ABSTRACT
Deposits formed after evaporation of sessile droplets, containing aqueous solutions of poly(ethylene oxide), on hydrophilic glass substrates were studied experimentally and mathematically as a function of the initial solution concentration. The macrostructure and micro/nanostructures of deposits were studied using stereo microscopy and atomic force microscopy. A model, based on thin-film lubrication theory, was developed to evaluate the deposit macrostructure by estimating the droplet final height. Moreover, the model was extended to evaluate the micro/nanostructure of deposits by estimating the rate of supersaturation development in connection with the driving force of crystallization. Previous studies had only described the macrostructure of poly(ethylene oxide) deposits formed after droplet evaporation, whereas the focus of our study was the deposit micro/nanostructures. Our atomic force microscopy study showed that regions close to the deposit periphery were composed of predominantly semicrystalline micro/nanostructures in the form of out-of-plane lamellae, which require a high driving force of crystallization. However, deposit central areas presented semicrystalline micro/nanostructures in the form of in-plane terraces and spirals, which require a lower driving force of crystallization. Increasing the initial concentration of solutions led to an increase in the lengths and thicknesses of the out-of-plane lamellae at the deposits' periphery and enhanced the tendency to form spirals in the central areas. Our numerical study suggested that the rate of supersaturation development and thus the driving force of crystallization increased from the center toward the periphery of droplets, and the supersaturation rate was lower for solutions with higher initial concentrations at each radius. Therefore, periphery areas of droplets with lower initial concentrations were suitable for the formation of micro/nanostructures which require higher driving forces, whereas central areas of droplets with higher initial concentration were desirable for the formation of micro/nanostructures which require lower driving forces. These numerical results were in good qualitative agreement with the experimental findings.
ABSTRACT
Faraday made investigations into behaviour of ice and snow. He was ahead of his time. Our paper briefly describes the current state of knowledge in ice friction and adhesion and its historical development. Important aspects of these phenomena in engineering, winter sports and the natural environment are considered. We report new results for static and dynamic friction of a metal (steel) and a polymer (PMMA) on ice over a range of temperatures (-3 to -13 degrees C), and interpret the behaviour by considering processes that operate at the interface and in the bulk of the materials. Clearly the chemical and thermomechanical properties of steel, PMMA and ice differ. The thermomechanical properties of ice itself also vary within the temperature range examined. We find higher static friction with increasing time, and a curious difference in the behaviour of the metal and polymer with temperature. We explain the results by considering the materials' stiffness, plastic deformation and creep, the ductile/brittle transition in ice, thermal properties, physicochemical properties of the surfaces and the real area of contact.
