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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Viscosity of complex polar fluids, electrolytes, and ionic liquids

Alternative title: Viskositet av komplekse polarvæsker, elektrolytter og ioniske væsker

Awarded: NOK 7.8 mill.

Friction between solid surfaces is an important phenomenon in everyday life. Without it, even something as simple as holding a piece of paper would be impossible. A large portion of the total energy production in industrialised countries is lost through friction and wear. With the increasing need for clean, sustainable production and use of energy, there is an urgent need for better technologies to reduce friction and for greater understanding of friction and lubrication (tribology), both at macroscopic and at microscopic scales. A new frontier in friction research and a promising basis for better lubricants are electrolyte systems: water-based electrolyte (salt) solutions and ionic liquids (liquid salts). Water-based synovial fluid, for example, lubricates the joints of living creatures with incredible effectiveness. These systems can be created with a rich variety of characteristics and the possibilities for innovative applications are endless. Water-based lubricants are also cheaper, and can be made environmentally friendlier and easier to clean up than mineral oils. Unfortunately, development of new technologies is hampered by our lack of systematic theoretical understanding. The challenge lies in the same thing that make these systems so interesting and potentially useful: the complicated interactions and geometry of the molecules. The aim of this project has been to remedy this and gain theoretical understanding, connecting the properties of electrolyte molecules to the viscosity of a lubricant that contains them. This project has taken a general and powerful approach based on kinetic theory of gasses and liquids, which describes the relation between transport properties and the dynamics of interacting molecules. The project combines pen-and-paper calculations with simulations of simple model fluids. We have developed a theoretical description for the viscosity of dipolar hard spheres. This theory is able to capture the effects of clustering of particles, which other theories so far had not, and which is important for understanding the viscosity of fluids with hydrogen bonding, such as water. Our approach is quite general and we have also shown that it can be used to understand the viscosity of other fluids. We have investigated the effect of external electric fields the cluster formation and how this affects the viscosity. Finally, we have extended our theory to mixtures, since many lubricants and other fluids of interest are mixtures. These results enable us to understand the effects of electrostatic interactions in a number of conditions that are relevant for applications.

The main outcome of this project has been the development of theory and understand of the viscosity of polar and other complex fluids. The theoretical approaches we have developed are quite general, and have allowed us to understand the effects of different long-range interactions, external fields, and mixtures. The push for environmentally-friendly water-based (polar) lubricants is intensifying, and new lubricants are needed for new applications, such as electric cars. These results will have an impact on these developments, as they will allow us to determine more quickly and easily what the consequences of mixing specific compounds together will be for the viscosity of the lubricant.

This project is concerned with the transport properties, especially the viscosity, of electrolyte systems: aqueous electrolyte (salt) solutions and ionic liquids (liquid electrolytes). These systems are becoming a new frontier in friction research and are very promising for low-friction applications, as demonstrated for instance by the amazing effectiveness with which water-based synovial fluid lubricates our joints. Development and implementation of new technologies, however, is blocked by our lack of systematic theoretical understanding. We do not yet have sufficient understanding of the viscosity of these complex liquids to be able to control it. The aim of this project is to remedy this situation and develop general understanding and practical methods for calculating the viscosity of electrolyte systems. The challenge lies in the large number of parameters and the complicated dynamics that result from the presence of electrostatic interactions. As a result, theoretical work has so far been limited to detailed atomistic simulations. These are extremely demanding and can only give insight into specific systems, but it is simply not feasible to simulate enough different systems to deduce general trends. This project will instead use a more challenging but ultimately much more powerful analytical approach based on kinetic theory of fluids. New theory will be developed and used to compute viscosities of complex electrolyte liquids. The theoretical development will be combined with simulations of simple model fluids that will be used to test the validity of approximations and check results. Along with this, there will be close interaction with experimental and theoretical groups in Stockholm and at Imperial College London.

Funding scheme:

FRINATEK-Fri prosj.st. mat.,naturv.,tek