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 Flotation Oil 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 tension effects steric effects Oil k c ·  z c = F k d (  z d ) ·  z d = F zczc S = ? zdzd F(S) zz

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

fluid medium r z F  (z(r))  (r,z) PoPo D(r) DoDo 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.

Several properties affect drop profile evolution: Water Oil 1. Initial drop curvature 2. Particle size 3. Interfacial tension 4. Electrostatics 5. Approach velocity  P = P o  P > P o 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. Includes hard wall repulsion Empirical fit Moderately strong, asymmetric double-layer overlap Hydrodynamic lubrication - Reynolds pseudo-steady state drainage. * Added functionality for varied boundary conditions

Theoretical Oil Drop-Sphere Interactions Drop radius, R d Particle radius, R s Approach velocity, |v| Interfacial tension,  Electrolyte conc. Surface charge, decreases increases ~constant constant increases decreases increases As These Increase Drop StiffnessFilm Thickness [NaNO 3 ] Polysytrene/Hexadecane in Salt Solutions |v| = 100 nm/s  = 52 mN/m R d = 250  m R s = 10  m A 132 = 5 x J = =  C/cm 2

Oil-PS Experimental Profiles 0.1 mM NaNO 3 Hydrophobic effect C 1 = -2 mN/m = 3 nm |v| = 120 nm/s  = 52 mN/m R d = 250  m R s = 10  m A 132 = 5 x J = =  C/cm 2

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

Oil Drop with Cationic Starch Adlayers  P = P o  P < P o 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? Long-range attraction without wetting = depletion? 0.1 M NaNO 3

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