The objective of this work is systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance is to understand the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we are seeking to develop new methods to control and manipulate the properties of their interfacial films.
Specifically, the dynamic mechanical properties of the resulting interfacial film are being studied directly with special emphasis on how they depend on both vibration frequency and strain rate. A homebuilt apparatus is employed to this purpose with the following unique properties:
• Surface-surface spacing are variable from thousands of Ångstroms to molecular contact. The surface force needed to produce this separation is measured while at the same time measuring shear nanorheology. The tip is imaged in situ directly during each experiment, therefore force can be normalized by area to produce stress.
• Oscillatory deformations with variable frequency in the range 0.01 to 103 rad-sec-1 can be applied, with deformations either in the shear direction or in the normal (pumping) direction.
• The amplitude of deformation can be varied from sub-Ångstrom to microns. The reason to use very small deformations is to produce a linear viscoelastic response (representative of the rest state, because this can be studied by methods of equilibrium statistical thermodynamics). The reason to use very large deformations is to produce strongly nonlinear deformations characteristic of very high shear rates.
To the best of our knowledge, no other instrument with these properties exists in any other laboratory in the world. We would like to take this opportunity to encourage IFPRI members to suggest new systems that would be interesting to study using these unique methods. The main finding during Year I was to develop criteria with predictive power to understand whether opposed particles will move past one another with intermittent stick- slip motion or with smooth sliding. We found that stick-slip motion occurred only when 2 thin films were deformed faster than their intrinsic relaxation time. The observation offered a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film. The main findings during Years II and III concerned methods to control suspension rheology with surface coatings. First, we found that because the interparticle potential is an equilibrium quantity, whereas nanorheological responses depend on rate- dependent processes, the interparticle potential failed to correlate directly with shear nanorheology when dealing with films that showed a viscoelastic response. We initiated studies concerning forces in a “tapping” mode – to hydrodynamics when particle surfaces come together and are pulled apart with fluid media in between. These studies focused on nonaqueous systems and some preliminary studies were undertaken in aqueous media. During Year IV, studies were initiated concerning the boundary condition, ‘no-slip’ or ‘partia lslip’ of continuum hydrodynamics. Preliminary findings indicated massive breakdown of the conventional ‘no-slip’ assumption, but those conclusions were tempered by the fact that the experiments at that point had concerned only surfaces that were atomically smooth.