solid-liquid interface
v(z)
OK at a macroscopic scale and for simple fluids
Phenomenological origin: derived from experiments on low molecular mass liquids
Goldstein 1938
Goldstein S. 1969. Fluid mechanics in the first half of this century. Annu. Rev. Fluid Mech 1:1–28
Batchelor, An introduction to fluid dynamics, 1967
The nature of hydrodynamics bc’s has been widely debated in 19th century
Lauga & al, in Handbook of Experimental Fluid Dynamics, 2005
M. Denn, 2001 Annu. Rev. Fluid Mech. 33:265–87
And alsoBulkley (1931),Chen & Emrich (1963), Debye & Cleland (1958)…
… and some time suspected on non-wetting surfaces
But wall slippage occurs in polymer flows…
Pudjijanto & Denn 1994 J. Rheol. 38:1735
Shark-skin effect in extrusion of polymer melts
C. Chan and R. Horn J. Chem. Phys. (83) 5311, 1985
mica
Ag
no-slip flow over a « trapped » monolayer various organic liquids / mica
Ag
J.N. IsraelachviliJ. Colloid Interf. Sci. (110) 263, 1986 Water on mica: no-slip within 2 Å
George et al., J. Chem. Phys. 1994
no-slip flow over « trapped » monolayervarious organic liquids/ metal surfaces
Drainage experiments with SFA
∆P
N.V. Churaev, V.D; Sobolev and A.NSomovJ. Colloid Interf. Sci. (97) 574, 1984
Water slips in hydrophobic capillariesslip length 70 nm
z
VS ≠ 0
v(z)
b
VS : slip velocity
S : tangential stress at the solid surface
b : slip length
: liquid-solid friction coefficient
: liquid viscosity
€
b =η
λ
Partial slip and solid-liquid friction
Navier 1823Maxwell 1856
∂V∂z
= : shear rate
Tangentiel stress at interface
€
S = η∂V
∂z= λ VS
€
VS = b∂V
∂z
Interpretation of the slip length
From Lauga & al, Handbook of Experimental Fluid Dynamics, 2005
b
The bc is an interface property. The slip length has not to be related to an internal scale in the fluid
The hydrodynamic b.c. is fully characterized by b()
The hydrodynamic bc is linear if the slip length does not depend on the shear rate.
On a mathematically smooth surface, b=∞ (perfect slip).
Some properties of the slip length
No-slip bc (b=0) is associated to very large liquid-solid friction
The no-slip boundary condition (bc): a long lasting empiricism regularly questionned
Theory of the h.b.c. for simple liquids
Some examples of importance of the b.c. in nanofluidics
Pressure drop in nanochannelsElektrokinetics effectsDispersion & mixing
Pressure drop in nanochannels
d
∆P
x
zb
Slit
r
Tube
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Exemple 1: slit d=1 µm
gain in flow rate : 12%Change from no-slip to b=20nm
Exemple 2: tube d= 2 nm
Change from no-slip to b=20nm gain in flow rate : 8000%
(2 order of magnitude)
B. Lefevre et al, J. Chem. Phys 120 4927 2004 Silanized MCM41of various radii (1.5 to 6 nm)
10nm
Forced imbibition of hydrophobic mesoporous medium
The intrusion-extrusion cycle of water in hydrophobic MCM41
mesoporous silica: MCM41
quasi-static cycle does not depend on frquency up to kHz
Exemple 3
L ~ 2-10 µm
Porous grain
Dispersion of transported species - Mixing
t=0 injection
d
time t
Taylor dispersion
Without molecular diffusion: QuickTime™ et un
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QuickTime™ et undécompresseur TIFF (non compressé)sont requis pour visionner cette image.Molecular diffusion spreads the solute through the width within
Solute motion is analogous to random walk: QuickTime™ et un
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With partial slip b.c.
t=0
d
time t
b
With partial slip b.c.
