Coating functional sol–gel films inside horizontally-rotating cylinders by rimming flow/state

The fabrication of uniform sol–gel coatings with embedded functional nanomaterials inside cylinders requires detailed understanding of the gelation behavior. For sol–gel systems the viscosity is a function of gelation time that affects sol–gel coatings on the inside of a slowly, horizontally rotating cylinder. Therefore the angular velocity has to be adjusted to this time dependence. The higher the viscosity the more liquid is dragged along with the moving cylinder wall while the balance of gravity and drag limits the layer thickness. In addition, inertial forces and surface tension can create instabilities within the coated layer. Here, we show that it is important to suppress these instabilities by transitioning the viscous sol directly to a velocity that allows for the formation of an almost uniform layer. In this regime, which is the so-called rimming state, the recirculation of the gel precursor solution is strongly reduced which allows to fabricate coatings with shear sensitive sol–gel chemistries. Here, we tested this approach with 4 different aerogel systems, with low-density CH-based-, TiO 2 -, SiO 2 - and Fe 2 O 3 -aerogels, that represent a wide variety of different sol–gel behaviors. We show that the required rotational velocities for these aerogel systems can be predicted with a simple analytical approximation, and we performed computational fluid dynamics simulations to predict local shear and thickness uniformity.