ENERGY DISSIPATION DURING LIQUID ADSORPTION/DESORPTION IN/FROM LIQUID-REPELLENT NANOCHANNELS

摘要

Ability of viscous fluids, flowing in narrow interstices, to dissipate the mechanical energy of shock and vibration is well known. In recent years, connected to the nano-technological development, solid-liquid interfaces have been used to dissipate surface energies, in systems where the solid is liquid-repellent; such interfaces are able to store, release or transform the energy. Thus, the contact angle hysteresis can be applied to dissipate the mechanical energy, and this kind of energy loss, in which not the viscosity but the surface tension of the liquid plays the main role, is called surface dissipation. In fact a liquid nano-porosimeter that exhibits nano-damping ability, when applied to mechanical systems is called colloidal damper. Concretely, during the cyclical adsorption/desorption of the liquid (e.g., water or aqueous solutions) in/from the liquid-repellent nanochannels (e.g., modified nanoporous silica gel) the energy is dissipated. Such absorber is convenient from the ecological standpoint since it is oil-free and since both the silica gel (artificial sand with controlled architecture) and the liquid are environment-friendly. Connected to this attractive kind of energy loss, one of the problems awaiting solution is that a theoretical model of the surface dissipation remains to be developed and validated by tests. Accordingly, in this work, based on a detailed discussion of the mechanism of surface dissipation one reveals that the parameters which determine the magnitude of the energy loss are the silica gel mass, the liquid and solid surface tensions, and an integral function (specific pore surface) which is related to the nano-architecture of the liquid-repellent coating, to the silica gel pore architecture and to the maximum applied pressure. Silica gel particles are supposed to be obtained through the aggregation of nano-particles, producing rough nanochannels of variable radius, and normal distribution fits quite well the measured pores size distributions. Heterogeneous molecules of the liquid-repellent coating have a methyl group as head, and a body consisted of methylene groups; they produce a nanopillar structure on the silica gel surface. Maximization of the surface dissipation for imposed working liquid or imposed coating molecule is discussed. Test rig is a compression-decompression chamberrnused to validate the theoretical findings. Results obtained are useful in general for the appropriate design of liquid-repellent nanochannels with technological applications, and in particular for the absorber optimum design under imposed requirements.