10. 3 Effective interaction Area of two spheres 10.3 Effective interaction Area of two spheres : The Langbein Approximation The effective area of interaction of a sphere with a surface = the circular zone centred at a distance–D from the surfaces( inside the sphere ) for n=6 The interaction of a sphere and a surface = the same as that of two planar surfaces at the same surface separation D

10.4 Interactions of large bodies compared to those between molecules For two macroscopic bodies, the interaction energy generally decays much more slowly with distance  the van der Waals energy between large condensed bodies decays is effectively of much longer range

10.4 Interactions of large bodies compared to those between molecules In contact b/n a small molecule and a wall In contact b/n a sphere of atomic dimensions In contact b/n two spheres Increasing of the size of a sphere above atomic dimensions

10. 5 Interaction Energy and Interaction 10.5 Interaction Energy and Interaction Forces : Derjagun Approximation [ Assumption ] Two large spheres of radii R1 and R2 R1>>D and R2>>D By integrating the force between small circular regions of area on one surface Surface to be locally flat

10. 5 Interaction Energy and Interaction 10.5 Interaction Energy and Interaction Forces : Derjagun Approximation The force b.t.n two spheres is expressed in terms of the energy per unit area of two flat surfaces at the same separation D The distance dependence of the force b.t.n two curved surfaces can be quite different from that b.t.n two surfaces even though the same type of force is operating in both.

Fig. 10. 4 Force laws betweem two curved Fig. 10. 4 Force laws betweem two curved surfaces and two flat surfaces Useful theoretical tool to derive

16.1 Indirect access for W(D) Thermodynamic data on gases, liquids and solids Physical data on gases, liquids and solids Thermodynamic data on liquids and liquid mixtures PVT data, B.P Latent heats of vaporization lattice energy Viscosity, diffusion. Compressibility, NMR X-ray, molecular beam scattering experiment Phase diagrams solubility Partitioning, miscibility osmotic pressure Short-range attractive potentials b.t.n molecules Short-range interactions of molecules, especially repulsive forces giving molecular size, shape and structural role in condensed phase Short-range silute-solvent and solute-solute interactions Useful theoretical tool to derive

16.2 Direct access for W(D) Useful theoretical tool to derive

Practical Applications 16.2 Direct access for W(D) Types Practical Applications Information Adhesion measurement Xerography, particle adhesion. Powder technology, ceramic processing Particle adhesion forces and the adhesion energies of solid surfaces in contact ( attractive short-range forces) Peeling measurement Adhesive tapes, material fracture and crack propagation Force-measuring spring or balance Testing theories of intermolecular forces The force macroscopic surfaces as a function of surface separation The full force law of an interaction Useful theoretical tool to derive

Practical Applications 16.2 Direct access for W(D) Types Practical Applications Effects Contact Angle Testing wettability and stability of surface films, foams Liquid-Liquid or Liquid-Solid adhesion energy Information of states and adsorbed films, and of molecular reorientation time at interfaces Equilibrium thickness of thin free films Soap films, foams A function of salt conc. or vapour pressure The long-range repulsive forces stabilizing thick wetting films Equilibrium thickness of thin absorbed films Wetting of hydrophilic surface by water, adsorption of molecules from vapor, protective surface coatings and lubricant layers, photographic films Useful theoretical tool to derive

Practical Applications 16.2 Direct access for W(D) Types Practical Applications Effects Interparticle spacing in liquids Colloidal suspensions, paints, pharmaceutical dispersions The interparticle forces By changing the solution conditions and their mean separation By changing the quantity of solvents Limits to measure only the repulsive parts Sheet-like particle spacing in liquids Clay and soil swelling behavior, microstructure of soaps and biological membranes Coagulation studies Basic experimental technique for testing the stability of colloidal preparations Information on the interplay of repulsive and attractive forces between particles in pure, sulfactant and polymer solutions Useful theoretical tool to derive

10.7 Direct measurements of Surface and Intermolecular Forces The most unambiguous way to measure a force-law  to position two bodies close together and directly measure the force between them  very straightforward  very weak challenge coming at very small intermolecular interaction  surface separation controlled and measured to within 0.1nm Useful theoretical tool to derive

The Surfaces Forces Apparatus (SFA) Measuring surface forces in controlled vapor or immersed in liquids is directly measured using a variety of interchangeable force-measuring springs Both repulsive and attractive forces are measuring and a full force law can be obtained over any distance regimes Useful theoretical tool to derive

The Surfaces Forces Apparatus (SFA) The distance resolution about 0.1nm (angstrom level) the force sensitivity about 10-8 N In Surfaces Two curved molecularly smooth surfaces of mica in a crossed cylinder configuration The separation is measured by use of an optical technique using multiple beam interference fringes The distance is controlled by use of a three-stage mechanism of increasing sensitivity ( the coarse control 1µm – the medium control 1nm – a piezoelectric crystal tube 0.1nm ) Useful theoretical tool to derive

The Surfaces Forces Apparatus (SFA) The force measurement The force A measured by expanding or contracting the piezoelectric crystal by a known amount The force B measured by optically how much the two surfaces have actually moved The difference of force b.t.n two positions = [ Force A – force B ] * the stiffness of the force-measuring spring Useful theoretical tool to derive

The Surfaces Forces Apparatus (SFA) The force and the interfacial energy The force b.t.n two curved surfaces scale = R The adhesion or interfacial energy E per unit area two flat surfaces by the Derjaguin approximation For given R and sensitivity F, getting E ( an interfacial energy) Useful theoretical tool to derive

The Surfaces Forces Apparatus (SFA) The use of SFA Identifying and quantifying most of fundundamental interactions occuring between surfaces on both aqueous solutions and nonaqueous liquids Including the attractive van der Waaals and repulsive electrostatic ‘double –layer’ forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems and capillary and adhesion The extension of measurement into dynamic interaction and time-dependent effects and the fusion of lipid bilayers . etc Useful theoretical tool to derive

Total Internal Reflection Microscopy(TIRM) Measuring minute forces( <10-15 N) between a colloidal particle and a surface Measuring the distance between an individual colloidal particle of diameter ~10 µm hovering over a surface. A laser beam is directed at the particle through the surface made of transparent glass From the intensity of reflected beam deducing the equilibrium separation D0 Providing data on interparticle interactions under conditions closely paralleling those occurring in colloidal systems Useful theoretical tool to derive

The Atomic Force Microscope(AFM) Measuring atomic adhesion forces (10-9~10-10 N) between a fine molecular-sized tip and a surface ( 1µm < Tip radii < atom size ) At finite distances, using very sensitive force-measuring springs (spring stiffness=0.5 Nm-1) and very sensitive ways for measuring the displacement (0.01nm) very short-range forces , but not longer range forces Interpreting the results is not always straightforward and exact due to the tip geometry Useful theoretical tool to derive