Odd Friction Behavior Of Droplets

More force is required to slide a droplet resting on a surface than one suspended from a surface-right? Not so, say Rafael Tadmor, Prashant Bahadur, and coworkers of Lamar University, in Beaumont, Texas. The counterintuitive finding could help scientists understand the behavior of lubricants in low-gravity environments. Classic textbooks teach that more force is needed to push a heavy box than a lighter box across a floor because the additional weight increases the contact area between the microscopically rough surfaces, which increases friction. To study droplets' frictional behavior, which appears not to follow those principles, the Lamar team built a novel device that features a wireless camera positioned next to a droplet stage on a tiltable rotating arm. The device independently varies and measures the forces acting on the droplets parallel and perpendicular to the surface while monitoring droplet sliding. It is unclear why hanging droplets require 27% more centrifugal force than ones at rest to begin sliding, the team notes. But the results, which have been accepted for publication in Physical Review Letters, may be driven by force-related reorientation of molecules at the interface, they say.