Spreading Kinetics of Lubricant Droplets on Magnetic Recording Disks

The spreading of droplets of perfluoropolyether lubricants is studied on unlubricated, carbon-overcoated disk surfaces, the type used in magnetic recording disk drives. As the number of hydroxyl end groups on the Fomblin-Z backbone is increased from zero to two to four (Z → Zdol → Ztetraol), the spreading kinetics slows down dramatically for each addition of two hydroxyl end groups, and with eight hydroxyl groups, the lubricant ZTMD no longer spreads. Faster spreading kinetics is observed initially when a central droplet is still available to feed lubricants into the molecularly thin lubricant film. Much slower spreading kinetics is observed once this central droplet has dissipated to form a pancake-shaped film less than a monolayer thickness. During the initial spreading stage with the central droplet, the radius R of the area covered by the spreading film grows as R ~ t ν , with ν ~ 1/3, while during the final spreading stage, ν ~ 0.1. The faster spreading mechanism during the initial spreading stage is attributed to the en masse flow of lubricant molecules in the lubricant film driven by the backing pressure of the lubricant remaining in the droplet. The slower spreading kinetics once the central droplet has dissipated is well described by a thickness-dependent diffusion coefficient D(h) obtained by modeling lubricant migration as Poiseuille flow driven by the disjoining pressure gradient.