Capillary sorting of fiber suspensions by dip coating

Sorting elongated anisotropic particles, such as fibers, dispersed in suspensions poses significant challenges, as they present two characteristic dimensions: length and diameter. Fibers in suspensions usually align with the flow, leading to diameter-based filtration when passing through a sieve. Modifying the flow conditions by introducing more mixing, so that fibers are arbitrarily oriented, can lead to sorting by diameter and length simultaneously, resulting in a lower filtration quality. Here, we demonstrate that capillary filtration by dip coating can be utilized to selectively sort fibers by length or by diameter in a controlled manner. Using the withdrawal of a flat substrate from a fiber suspension, we demonstrate that fibers are primarily sorted by their diameters. When considering cylindrical substrates, fibers can be sorted by length under appropriate conditions due to the orientation adopted by the fibers during their entrainment. We report guidelines for designing this filtration process and obtaining good sorting efficiency.