Effects of Substrate Temperature on Spreading and Adhesion of Colloidal Droplets in Inkjet 3D Printing: An Experimental Study

Inkjet-based additive manufacturing (AM) technology has evolved into a competent tool for manufacturing complex and advanced structures of various materials, yet it suffers from several drawbacks, such as coarse resolution, lack of adhesion, manufacturing inconsistency, and uncertain final part mechanical properties. These undesirable effects are due to the droplet impact dynamics and the uncontrolled flows inside of colloidal suspension droplets while being kept in the liquid state during printing and drying, creating irregular and non-uniform deposit distribution patterns on the printed surface. We have developed a freezing-sublimation-based method to overcome these defects by rapidly freezing printed colloidal droplets on substrates, followed by a sublimation-based drying process to remove water from these droplets. A much more uniform deposit distribution was achieved with this novel approach. However, due to the lack of knowledge about colloidal droplets impacting, spreading, and freezing on substrates at freezing temperatures, the operational conditions of this freezing-sublimation-based inkjet 3D printing method remain under development. In the present study, an experimental study was conducted to investigate the colloidal droplet impinging and spreading morphologies on precooled hydrophilic and hydrophobic surfaces with different Webber numbers of droplet impact. It was found that, with a surface temperature lower than the freezing temperature, the bottom layer of the droplet at the interface was rapidly solidified, pinning the contact line of the droplet and providing increased adhesivity of the droplet on the substrate even on hydrophobic surfaces. In addition, various spreading patterns of colloidal droplets upon impact were observed, which were significantly influenced by the surface temperature. A comparison of the printed patterns of droplets on substrates with different temperatures was depicted and elucidated for the suggested printing method on different surfaces.