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Fluid Dynamics at the Interface between Soft Elastic Solids

Subject Area Polymer Materials
Term from 2015 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 272908054
 
Final Report Year 2018

Final Report Abstract

We performed adhesion and friction studies for a set of well prepared and broadly characterized (e.g. viscoelasticity, surface energies and contact angle, etc.) rubbers with technical importance like NBR, EPDM, GECO and PDMS in the dry state and in water. We studied the contact mechanics and friction between the rubber materials and different counter surfaces (glass, polymers, metals) in fluids, mainly water but also different types of oil. The friction and the leakage of rubber seals was found to depend on the adhesion between the materials in the fluid. If adhesion is observed in the fluid, the fluid is likely expelled from the rubber asperity contact area (dewetting), which we found result in high friction (for rubber sliding on the counter surface), and low fluid leakage (for rubber seals). The latter we interpreted as resulting from blocking of fluid flow channels at the interface by micrometer-sized gas bubbles. We also studied the contact mechanics between rubber and different counter surfaces at low temperatures. Even if negligible adhesion between the materials was observed at room temperature because of the influence of the surface roughness, very strong adhesion was observed if the contact was cooled below the glass transition temperature of the rubber material. The origin of this is the frozen-in deformations in the rubber material, i.e., at low temperatures the elastic deformation energy is not “given back” when the solids are separated, resulting in the strong observed adhesion. We showed that this has a strong influence on the fluid (or gas) leakage of rubber seals (as also manifested in the Challenger disaster). This observation is important for use of rubber seals at low temperature e.g. in some oil and gas exploration applications. We also studied how the adhesion between a glass ball and rubber evolves with time as the rubber crosslink density increases, converting it from a (high viscosity) liquid to a solid. We found that the adhesion (or pull-off force) exhibit a sharp and huge maximum at the time when the material make a transition from fluid to solid (the so called gel-point). This result may be of interest for the development of tacky materials, e.g. pressure sensitive adhesives.

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