Controlling Rheology by Tuning Colloidal Interactions

由范德华粒子间力维系在一起的胶态粒子链状结构正在被构建。这些链中带电粒子之间的相互作用能可以通过添加盐来控制和调节，从而可以很容易地构建出粒子-粒子相互作用能在2-50 kT之间的链状结构。这是通过将两个平面退火到球形聚苯乙烯胶体上来完成的。通过添加适当的盐浓度，这些带电的胶体可以聚集在一起，使得两个平面与其他粒子上的平面结合，在粒子之间的次级能量最小，形成由两个平面的胶体组装而成的柔性聚合物样结构。这使得研究这些链的悬浮液的流变学成为可能。粒子的体积分数和离子强度将被调整，以调整胶体的相互作用。如果可以确定使胶体聚合物表现出流变性（我们在球状蛋白和滑液溶液中观察到的流动下聚集）的条件，那么将在有和没有流变性的条件下研究它们的润滑性能。这样做是为了确定流变性与弱结构流体中优越的润滑之间的联系，从而确定润滑的最佳相互作用能。具有两个平面的球形带电胶体是一个具有净电荷排斥和球状蛋白质链状聚集特征的模型系统，但胶体允许通过简单地改变离子强度来调整相互作用，并且足够大，可以在光学显微镜下看到。确定相互作用能、流变学和润滑之间的联系将对生物学的基本理解和合成润滑剂的迂腐设计产生深远的影响。滑液中的球状蛋白是否恰好具有必要的相互作用，使滑液具有随时间变化的流变性（流变性），并使滑液成为我们知道如何合成的任何东西的优质润滑剂？那么，基于这种新的认识，是否可以设计出更好的润滑剂呢？通过研究小颗粒的模型系统，人们希望能够理解为什么哺乳动物的关节液体比合成液体是更好的润滑剂。这对于了解关节流体的功能以及如何构建更好的合成润滑剂具有实际意义。这些研究活动将与招收波多黎各高中生进入大学和从事科学和工程工作的努力相结合。参与该项目的化学工程博士Laura Mely Ramírez计划每年回到她的母校María波多黎各圣胡安Reina （AMR）女子高中，与那里的学生讨论科学和工程提供的机会。波多黎各目前的高中辍学率为51%，这可以通过在Miss. Ramírez为学生提供良好的榜样来改善。AMR高中每年都有职业日，Ramírez小姐计划访问学校，花一天时间与学生讨论并展示纳米技术。

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 Ramírez, the Chemical Engineering Ph.D. student on this project, plans annual returns to her alma mater, Academia María 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. Ramírez. AMR High School has yearly Career Days, and Miss. Ramírez plans to visit the school and spend a day discussing with students and showing demonstrations of nanotechnology.