Elastocapillary painting: deposition of fluid trapped within fiber arrays

When a brush is dipped in a fluid, capillary forces etrain the liquid within the dense forests of hairs as individual bristles coalesce. As the brush is subsequently brought against a surface, as in processes such as painting, the bundles of bristles deform as the entrained fluid transfers from the brush onto the substrate. While capillary wicking within elastic porous media has been well-studied, the transfer of fluid from the deformation of an elastocapillary bundle remains relatively unexplored. Here we explore this transfer process using simplified paintbrushes composed of closely-packed arrays of elastic beams. Upon wetting a brush with oil, we study two modes of painting: stamping, in which a brush is pressed vertically onto a plate, and brushing, where it moves parallel to the plate after contact. In both cases, the displacement of the brush against the substrate strongly affects drainage from the elastic medium. In the former case, we find that the speed of retraction can affect the proportion of fluid transferred due to liquid bridge breakup. Meanwhile, in the latter case the initial fluid layer can fully dewet from bristles at rates dependent upon brush geometry and the speed of dragging.