To coat the interior of a capillary tube, you typically fill the tube with a viscous liquid, then pump air in to displace the liquid, leaving behind a thin film of the viscous fluid. Keeping that film uniform and thin is a challenge, though, since the pumps used often struggle to keep a consistent low flow rate. Instead, a team of researchers used spin coating to treat the interior of capillary tubes.
Their apparatus consisted of a repurposed computer fan, stripped of its blades and fitted with a 3D-printed platform that could hold capillary tubes (left). When spinning, an oil slug inside each tube gets forced outward from the center of the platform, leaving behind a thin, uniform film coating in the tube. The group found that some fluids develop a wavy, Plateau-Rayleigh instability in the film once spinning stops (right), which is useful for creating a consistent wavy interior for the tube, particularly when using curable polymers for the coating. (Image, research, and submission credit: B. Primkulov et al.)