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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
      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      kc · Dzc = F

                      S=?          F(S)

              Oil         Dzd      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
                                                   He-Ne                            Optical
                                                    laser                          objective


                                                                           Glass
                                                                           walls
                                                                Water


                                     Oil


                              x-y-z
                             Scanner


Prove AFM utility based on theoretical modeling.

         AFM F(z) Data                                               Classic Force Profile

                                             Approach (mN/m)                                                 Theory (mN/m)


                                                          -21
                             A = 9.5 x 10                       J                                                        -21
                                                                                                A = 9.5 x 10                   J
 Force




                                     121
                                                                    F/R




                                                                                                    121
                             y = y = -22 mV
                                     o1       o2                                               y = y = -22 mV
                                                                                                    o1        o2
                                 0.04 mM NaNO
                                                                3                                   0.04 mM NaNO
                                                                                                                               3
                                     |v| = 100 nm/s
                                                                                                    |v| = 100 nm/s
                                   -22/-22 mV, 0.04 mM                                            -22/-22 mV, 0.04 mM
                                 100 nm/s, k = 0.0085 N/m                                       100 nm/s, k = 0.0085 N/m
                                                            -2/3                                                           -2/3
                             k =0.0092 N/m, k =50 nm                                           k =0.0092 N/m, k =50 nm
                                 1                  2                                           1                  2

         Displacement (mm)                 k' = 0.00015
                                                                          Separation (nm)                 k' = 0.00015
Exact Solution for Droplet Deformation

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


      z(r )                  z(r )     
                                          gDz (r )  [ P  P ( D(r ))]
 
                                 (r ) 2 
                  3                                           o
  1  z(r )
              2       2     r 1 z         



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



             AFM probe                                          r
                                          F
                                                         z

                  P(z(r))                                     fluid
                                   Do             D(r)
                                                             medium
                                         Po
                                              k(r,z)
 Qualitative Sphere-Drop Interactions
Several properties affect drop profile evolution:

   1. Initial drop curvature
   2. Particle size
                                  Water
   3. Interfacial tension
                                              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   A 1  ro6 
             2  8                           Includes hard
                                                 wall repulsion
       R     6 D  D 

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

           
                   
        Fel 2 212 e kD  ( 1  2 )e 2kD
                                         2        2
                                                              
        R           ok (1  e 2kD )
          Moderately strong, asymmetric double-layer overlap


Hydrodynamic lubrication - Reynolds pseudo-steady state
drainage.

       FH    6 Reff dD *                    * Added functionality for
                       f                   varied boundary conditions
       R       D      dt


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
 As These Increase                  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



 Polysytrene/Hexadecane in Salt Solutions
   Rd = 250 mm                  A132 = 5 x 10-21 J           |v| = 100 nm/s
   Rs = 10 mm                  1=  2= -0.25 mC/cm2          = 52 mN/m
                          1
                     10
                                                   [NaNO3]
                          0
                     10                               0.01 mM
                                                      0.1 mM
        F/R (mN/m)




                                                      1 mM
                                                      10 mM                y = y = -18
                          -1                                                o1   o2
                     10                               100 mM          R = 10 mm, R =
                                                                       s              d

                          -2
                     10

                          -3
                     10
                           0   20     40      60    80          100
                                    Separation (nm)
               Oil-PS Experimental Profiles
Rd = 250 mm               A132 = 5 x 10-21 J              |v| = 120 nm/s
Rs = 10 mm               1=  2= -0.32 mC/cm2             = 52 mN/m

              0.3                 0
                             10

             0.25                              0.1 mM NaNO3
                              -1
                             10
                                                                                -18/-18 m
              0.2                                                                   k =0.0
                                                                                    1
F/R (mN/m)




                                                                                   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         100    120
                                      Separation (nm)

Hydrophobic effect

             Fh          D                           C1 = -2 mN/m
                 C1 exp    
             R                                          = 3 nm
              Dynamic Interfacial Tension - SDS
                                 5
                                                                0.1 mM                                  0.01 m
                                                                                                        48 mN
                                 4                                                                      0.1 m
                                                                                                        46 mN
                                                                                                        1 mM
                    F/R (mN/m)



                                 3           1 mM                 0.01 mM                               33 mN
                                                                                                        10 mM
                                                                                                        8 mN/
                                 2
                                                    10 mM                                          -3
                                                                                                 10 M NaN
                                 1
                                          |v| ~ 14 mm/s
                                  0                                                                |v| ~ 14
                                 -1500          -1000          -500         0         500        k = 0.008
                                                                                                  eff
                                                          Distance (nm)
• Oil-water interfacial tension above the CMC for SDS decreases
with continued deformation of the droplet.

               3
                                                                                                 Model 6 mN
                                                                                                 Run #1
              2.5                                                                                Run #2
                                                                                                 Run #3
                                                                                                 Run #4
               2
 F/R (mN/m)




              1.5                                         6 mN/m
                                                            Fit
               1                     -2
                                 10 M SDS                                                   -3
                                                                                       10 M NaNO
                                                                                                        3
              0.5                |v| ~ 14 mm/s

               0
               -400                   -200        0      200        400         600
                                                Distance (nm)
  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
                                      max ~ 3.4 mN/m                                                          1f-i
                                                                                                     Ps01/ 1f-i
                                                                                                        Ps01/          ps01f.clp
                                                                                                                                   ps01f.clp
                                                                     0.1                                               ps01g.clp

                                                                                                                       ps01h.clp
                                                       F/R (mN/m)




                            1.5
                                                                    0.05
                                                                                                                              NaNO
                                                                                                                        0.1 M ps01g.clp 3
                                                                                                                       ps01i.clp


                                                                      0
               F/R (mN/m)




                                                               -0.05
                                                                                                                                   ps01i.clp
                             1                                      -0.1
                                                                       200   250     300    350    400   450   500
                                                                                       Distance (nm)




                            0.5                                               k = 0.0104 N/m
                                                                                   eff
                                                                                         |v| ~ 6 mm/s
                             0

                                  0       100     200       300                                 400              500
                                                  Distance (nm)


• 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.

								
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