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)

Quantify the influence of non-DLVO forces on colloidal behavior:

1. Hydrophobic attraction

2. Hydrodynamic repulsion

3. Steric, depletion, etc.

Gain knowledge about oil agglomeration and air flotation through studies of single particle/oil-drop interactions.

Air FlotationOil Agglomeration

Colloidal AFM

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.

Proper accounting of DLVO and hydrodynamic

effects

hydrophobic effects

Interfacial tensioneffects

steric effects

Oil

kc · zc = F

kd(zd) · zd = F

zc

S = ?

zd

F(S)

z

Theory (mN/m)

-22/-22 mV, 0.04 mM100 nm/s, k = 0.0085 N/m

k1=0.0092 N/m, k

2=50 nm-2/3

k' = 0.00015

A121

= 9.5 x 10-21 J

=

= -22 mV

0.04 mM NaNO3

|v| = 100 nm/s

Approach (mN/m)

-22/-22 mV, 0.04 mM100 nm/s, k = 0.0085 N/m

k1=0.0092 N/m, k

2=50 nm-2/3

k' = 0.00015

A121

= 9.5 x 10-21 J

=

= -22 mV

0.04 mM NaNO3

|v| = 100 nm/s

AFM Experimental Design

Direct interfacial force measurements with AFM.

Optical objective

Photodetector

Oil

Water

x-y-zScanner

He-Ne laser

Glass walls

Prove AFM utility based on theoretical modeling.

Classic Force ProfileAFM F(z) Data

Displacement (m) Separation (nm)

F/R

For

ce

fluidmedium

r

zF

(z(r))

(r,z)

Po

D(r)Do

AFM probe

Exact Solution for Droplet Deformation

The relationship between drop deflection and force is not fit by a single function.

Drop profile calculated from augmented Young-Laplace equation: includes surface and body forces.

))](([)(

)(1

)(

)(1

)(22

32

rDPrzgrzr

rz

rz

rzo

Several properties affect drop profile evolution:

Water

Oil

1. Initial drop curvature

2. Particle size

3. Interfacial tension

4. Electrostatics

5. Approach velocity

P = PoP > Po

Qualitative Sphere-Drop Interactions

Liquid interface can become unstable to attraction.

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.

Electrostatic double-layer - often longer-ranged than dispersion forces.

Hydrophobic effect - observed attraction unexplained by DLVO theory or an additional, singular mechanism.

8

6

2

1

6 D

r

D

A

R

F ovdW

D

CR

Fh exp1

Includes hardwall repulsion

Empirical fit

)1(

)(222

222

Do

DDel

e

ee

R

F

Moderately strong, asymmetric double-layer overlap

Hydrodynamic lubrication - Reynolds pseudo-steady state drainage.

*6f

dt

dD

D

R

R

F effH

* Added functionality for

varied boundary conditions

10-3

10-2

10-1

100

101

0 20 40 60 80 100

0.01 mM0.1 mM1 mM10 mM100 mM

F/R

(m

N/m

)

Separation (nm)

=

= -18 mV

Rs = 10 m, R

d = 250 m

Theoretical Oil Drop-Sphere Interactions

Drop radius, Rd

Particle radius, Rs

Approach velocity, |v|

Interfacial tension,

Electrolyte conc.

Surface charge,

decreases

increases

increases

increases

~constant

~constant

constant

increases

increases

decreases

decreases

increases

As These Increase Drop Stiffness Film Thickness

[NaNO3]

Polysytrene/Hexadecane in Salt Solutions

|v| = 100 nm/s = 52 mN/m

Rd = 250 m

Rs = 10 m

A132 = 5 x 10-21 J

= = -0.25 C/cm2

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100 120

Approach (mN/m)0.1 mM

F/R

(m

N/m

)

Separation (nm)

-18/-18 mV, 4x10-5 Mk

1=0.0094 N/m

k2= 0.003 nm-1

-22/-22 mV, 0.04 mM100 nm/s, k = 0.0085 N/m

k1=0.0092 N/m, k

2=50 nm-2/3

k' = 0.00015

10-3

10-2

10-1

100

0 20 40 60 80 100 120

Approach (mN/m)0.1 mM

-18/-18 mV, 4x10-5 Mk

1=0.0094 N/m

k2= 0.003 nm-1

-22/-22 mV, 0.04 mM100 nm/s, k = 0.0085 N/m

k1=0.0092 N/m, k

2=50 nm-2/3

k' = 0.00015

Oil-PS Experimental Profiles

0.1 mM NaNO3

D

CR

Fh exp1

Hydrophobic effect

C1 = -2 mN/m

= 3 nm

|v| = 120 nm/s = 52 mN/m

Rd = 250 m

Rs = 10 m

A132 = 5 x 10-21 J

= = -0.32 C/cm2

0

0.5

1

1.5

2

2.5

3

-400 -200 0 200 400 600

Model 6 mN/mRun #1Run #2Run #3Run #4

F/R

(m

N/m

)

Distance (nm)

|v| ~ 14 m/s

10-2 M SDS 10-3 M NaNO3

Dynamic Interfacial Tension - SDS

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

6 mN/mFit

0

1

2

3

4

5

-1500 -1000 -500 0 500

0.01 mM SDS48 mN/m0.1 mM SDS46 mN/m1 mM SDS33 mN/m10 mM SDS8 mN/m

F/R

(m

N/m

)

Distance (nm)

|v| ~ 14 m/sk

eff = 0.0087 N/m

10 mM10-3 M NaNO

3

0.01 mM

0.1 mM

1 mM

|v| ~ 14 m/s

Oil Drop with Cationic Starch Adlayers

P = PoP < Po

• Cationic starch electrosterically stabilizes against wetting.

• Even at high salt, steric hindrance alone maintains stability.

• What is the minimum adlayer condition for colloid stability?

• Why does cationic starch seem not to inhibit air flotation?

0

0.5

1

1.5

2

0 100 200 300 400 500

ps01f.clp

ps01g.clp

ps01i.clp

F/R

(m

N/m

)

Distance (nm)

Ps01/ 1f-imax ~ 3.4 mN/m

keff

= 0.0104 N/m

|v| ~ 6 m/s

-0.1

-0.05

0

0.05

0.1

0.15

200 250 300 350 400 450 500

ps01f.clp

ps01g.clp

ps01h.clp

ps01i.clp

F/R

(m

N/m

)

Distance (nm)

Ps01/ 1f-i

Long-range attraction without wetting = depletion?

0.1 M NaNO3

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.

• FI-AFM greatly expands our ability to explore fluid interfaces on an ideal scale.

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