t=0
d
time t
b
Same channel, same flow rate
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Hydrodynamic dispersion is significantly reduced if b ≥ d
b = 0.15 d reduction factor 2b = 1.5 d reduction factor 10
Electric fieldelectroosmotic flow
Electrostatic double layernm 1 µm
Electrokinetic phenomena
Electro-osmosis, streaming potential… are determined by interfacialhydrodynamics at the scale of the Debye length
Colloid science, biology, …
The no-slip boundary condition (bc): a long lasting empiricism regularly questionned
Theory of the h.b.c. for simple liquids
Some examples of importance of the b.c. in nanofluidics
Pressure drop in nanochannelsElektrokinetics effectsDispersion & mixing
locally: perfect slip
Far field flow : no-slip
Effect of surface roughness
roughness « kills » slip
Richardson, J Fluid Mech 59 707 (1973), Janson, Phys. Fluid 1988
Fluid mechanics calculation :
Robbins (1990) Barrat, Bocquet (1994, 1999)Thomson-Troian (Nature 1997)
Slip at a microscopic scale : molecular dynamics on simple liquids
b
Thermodynamic equilibrium determination of b.c.with Molecular Dynamics simulations
Be j(r,t) the fluctuating momentum density at point r
Assume that it obeys Navier-Stokes equation
And assume Navier boundary condition
Bocquet & Barrat, Phys Rev E 49 3079 (1994)
Then take its <x,y> average
And auto-correlation function
b
C(z,z’,t) obeys a diffusion equation
with boundary condition
and initial value given bythermal equilibrium
2D density
C(z,z’,t) can be solved analytically and obtained as a function of b
b can be determined by ajusting analytical solution to datameasured in equilibrium Molecular Dynamics simulation
b
Green-Kubo relation for the hydrodynamic b.c.:
(assumes that momentum fluctuations in fluid obey Navier-Stokesequation + b.c. condition of Navier type)
Slip at a microscopic scale : linear response theory
Liquid-solid Friction coefficient total force exerted
by the solid on the liquid
canonicalequilibrium
Friction coefficient (i.e. slip length) can be computed at equilibrium fromtime decay of correlation function of momentum tranfer
Bocquet & Barrat, Phys Rev E 49 3079 (1994)
« soft sphere » liquid interaction potential (r) = (r)12 molecular size :
u
q = 2
u/ b/
00.01>0.03>0.03
∞400-2
very small surface corrugation isenough to suppress slip effects
Slip at a microscopic scale : molecular dynamics
Barrat, Bocquet, PRE (1994)
hard wall corrugation z=u cos qx
attractive wall
interaction potential z)= sf (1/z9-1/z3)
Strong wall-fluid attraction induces an immobile fluid layer at wall
sf =15
Effect of liquid-solid interaction
D
€
vαβ (r) = 4εσ
r
⎛
⎝ ⎜
⎞
⎠ ⎟
12
− cαβ
σ
r
⎛
⎝ ⎜
⎞
⎠ ⎟6 ⎡
⎣ ⎢
⎤
⎦ ⎥
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
= {fluid,solid}
Simple Lennard-Jones fluid with fluid-fluid and fluid-solid interactions
Barrat et al Farad. Disc. 112,119 1999
c parameter controls wettability
Wettability is characterized by contact angle (c.a.)
cFS=1.0 : =90°
cFS=0.5 : =140°
cFS=0 : =180°
Two types of flow
Here : =140°, P~7 MPaSlip length b=11 is found (both case)
Poiseuille flow
V(z)
z/
F0
b=0
Couette flow
V(z)
z/
U
b=0
Linear b.c. up to ~ 108 s-1
Slip at a microscopic scale: liquid-solid interaction effect
=140°
130°
=90°
b/
P/P0P0~MPa
substantial slips occurs on strongly non-wetting systems
slip length increases with c.a.
essentially no (small) slip in partial wetting systems ( < 90°)
slip length increases stronly as pressure decreases
Po ~ MPa
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Slip increases with reduced fluid density at wall.However slippage does not reduce to « air cushion » at wall.
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fluid density profile across the cell
Soft spheres on hard repulsive wall Lennard-Jones fluid = 137°
Slip at a microscopic scale: theory for simple liquids
Analytical expression for slip length
Depends only on structural parameters, no dynamic parameter
density at wall,depends onwetting properties
fluid struct.factorparallel to wall
wall corrugationa exp(q// • R//)
molecular size
//
Barrat et al Farad. Disc. 112,119 1999
Theory for intrinsic b.c. on smooth surfaces : summary
substantial slips in strongly non-wetting systems slip length increases with c.a. slip length decreases with increasing pressure
no-slip in wetting systems (except very high shear rate < 108 s-1 )
slip length is moderate (~ 5 nm at )
.
slip length does not depend on fluid viscosity (≠ polymers)
non-linear slip develops at high shear rate (~ 109 s-1 )
.
(obtained with LJ liquids, some with water)
1
10
100
1000
slip length (nm)
150100500
Contact angle (°)
Tretheway et Meinhart (PIV) Pit et al (FRAP) Churaev et al (perte de charge) Craig et al(AFM) Bonaccurso et al (AFM) Vinogradova et Yabukov (AFM) Sun et al (AFM) Chan et Horn (SFA)
Zhu et Granick (SFA) Baudry et al (SFA) Cottin-Bizonne et al (SFA)
Some experimental results….
MD Simulations
Non-linear slip
Brenner, Lauga, Stone 2005