  The dip coating of suspensions made of monodisperse non-Brownian spherical
particles dispersed in a Newtonian fluid leads to different coating regimes
depending on the ratio of the particle diameter to the thickness of the film
entrained on the substrate. In particular, dilute particles dispersed in the
liquid are entrained only above a threshold value of film thickness. In the
case of anisotropic particles, in particular fibers, the smallest
characteristic dimension will control the entrainment of the particle.
Furthermore, it is possible to control the orientation of the anisotropic
particles depending on the substrate geometry. To test the hypotheses, we
performed dip-coating experiments with dilute suspensions of non-Brownian
fibers with different length-to-diameter aspect ratios. We characterize the
number of fibers entrained on the surface of the substrate as a function of the
withdrawal velocity, allowing us to estimate a threshold capillary number below
which all the particles remain in the liquid bath. Besides, we measure the
angular distribution of the entrained fibers for two different substrate
geometries: flat plates and cylindrical rods. We then measure the film
thickness for more concentrated fiber suspensions. The entrainment of the
fibers on a flat plate and a cylindrical rod is primarily controlled by the
smaller characteristic length of the fibers: their diameter. At first order,
the entrainment threshold scales similarly to that of spherical particles. The
length of the fibers only appears to have a minor influence on the entrainment
threshold. No preferential alignment is observed for non-Brownian fibers on a
flat plate, except for very thin films, whereas the fibers tend to align
themselves along the axis of a cylindrical rod for a large enough ratio of the
fiber length to the radius of the cylindrical rod.
