Nanoscale Transport Phenomena at the Interface of Hard and Soft Matter

Hard and soft matter can be distinguished by the energy of chemical bonds in comparison with kBT. At the interface of hard and soft matter, there exists a region of transition between strong (covalent/ionic/metallic) bonds in solids and weak (van der Waals/hydrogen/electrostatic) interactions in liquids and polymers. Transport of energy and mass at such interfaces is yet to be fully explored, but seems both rich in science and of technological importance. This paper discusses some fundamental issues as well as some technological implications. Introduction The purpose of this paper is not to present any new results, but instead to use the work of others from various disciplines and identify a common theme for future research that is both rich in science and could have significant impact on technology. This theme has to do with transport phenomena at the interface between hard and soft matter. The hardness and softness of matter is essentially a comparison of the chemical binding energy of molecules to the entropic driving energy, kBT. In solids, the bond strengths arising from covalent, metallic or ionic bonds are generally on the order of 1-5 eV whereas at room temperature kBT ≈ 26 meV. On the other hand, the binding energy in liquids due to van der Waals, electrostatic or hydrogen bond interactions fall in the range of 30-100 meV. Hence, entropic forces can lead to molecular mobility and transport properties such as viscosity. Polymers are a class of materials where some bonds covalent with binding energy of 1-5 eV but others are rather weak. This gives rise to flexibility and thereby fluctuations due to kBT. Given this vast difference in the behavior of hard and soft matter, natural questions with regards to transport phenomena would be: How are heat and mass transported at their interfaces? Are there practical applications where such phenomena become important? Before we go any further, lets us first look at the interface between a solid and a liquid. In particular, I will focus on water in contact with a hydrophilic surface. Next, I will look at polymers in contact with solid surfaces. Solid-Liquid Interfaces Hydration Forces Solid surfaces such as silica, mica or any other oxides or biological molecules such as DNA or proteins that contain a polar group, are hydrophilic and form hydrogen bonds with water. The H-