Multiscale landscaping of droplet wettability on fibrous layers of facial masks.

Liquid mobility is ubiquitous in nature, with droplets emerging at all size scales, and artificial surfaces have been designed to mimic such mobility over the past few decades. Meanwhile, millimeter-sized droplets are frequently used for wettability characterization, even with facial mask applications, although these applications have a droplet-size target range that spans from millimeters to aerosols measuring less than a few micrometers. Unlike large droplets, microdroplets can interact sensitively with the fibers they contact with and are prone to evaporation. However, wetting behaviors at the single-microfiber level remain poorly understood. Herein, we characterized the wettability of fibrous layers, which revealed that a multiscale landscape of droplets ranged from the millimeter to the micrometer scale. The contact angle (CA) values of small droplets on pristine fibrous media showed sudden decrements, especially on a single microfiber, owing to the lack of air cushions for the tiny droplets. Moreover, droplets easily adhered to the pristine layer during droplet impact tests and then yielding widespread areas of contamination on the microfibers. To resolve this, we carved nanowalls on the pristine fibers by plasma etching, which effectively suppressed such wetting phenomena. Significantly, the resulting topographies of the microfibers managed the dynamic wettability of droplets at the multiscale, which reduced the probability of contamination with impact droplets and suppressed the wetting transition upon evaporation. These findings for the dynamic wettability of fibrous media will be useful in the fight against infectious droplets.