Transcript Fluid Interface Atomic Force Microscopy (FI-AFM) D. Eric Aston Prof. John C. Berg, Advisor Department of Chemical Engineering University of Washington.

Fluid Interface
Atomic Force Microscopy
(FI-AFM)
D. Eric Aston
Prof. John C. Berg, Advisor
Department of Chemical Engineering
University of Washington
Fluid Interface AFM (FI-AFM)
Gain knowledge about oil agglomeration and air flotation
through studies of single particle/oil-drop interactions.
Oil Agglomeration
Air Flotation
Quantify the influence of non-DLVO forces on colloidal
behavior:
Colloidal AFM
1. Hydrophobic attraction
2. Hydrodynamic repulsion
3. Steric, depletion, etc.
Ultimately, standardize an analytical technique for colloidal
studies of fluid-fluid interfaces with AFM.
Objectives for Deforming Interfaces
Determine drop-sphere separation with theoretical modeling.
Dzc
S=?
Dzd
Oil
kc · Dzc = F
F(S)
kd(Dzd) · Dzd = F
Dz
Proper accounting of
DLVO and hydrodynamic
effects
hydrophobic effects
steric effects
Interfacial tension
effects
AFM Experimental Design
Direct interfacial force measurements with AFM.
Photodetector
Optical
objective
He-Ne
laser
Water
Glass
walls
Oil
x-y-z
Scanner
Prove AFM utility based on theoretical modeling.
AFM F(z) Data
Classic Force Profile
Approach (mN/m)
-21
121
J
y = y = -22 mV
o1
o2
0.04 mM NaNO
A = 9.5 x 10
F/R
Force
A = 9.5 x 10
Theory (mN/m)
o1
-22/-22 mV, 0.04 mM
100 nm/s, k = 0.0085 N/m
-2/3
k =0.0092 N/m, k =50 nm
k =0.0092 N/m, k =50 nm
Displacement (mm)
3
|v| = 100 nm/s
-2/3
2
o2
0.04 mM NaNO
3
-22/-22 mV, 0.04 mM
100 nm/s, k = 0.0085 N/m
k' = 0.00015
J
y = y = -22 mV
|v| = 100 nm/s
1
-21
121
Separation (nm)
1
2
k' = 0.00015
Exact Solution for Droplet Deformation
Drop profile calculated from augmented Young-Laplace
equation: includes surface and body forces.

z(r )


2

1

z
(
r
)



3

  gDz (r )  [ P  P( D(r ))]

o
r 1  z(r ) 2 
z(r )
2
The relationship between drop deflection and force is not
fit by a single function.
AFM probe
r
F
z
P(z(r))
D(r)
Do
Po
k(r,z)
fluid
medium
Qualitative Sphere-Drop Interactions
Several properties affect drop profile evolution:
1. Initial drop curvature
2. Particle size
3. Interfacial tension
Water
Oil
4. Electrostatics
5. Approach velocity
Liquid interface can become unstable to attraction.
DP = Po
DP > Po
Drop stiffness actually changes with deformation:
• Weakens with attractive deformation.
• Stiffens with repulsive deformation.
Long-Range Interactions in Liquids
van der Waals interaction - usually long-range attraction.
FvdW
ro6 
A 1
  2  8
R
6 D
D 
Includes hard
wall repulsion
Electrostatic double-layer - often longer-ranged than dispersion
forces.

