Droplet impact dynamics on textiles.

The development of textiles that repel droplets following droplet impact at a high velocity is a common requirement in a number of applications, ranging from waterproof clothing to inkjet printing, yet the underpinning physical mechanisms are not entirely understood. The impact of a droplet on the surface of a textile produces two simultaneous yet separate flows, occurring above and below the surface, and which are associated with the spreading and penetration dynamics. In this paper, we study the temporal evolution of the lateral spreading diameter of a droplet impacting both hydrophobic and hydrophilic textiles. We show that the impact on textiles at short timescales involves no deformation of the droplet shape if the textile's porosity is sufficiently low. We show that the early-stage impact penetration is solely driven by inertia and no lamella is visible. We also show that for hydrophilic textiles, depending on the impact conditions, a droplet can be captured by the textile or penetrate it. We show by balancing the dynamic impact and capillary pressures that the penetration behaviour is governed by a threshold pore size, the liquid characteristics and the droplet diameter. Our conclusions highlight that the ability of a textile to repel water is controlled by the mesh size. Our experiments and analysis were carried out on coated hydrophobic and non-coated hydrophilic textiles with four corresponding mesh sizes, and are in agreement with the previous findings on hydrophobic metallic (copper) meshes.

X-ray Imaging of Transplanar Liquid Transport Mechanisms in Single Layer Textiles.

Understanding the penetration of liquids within textile fibers is critical for the development of next-generation smart textiles. Despite substantial research on liquid penetration in the plane of the textile, little is known about how the liquid penetrates in the thickness direction. Here we report a time-resolved high-resolution X-ray measurement of the motion of the liquid-air interface within a single layer textile, as the liquid is transported across the textile thickness following the deposition of a droplet. The measurement of the time-dependent position of the liquid meniscus is made possible by the use of ultrahigh viscosity liquids (dynamic viscosity from 105 to 2.5 × 106 times larger than water). This approach enables imaging due to the slow penetration kinetics. Imaging results suggest a three-stage penetration process with each stage being associated with one of the three types of capillary channels existing in the textile geometry, providing insights into the effect of the textile structure on the path of the three-dimensional liquid meniscus. One dimensional kinetics studies show that our data for the transplanar penetration depth ΔxL vs time do not conform to a power law, and that the measured rate of penetration for long times is smaller than that predicted by Lucas-Washburn kinetics, challenging commonly held assumptions regarding the validity of power laws when applied to relatively thin textiles.