Towards an improved understanding of liquid transportation along a hair fiber: ratchet-like microstructure induced capillary rise

Abstract

To understand sweat transportation along a single hair fiber, its capillary rise was observed. But the effects of hair cuticular cells on the process were difficult to identify due to their submicron scales. In order to deal with the problem, an analogical model of a conical frustum with ratchet-like sides was developed. Side microstructures of the model block the rise of water; moreover, the contact line and contact angle (θ) are more sensitive to such microstructures than wetting height, and there holds the relation between θ and time (t): θ ∼ t⁻ⁿ (n > 0.5). Similarly, the dynamic θ is verified to fit the wetting of the hair fiber. In contrast with a smooth artificial fiber, the introduction of cuticular cells positively speeds up the transportation of liquid (e.g. sweat) along the hair fiber. In addition, hair cleaning is emphasized because oily liquid easily covers the hairs, preventing the transportation of sweat. We believe that our findings are of important significance for cosmetic applications.