Fel 2 212 e kD  (1  2 )e2kD

R
 ok (1  e2kD )
2
2

Moderately strong, asymmetric double-layer overlap
Hydrodynamic lubrication - Reynolds pseudo-steady state
drainage.
6Reff dD *
FH

f
R
D
dt
* Added functionality for
varied boundary conditions
Hydrophobic effect - observed attraction unexplained by
DLVO theory or an additional, singular mechanism.
Fh
D
 C1 exp

R



Empirical fit
Theoretical Oil Drop-Sphere Interactions
Drop Stiffness
Film Thickness
Drop radius, Rd
decreases
constant
Particle radius, Rs
increases
increases
Approach velocity, |v|
increases
increases
Interfacial tension, 
increases
decreases
Electrolyte conc.
~constant
decreases
Surface charge, 1 2
~constant
increases
As These Increase
Polysytrene/Hexadecane in Salt Solutions
Rd = 250 mm
Rs = 10 mm
A132 = 5 x 10-21 J
1= 2= -0.25 mC/cm2
|v| = 100 nm/s
 = 52 mN/m
1
10
[NaNO3]
0
F/R (mN/m)
10
10
0.01 mM
0.1 mM
1 mM
10 mM
100 mM
-1
y = y = -18
o1
R = 10 mm, R = 2
s
10
-2
10
-3
0
20
40
60
80
Separation (nm)
o2
100
d
Oil-PS Experimental Profiles
Rd = 250 mm
Rs = 10 mm
A132 = 5 x 10-21 J
1= 2= -0.32 mC/cm2
0.3
|v| = 120 nm/s
 = 52 mN/m
0
10
0.1 mM NaNO3
0.25
-1
F/R (mN/m)
10
-18/-18 m
k =0.
0.2
1
k =0
2
-2
10
0.15
-22/-22 m
100 nm/s, k
k =0.0092 N/
1
0.1
-3
10
k' =
0
20
40
60
80
100
120
1
0.05
k
1
0
0
20
40
60
80
Separation (nm)
100
120
Hydrophobic effect
Fh
D
 C1 exp

R
  
C1 = -2 mN/m
 = 3 nm
Dynamic Interfacial Tension - SDS
5
0.1 mM
0.01 m
48 mN
0.1 m
46 mN
1 mM
33 mN
10 mM
8 mN/
F/R (mN/m)
4
3
1 mM
0.01 mM
2
10 mM
-3
10 M NaN
1
|v| ~ 14 mm/s
0
-1500
-1000
-500
Distance (nm)
0
|v| ~ 14
k = 0.008
500
eff
• Oil-water interfacial tension above the CMC for SDS decreases
with continued deformation of the droplet.
3
Model 6 mN
Run #1
Run #2
Run #3
Run #4
F/R (mN/m)
2.5
2
1.5
1
0.5
0
-400
6 mN/m
Fit
-2
-3
10 M SDS
|v| ~ 14 mm/s
-200
0
200
Distance (nm)
10 M NaNO
3
400
600
Oil Drop with Cationic Starch Adlayers
• Cationic starch electrosterically stabilizes against wetting.
• Even at high salt, steric hindrance alone maintains stability.
DP < Po
DP = Po
Long-range attraction without wetting = depletion?
2
0.15
Ps01/ 1f-i
Ps01/
1f-i
max ~ 3.4 mN/m
ps01f.clp
ps01f.clp
ps01g.clp
0.1
F/R (mN/m)
ps01h.clp
F/R (mN/m)
1.5
0.1 M ps01g.clp
NaNO3
0.05
ps01i.clp
0
-0.05
1
-0.1
200
0.5
ps01i.clp
250
300
350
400
Distance (nm)
450
500
k = 0.0104 N/m
eff
|v| ~ 6 mm/s
0
0
100
200
300
Distance (nm)
400
500
• What is the minimum adlayer condition for colloid stability?
• Why does cationic starch seem not to inhibit air flotation?
Conclusions
• Expectation of a dominant hydrophobic interaction
is premature without thorough consideration of the
deforming interface.
• Several system parameters are key for interpreting
fluid interfacial phenomena, all affecting drop
deformation.
1. Surface forces - DLVO, hydrophobic, etc.
2. Drop and particle size - geometry of film drainage
3. Interfacial tension - promotion of film drainage
4. Approach velocity - resistance to film drainage
• FI-AFM greatly expands our ability to explore fluid
interfaces on an ideal scale.