A single-faced superhydrophobic lyocell fabric maintaining its inherent high moisture absorbing bulk property was produced by oxygen plasma-based nanostructuring and a subsequent coating with a lowsurface-energy material. After 5 minutes of oxygen plasma etching, followed by 30 seconds of a plasma polymerized hexamethyldisiloxane coating, the treated surface of lyocell turned into a superhydrophobic surface with a static contact angle greater than 160 and a sliding angle less than 2 ; however, the backside was hydrophilic, untreated lyocell fabric. As a result of oxygen plasma etching, dual hierarchical roughness was formed on the lyocell fabric as nano scale pillars or hairs were added onto the lyocell fabric surface with micro scale roughness. Extremely opposite wetting behavior was observed, when a water droplet was deposited on the face and backside of the plasma-treated lyocell fabric. A water droplet was immediately absorbed and spread out on the untreated backside, while it rolled off the treated surface, demonstrating a bouncing effect.
This study investigated moisture management properties of a single-faced superhydrophobic fabric. A single-faced superhydrophobic lyocell fabric, where one face of the surface is superhydrophobic and the opposite face is hydrophilic, was produced by a two-step plasma process on one side of the fabric: (1) the addition of nano-scale roughness by 5 minutes of O2 plasma etching; (2) subsequent 30 seconds of plasma enhanced chemical vapor deposition with hexamethyldisiloxane to lower the surface energy of lyocell fibers. As a result, the superhydrophobic lyocell fabric exhibited water repellency with a static water contact angle greater than 161° on the treated surface, allowing water absorption from the untreated face. The nanometer depth of the superhydrophobic layer in the hydrophilic textile affected water absorption capacity, drying rate, vertical wicking rate, and moisture management properties. The air permeability and water vapor transmission rate of the superhydrophobic treated lyocell fabric were hardly changed. The superhydrophobic properties were maintained after a gentle wash cycle, although the level of superhydrophobicity was reduced, especially when it was washed with detergent. This superhydrophobic and moisture managing textile would be relevant for an application that requires a water repellent property on one face and water absorbing property on the opposite face, such as medical operation gowns, wound dressings, and hygienic products.
A trapezoidal structure is introduced to fill microchannels with a liquid by simple dip-coating without leaving any residue on the other parts of the surface. This selective filling is in turn used for patterning by direct transfer of the filled liquid onto a substrate followed by curing. This patterning leaves no residual layer that most of the lithographies based on a mold have to contend with. For the selective filling, the liquid contact angle has to be high, but a high contact angle can cause instability of the liquid filament in the channel. The trapezoidal structure widens the window of the stability in terms of the contact angle and the channel aspect ratio. A theoretical result is obtained for the stability. Experimental results are also presented to show that the selective filling and patterning can be realized for the channels with an aspect ratio lower and a contact angle higher than those allowed for rectangular channels. The surface curvature of the filled liquid can be controlled by manipulating the drawing speed in the dip coating.
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