Geometric impact of void space in woven fabrics on oil resistance or repellency

An oil-repellent surface is challenging to achieve due to the low surface tension of oil. The critical factor is to create void spaces at multiple scales (i.e., macro, micro, and nano) to increase the energy barrier that the liquid-vapor interface must overcome to transit from the Cassie-Baxter state to the Wenzel state, in which the surface is completely wet. To obtain an oil-resistant or repellent surface, one must create a structural geometry with void spaces, and the solid surface energy must be low relative to the oil. Knowing the impact of void space is important to enable a rational design of such surfaces. Woven fabric inherently consists of multiscale structures by its construction: nanoscale in fiber, microscale in yarn, and macroscale in fabric. In this study, theoretical modeling and experiments with actual fabric samples were utilized to determine the impact of void space in woven fabric. The ratio of the void space between two adjacent yarns to the yarn diameter, T-y, was integrated into the lenticular Cassie-Baxter model of woven fabric (i.e., plain structure). Then, the role of void space resisting or repelling oil was quantified by measuring the contact angle of dodecane (gamma(LV) = 25.3 mN/m) on the surface of the fabric samples with varied void spaces. The theoretical model predicted that the fabric's oil resistance or repellence increases as the void space increases, and the role of void space at the macroscale was more important than at the micro- or nanoscale. The predicted tendency of a fabric's apparent contact angle with oil, theta(F), was in good agreement with experiment and showed the value of incorporating T-y in the prediction of the liquid-resistant or repellent behavior. Contrary to the prediction, increasing T-y further caused the liquid drop on the surfaces to have a reduced contact angle, theta(F), due to the sagging of liquid into the void space.