Chain-like structures of colloidal particles, held together by van der Waals inter-particle forces, are being constructed. The interaction energy between the charged particles in these chains can be controlled and tuned by added salt, allowing chain-like structures with particle-particle interaction energies of 2-50 kT to be easily constructed. This is done by annealing two flat surfaces onto otherwise spherical polystyrene colloids. By adding the proper salt concentration, these charged colloids can be aggregated so that the two flats each associate with flats on other particles, in the secondary energy minimum between the particles, giving flexible polymer-like structures assembled from colloids with two flats. This allows study of the rheology of suspensions of these chains. The volume fraction of particles and the ionic strength will be adjusted in order to tune colloidal interactions. If the conditions that make colloidal polymers show rheopexy (aggregation under flow that we have observed in solutions of globular proteins and synovial fluids) can be identified, then their lubrication properties will be studied under conditions with and without rheopexy. This will be done in order to determine the connection between rheopexy and the superior lubrication in weakly structured fluids, thereby identifying the optimal interaction energy for lubrication.
Spherical charged colloids with two flats are a model system with both the net charge repulsion and chain-like aggregation character of globular proteins, but the colloids allow interactions to be tuned simply by changing ionic strength and are large enough to be seen in an optical microscope. Identifying the connections between interaction energy, rheology and lubrication will have far-reaching consequences on fundamental understanding in biology and pedantic design of synthetic lubricants. Do globular proteins in synovial fluid just happen to have the interactions necessary to make synovial fluid have time-dependent rheology (rheopexy) and allow synovial fluid to be a superior lubricant to anything we know how to synthesize? Can superior lubricants then be designed based on this new understanding?
By studying model systems of small particles, it is hoped to understand why the joint fluids of mammals are so much better lubricants than synthetic ones. There are practical implications for both understanding how joint fluids function and how better synthetic lubricants might be constructed. These research activities will be coupled with efforts to recruit Puerto Rican high school students into college and careers in science and engineering. Laura Mely RamÃrez, the Chemical Engineering Ph.D. student on this project, plans annual returns to her alma mater, Academia MarÃa Reina (AMR) High School for girls in San Juan, Puerto Rico, to talk with students there about the opportunities that science and engineering offer. The current high school dropout rate in Puerto Rico is 51%, and that can be improved upon by providing students with a good role model, in Miss. RamÃrez. AMR High School has yearly Career Days, and Miss. RamÃrez plans to visit the school and spend a day discussing with students and showing demonstrations of nanotechnology.
By changing one face of a spherical particle, scientists can get the particles to self-assemble into interesting structures. Our research project set out to do this in a new way, by flattening a spherical polymer particle on one side. That changes the interaction between two particles, such that under certain conditions they spontaneously self-assemble into flexible chains of particles. The particles are charged and hence all particles have a repulsive interaction from having the same charge. The magnitude of that repulsion is controlled by added salt in the surrounding water. More salt present lowers the repulsion. There is also an attraction between particles that depends on how close particles can get to each other. The flat side of the particle allows more attraction with another spherical particle than two spherical particles have, and this allows the flat-round interaction to be strong enough to form a flexible bond at a certain salt concentration, where the round-round interaction is weak. This means that at the right salt concentration, flexible colloidal chains can be created that are shown in the image. The graduate student, Laura Mely Ramirez, coined a clever name for these structures that are polymer chains assembled from colloidal particles; she dubbed them 'polloids'.
