Controlling the Wetting Behaviour of Wool Fabrics and 3D Printed Polymer Surfaces
Authors:   , Cook Michelle Jane 
Published: December 2021

<p><b>The wettability of a surface is determined by two factors; the chemistry of the surface, which determines the inherent chemical hydrophilicity or hydrophobicity, and the physicality of the surface, whereby the surface roughness enhances and exaggerates the inherent hydrophilicity or hydrophobicity of the surface. Hence, controlling the wettability of a surface requires manipulation of both the chemical nature of the surface and the surface roughness. This research predominantly investigated controlling the wettability of surfaces through creation and manipulation of surface roughness, but also studied alterations of the chemical hydrophilicity and hydrophobicity of surfaces. These studies were performed using two different substrates, for different potential commercial applications. Wool fabrics were studied, in order to increase their hydrophobicity and produce a stain and water repellent wool fabric. 3D printed polymers were also investigated in order to create surfaces with useful wetting behaviours, including superhydrophobicity and anisotropic wetting.</b></p>
<p>First, wool fabrics were manipulated through physical processing alone, in order to increase the surface roughness. This relied on altering the arrangement of the micrometre–sized wool fibres in the fabrics and creating a more disordered array. This was achieved through hand–felting of the wool fabrics and was found to successfully increase the surface roughness of the fabrics, and hence increase the hydrophobicity. However, it was found that the increased felting led to detectable increases in the macro–scale roughness of the surface, reducing the soft hand feel of the wool fabrics. This effect is an undesirable property in the textiles industry and another physical process of shearing protruding wool fibres was investigated in order to reduce this macro–scale roughness and increase the softness of the fabric. However, this processes was found to also reduce the micro-scale roughness of the surface and hence, the hydrophobicity of the fabrics was also decreased. This decrease in hydrophobicity was exacerbated due to the increased chemical hydrophilicity of the surface due to the exposure of more fibre ends, which are hydrophilic. Hence, the hydrophobicity of the sheared fabrics was reduced to a level lower than the untreated fabrics. Therefore, the use of physical processes to increase the roughness of wool fabrics was found to be successful in increasing the hydrophobicity of the surfaces. However, as the soft texture of the wool fabrics was compromised, the fields of application of these fabrics was determined to be somewhat limited.</p>
<p>Subsequently, the hydrophobicity of wool fabrics was increased using chemical modifications to increase the surface roughness, while not compromising the soft texture of the fabrics themselves. In these investigations, the surface roughness was again increased, but on a smaller scale, through the functionalisation of wool fibres with inorganic particles on the micro- or nano-scale. Three types of particles were investigated, gold nanoparticles, Ag/AgCl particles and CaCO3 particles. The gold nanoparticle functionalisation successfully increased the surface roughness on a very small scale, but the concurrent increase in chemical hydrophilicity was found to outweigh this increased surface roughness and result in a net decrease in hydrophobicity of the functionalised fabrics. The Ag/AgCl and CaCO3 functionalisations both successfully increased the hydrophobicity of the wool fabrics. Of these treatments, the Ag/AgCl functionalisation was found to have the greatest impact on increasing the hydrophobicity and Ag/AgCl functionalised fabrics showed increased water repellency in practical tests. The Ag/AgCl functionalisation also renders the fabrics antimicrobial, due to the known antimicrobial activity of silver nanoparticles (present as silver nanodomains on AgCl microparticles in Ag/AgCl composites). Hence, these investigations led to the development of water and stain repellent, and antimicrobial wool fabrics with great potential for commercial applications.</p>
<p>Polymer substrates and 3D printing were used in order to create surfaces with specific surface roughness patterns and changeable surface chemistry for the purpose of creating surfaces with controlled wetting behaviours. For these investigations, a fused deposition modelling (FDM) 3D printing process was used to facilitate rapid prototyping of surface roughness pattern designs and allow for optimisation of these designs. These surfaces were printed using acrylonitrile butadiene styrene (ABS). Firstly, a highly hydrophobic surface was designed. An array of droplet-shaped structures was printed onto a flat substrate. These droplet structures were on the size scale of a few hundred micrometres, but the process of melting and extruding the polymer in the printing process also gave the surface some degree of smaller scale roughness. This roughness was then increased further, by functionalising the surface with silver nanoparticles. This functionalisation was achieved in-situ and also rendered the surface antimicrobial. Due to the inherent chemical hydrophilicity of silver, the surfaces also required a subsequent treatment with a fluoroalkyl silane (FAS) to render them chemically hydrophobic. While these surfaces did not achieve the typically defined conditions of superhydrophobicity, they were found to be highly hydrophobic and were also antimicrobial. Hence, these surfaces were deemed to be potentially useful for commercial applications in healthcare industries.</p>
<p>FDM 3D printing was also used to create a surface with anisotropic wetting behaviour. This was achieved through printing a surface with 1-dimensional roughness in the form of protruding lines extending in one direction across the surface plane. These surfaces were printed using ABS or polylactic acid (PLA). Different shapes, sizes and spacings of these lines were investigated and these parameters were optimised. The resulting surface displayed significant anisotropy in its wetting behaviour with highly hydrophobic behaviour observed in the direction parallel to the protruding lines, but significantly more hydrophilic behaviour in the perpendicular direction. Hence, this surface allowed the movement of water in one direction across the surface but hindered movement in the perpendicular direction. This surface was then treated with a FAS, and subsequently treated with ion implantation in selected areas, using varied fluence of Ar+ or C+ ions. The FAS treatment served to change the surface chemistry from that of the printed polymer to a generic hydrophobic chemistry, using a surface treatment that is applicable to many different substrate materials. The ion implantation treatment significantly decreased the hydrophobicity of the treated areas of the FAS functionalised surface; in fact the ion implantation treatment was found to render the selected areas formally hydrophilic. This allowed for the design of a surface with both hydrophilic and hydrophobic areas, that allowed for movement of water in only one direction across the surface. Such a surface has potential applications in water harvesting, utilising the hydrophilic areas for condensation of water from the air, and the hydrophobic areas for movement of this water across the surface toward a collection tank.</p>
<p>The research carried out herein led to the successful increase in hydrophobicity of wool fabrics using both physical and chemical processes. 3D printing was also successfully utilised to create two different surfaces with controlled wetting behaviours. Firstly, a surface with high hydrophobicity and antimicrobial activity was produced. And secondly, a surface with anisotropic wetting behaviour and areas of hydrophilicity and areas of hydrophobicity was achieved. Possible applications of these surfaces were considered.</p>