THERMAL ORIENTATION OF MOLECULAR FLUIDS UNDER STRONG TEMPERATURE GRADIENTS : A COMBINED EXPERIMENTAL AND THEORETICAL INVESTIGATION

A large amount of heat is released as a result of human activities, but generally it is difficult to recover the waste heat as other useful forms of energy. Heat dissipation from machines, engines, heaters or our bodies, generate thermal gradients. Being able to convert this waste heat into other useful forms of energy is therefore an important objective in science and technology. Non-Equilibrium thermodynamics teaches us that thermal gradients couple to mass fluxes and electrical currents, leading to new physical effects. Two important effects, the so called Soret and Seebeck effects were discovered in the 19th century. These effects have enabled a number of technologies that we use nowadays (refrigerators, thermocouples). Applications of current interest include biothermal batteries to power heart pacemakers or conversion of electricity using waste heat in automobiles. A notable example of a machine that exploits thermoelectric conversion is NASA?s Voyager. This spaceship relies on radioisotope thermoelectric generators to convert the heat produced by radioactive decay into electricity. The Soret coefficient is also of considerable interest in modern technologies, as it provides a route to manipulate nanoparticles and molecules by moving them towards hot or cold sources. This effect is being used in thermo-analytical devices too. Despite considerable technological advances, we do not understand the physics behind these effects in full. As a matter of fact we discovered recently that water would polarise under a thermal gradient, a new effect we called thermo-molecular orientation (TMO). Water is a solvent of key importance in chemical reactions, biology, our body or the Earth. Our project focused on gaining a better understanding of the TMO effect, finding microscopic variables, such as a molecular mass and shape, that define the effect, and that can inform us on routes to control and modify the effect. We have also investigated the implications of TMO in the thermoelectricity of aqueous solutions, and to understand what happens when we apply a thermal gradient to e.g. a glass of salty water. Such common experimental system can provide information to design thermoelectric devices, as well as to understand why a nanoparticle or a protein move towards hot or cold regions. We investigated in our project salts such as LiCl. Lithium is of particular interest since this ion is widely used in many of the batteries we use everyday for our electrical devices. We found that the TMO effect in water represents a key contribution to quantify the Seebeck coefficient of aqueous solutions. Such effect has not been considered before, and hence it provides a new perspective of the thermoelectricity carried by ions in water, with potential applicantons in energy conversion devices. We have applied our algorithms to understand the behaviour of water mixtures, as water-urea, under thermal gradients. Urea is widely used to study protein denaturation, although the mechanism behind the denaturation is still under debate. Experimental studies performed during the project highlighted the exquisite sensitivity of thermodiffusion to solute-water interactions. We identified using computer simulations and experiments the onset of hydrophobicity of urea-water mixtures, which appears at a urea concentration regime (~5 M) corresponding to that where abrupt changes in the denaturation of proteins are commonly observed. The implications of TMO in this context are under investigation too. Overall, our work has laid out the physical background to understand and control TMO in water and aqueous solutions. Next steps can focus on using this effect to understand the physics of fluids in thermal fields, as we have done for the Seebeck effect of aqueous solutions; to understand the physical chemistry of solvation by using thermal gradients, as well as to evaluate the potential of the TMO effect in environmentally friendly energy conversion applications.

Temperature gradients induce coupling effects, such as electron transport, which can be used to generate electricity from waste heat. The flow of electrons under the temperature gradient is the basis of thermoelectric devices that are currently used in th e automobile industry, and specialized applications such as space probes. Very recently we have shown, using computer simulations and theoretical approaches that thermal gradients can induce a preferred orientation in molecular fluids. The degree of orien tation is proportional to the thermal gradient, and it is expected to be significant for gradient of ~10^6 K/m, which are achievable with current experimental set ups. This phenomenon called "Thermo-molecular orientation" is completely general, and it is observed in non-polar and polar fluids. We have predicted that in polar fluids molecular orientation can result in large electrostatic fields, of the order of the fields needed to operate liquid crystal displays. This project combines the expertise of exp erimental and theoretical groups to investigate this new physical phenomenon and its microscopic mechanism. In the present proposal we will investigate using computational methods and laser spectroscopy techniques thermal orientation of a wide range of fl uids in terms of their molecular architecture and chemical composition. By comparing different molecular structures we aim to uncover the microscopic rules that determine the strength of the thermal orientation effect. One central aspect of the project wi ll be the development of an experimental approach to validate and investigate thermal orientation phenomena in situ, by using laser heating and spectroscopy. This project will provide fundamental understanding on a novel physical phenomenon that in the lo ng term might provide the physical mechanism for new energy conversion devices. 1 PhD and 1 PostDoc will be trained in an international setting at NTNU with contributions from Imperial College.