Micro- and nanoscale devices and mechanisms are frequently covered with a thin film of a low surface energy polymer to protect against fouling and corrosion and to reduce adhesion and wear between contacting parts. When optimizing these films for lubrication, careful attention needs to be paid to the spreading kinetics of these molecular films as this determines how quickly the monolayer heals after being damaged during contact. Here, we report both experimental measurements of how fast lubricant films with a monolayer thickness spread over surfaces and molecular dynamics simulations of this spreading process. In contradiction to many previous studies, we show definitively that these polymers do not spread by simple diffusion, where the distance the film edge was predicted to move as distance ~ (time)1/2. Instead, the radius R of the area covered by the film spreading from a droplet is found to grow as R ~ (time)ν, with ν ~ 1/3 if a droplet is still present, and with ν ~ 0.1 once the droplet has spread into a pancake shape. The figure shows an example result from our molecular dynamics simulations. Molecular dynamics simulations indicate that pressure within the droplet drives the flow into the precursor molecular film and that entanglement of polymers within the spreading film impedes this flow. These studies also provide important insights into how slip occurs at the molecular level at liquid–solid interfaces.

Anomalous spreading kinetics of polymer lubricant films

253rd National Meeting (2017)

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