Microscopical Characterization of Metal-Coated Polymer Microspheres under Mechanical Compression
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- Institutt for fysikk 
We have performed compression experiments on both metal-coated and uncoated resorcinol-formaldehyde polymer microspheres by a experimental setup developed in-house. Through images obtained from optical microscopy, the experiments yield radius of the contact area between sphere and compressing plates, as well as on-axis compression and lateral expansion of the microspheres.Based on Tatara's solution to the sphere-contact problem, we have estimated the Poisson's ratio of uncoated particles to 0.314$\pm$0.0195, which corresponds well to existing literature. As the metal-coated particles are composite, it is not straightforward to find their Poisson's ratio. Uncertainties in the experiments for these particles are suggested to be mainly caused by delamination of the coating during compression and plastic deformation of the coating.The experimental setup proves to be excellent for determining amount of compression needed to fracture the spheres, where the yield point of the coated microspheres was found to be at 16$\%\pm$1.1$\%$ compression. The setup also appears to be good for direct observation of crack initiation and propagation, as one gains visualisation of the sphere surface through the optical microscope.