Development of Novel In Situ Microscopy Techniques for the Study of Water Interaction with Soft Materials

Abstract: The transport of water in soft materials can occur in liquid or gas phase and is highly dependent on the material microstructure and the structure dynamics. Understanding these relationships is the basis for the development of predictive models that can aid the design of new and improved functional materials. The environmental scanning electron microscope (ESEM) enables the visualisation of the effects of hydration or dehydration on a specimen down to the nanometre scale, facilitating the understanding of the structure-property relationships. However, the full potential of the ESEM has not yet been explored, especially when it comes to the transport of water in materials. The aim of this work was to develop new ESEM-based methods that enable the in situ study of water interaction with soft materials in a controlled manner. We designed a sample stage that uses a manipulator to bring the specimen in contact with a water reservoir in the ESEM, rendering the point of contact between water and specimen available for visual studies. In addition, coupled with a piezoresistive atomic force microscopy (AFM) sensor, the setup enables the local measurement of displacements in the nanometre range with millisecond temporal resolution through force spectroscopy. Thus, it provides a sensitive probe for swelling, which is an important effect of the water interaction for many soft materials. The potential of the developed methods has been demonstrated on three different materials systems and geometries. The absorption and transport of liquid water in individual cellulose fibres were imaged for the first time. The volumes of absorbed droplets were typically in the range of 0.02 nL to 0.2 nL and the rate of absorption varied between different fibres. The method was also applied to phase-separated polymer films intended as controlled-release coatings in oral pharmaceutical formulations and enabled the first studies of the water interaction in the initial stage of wetting. Simultaneous probing of the microstructure and the local water transport properties of the films provided previously inaccessible information about the structure-transport relationships and the microstructural evolution caused by the water interaction. In addition, measurements of the time-dependent osmotic swelling of yeast cells in the ESEM were demonstrated with a high spatial and temporal resolution. This type of measurement is valuable for the understanding of the water transport properties of cell membranes. The versatility of the setup allows the technique to be applied to a wide range of different materials systems and geometries where the interaction with water is of interest.