Fluid Interface Atomic Force Microscopy (FI-AFM)

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Fluid Interface Atomic Force Microscopy(FI-AFM)
D. Eric Aston Prof. John C. Berg,
Advisor Department of Chemical
Engineering University of Washington
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Fluid Interface AFM (FI-AFM)
Gain knowledge about oil agglomeration and air
flotation through studies of single
particle/oil-drop interactions.
Air Flotation
Oil Agglomeration
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.
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Objectives for Deforming Interfaces
Determine drop-sphere separation with theoretical
modeling.
Proper accounting of DLVO and hydrodynamic effects
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AFM Experimental Design
Direct interfacial force measurements with AFM.
Prove AFM utility based on theoretical modeling.
Classic Force Profile
AFM F(z) Data
F/R
Force
Displacement (mm)
Separation (nm)
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Exact Solution for Droplet Deformation
Drop profile calculated from augmented
Young-Laplace equation includes surface and
body forces.
The relationship between drop deflection and
force is not fit by a single function.
AFM probe
F
fluid medium
Do
P(z(r))
D(r)
Po
k(r,z)
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Qualitative Sphere-Drop Interactions
Several properties affect drop profile evolution
1. Initial drop curvature 2. Particle size 3.
Interfacial tension 4. Electrostatics 5.
Approach velocity
Water
Oil
Liquid interface can become unstable to
attraction.
DP gt Po
DP Po
Drop stiffness actually changes with deformation

• Weakens with attractive deformation.
• Stiffens with repulsive deformation.

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Long-Range Interactions in Liquids
van der Waals interaction - usually long-range
attraction.
Includes hard wall repulsion
Electrostatic double-layer - often longer-ranged
than dispersion forces.
Moderately strong, asymmetric double-layer overlap
Hydrodynamic lubrication - Reynolds pseudo-steady
state drainage.
Added functionality for varied boundary
conditions
Hydrophobic effect - observed attraction
unexplained by DLVO theory or an additional,
singular mechanism.
Empirical fit
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Theoretical Oil Drop-Sphere Interactions
Polysytrene/Hexadecane in Salt Solutions
NaNO3
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Oil-PS Experimental Profiles
0.1 mM NaNO3
Hydrophobic effect
C1 -2 mN/m l 3 nm
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Dynamic Interfacial Tension - SDS

• Oil-water interfacial tension above the CMC for
  SDS decreases with continued deformation of the
  droplet.

6 mN/m Fit
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Oil Drop with Cationic Starch Adlayers

• Cationic starch electrosterically stabilizes
  against wetting.
• Even at high salt, steric hindrance alone
  maintains stability.

DP lt Po
DP Po
Long-range attraction without wetting depletion?
0.1 M NaNO3

• What is the minimum adlayer condition for
  colloid stability?
• Why does cationic starch seem not to inhibit air
  flotation?

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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.