Dewetting

In most dewetting studies a thin polymer film is spin-cast onto a substrate. Even in the case of S the film does not dewet immediately if it is in a metastable state, e.g. if the temperature is below the glass transition temperature of the polymer. Annealing such a metastable film above its glass transition temperature increases the mobility of the polymer-chain molecules and dewetting takes place. The process of dewetting occurs by the nucleation and growth of randomly formed holes, which coalesce to form a network of filaments, before breaking into droplets. When starting from a continuous film, an irregular pattern of droplets is formed. The droplet size and droplet spacing may vary over several orders of magnitude, since the dewetting starts from randomly formed holes in the film. There is no spatial correlation between the dry patches that develop. These dry patches grow and the material is accumulated in the rim surrounding the growing hole. In the case where the initially homogeneous film is thin (in the range of 100 nm), a polygon network of connected strings of material is formed, like a Voronoi pattern of polygons. These strings then can break up into droplets, a process which is known as the Plateau-Rayleigh instability. At other film thicknesses, other complicated patterns of droplets on the substrate can be observed, which stem from a fingering instability of the growing rim around the dry patch. File:100nm hole (2).jpg|Circular hole formed in a 100 nm thick film of polystyrene. The blue color of the film is due to Structural coloration and depends on the film's thickness. File:AFM height profile for 100nm hole.jpg|AFM height profile of a dewetting hole's rim. Notice that the material that is removed by dewetting is accumulated at the rim around the hole; initial film thickness (height): 100 nm. File:100nm tessellation.jpg|Voronoi tessellation pattern of polygons achieved by the coalescence of dewetting holes. File:100nm droplets.jpg|If given enough time, this network of polygons decays into separate droplets. File:200nm unstable.jpg|Rim instability in the case of a thicker (200 nm) polystyrene film. == Dewetting of metal thin films ==

Solid-state dewetting of the metal thin films describe the transformation of a thin film into an energetically favoured set of droplets or particles at temperatures well below the melting point. The driving force for dewetting is the minimization of the total energy of the free surfaces of the film and substrate as well as of the film-substrate interface. The dedicated heating stage in SEM has been widely used to accurately control sample temperature through a thermocouple to observe the in-situ behaviour of the material, and can be recorded as a video format. Meanwhile, the two-dimensional morphology can be directly observed and characterised. ie. the partially dewetted Ni film is itself a workable fuel electrode for SOCs as it provides long triple phase boundary (TPB) lines if the structure is fine enough, the connectivity of the nickel and pore phases as well as the TPB lines can be used for SOFC characterisation. ==References==