Fluid Interface Atomic Force Microscopy (FI-AFM)

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

2. 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.

3. Dzc kc · Dzc = F F(S) S = ? Dzd kd(Dzd) · Dzd = F Oil Dz hydrophobic effects steric effects Interfacial tension effects Objectives for Deforming Interfaces Determine drop-sphere separation with theoretical modeling. Proper accounting of DLVO and hydrodynamic effects

4. Photodetector Optical objective He-Ne laser Glass walls Water Oil x-y-z Scanner AFM Experimental Design Direct interfacial force measurements with AFM. Prove AFM utility based on theoretical modeling. AFM F(z) Data Classic Force Profile F/R Force Displacement (mm) Separation (nm)

5. r z 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)

6. 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 > Po DP = Po Drop stiffness actually changes with deformation: • Weakens with attractive deformation. • Stiffens with repulsive deformation.

7. 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

8. Rd = 250 mm Rs = 10 mm A132 = 5 x 10-21 J = = -0.25 mC/cm2 |v| = 100 nm/s s = 52 mN/m Drop Stiffness Film Thickness As These Increase Drop radius, Rd Particle radius, Rs Approach velocity, |v| Interfacial tension, s Electrolyte conc. Surface charge, decreases increases increases increases ~constant ~constant constant increases increases decreases decreases increases Theoretical Oil Drop-Sphere Interactions Polysytrene/Hexadecane in Salt Solutions [NaNO3]

9. Rd = 250 mm Rs = 10 mm A132 = 5 x 10-21 J = = -0.32 mC/cm2 |v| = 120 nm/s s = 52 mN/m Oil-PS Experimental Profiles 0.1 mM NaNO3 Hydrophobic effect C1 = -2 mN/m l = 3 nm

10. Dynamic Interfacial Tension - SDS • Oil-water interfacial tension above the CMC for SDS decreases with continued deformation of the droplet. 6 mN/m Fit

11. 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? 0.1 M NaNO3 • What is the minimum adlayer condition for colloid stability? • Why does cationic starch seem not to inhibit air flotation?

12. 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.