Determining the Contact Angle of Inkjet Printed Drops on Textile Fibers

It is difficult to measure the true contact angle between a liquid drop and a textile surface because of the roughness generated by the curvature of the constituent fibers. Although it is possible to study the interaction between a liquid drop and a single fiber by measuring the equilibrium dimensions of the drop, this is problematic with textile fibers of radius <10 μm. Here we investigate whether measurements of the spreading length of a dried drop of known volume can be used to determine the contact angle between a liquid and a textile fiber within a fabric structure. Our hypothesis is that, if the liquid is a solution or suspension of a solid phase, the presence of the deposit represents the maximum extent of spreading and thus the length of the equilibrium drop shape before fluid evaporation and drying. By using inkjet printing to deposit drops of identical volume and applying an analytical model for the equilibrium drop shape, it is possible to compute the equilibrium wetting angle from measurements of the mean wetted length, if the drop volume is known. Here, we us this method to measure fiber/liquid contact angles within textile structures in two cases: (i) a plain weave polyester fabric with a Ag nanoparticle ink, (ii) a nonwoven carboxymethyl cellulose fabric with a Ag compound solution ink. We find that the measured values are consistent with bulk behavior and demonstrate that our hypothesis provides an upper bound to the contact angle.