Session: 04-07-02: Interfacial Phenomena and Flows

Paper Number: 131036

131036 - An Experimental Study on the Sublimation-Induced Porous Structure Formation in a Freezing-Based Inkjet 3D Printing Method 

Colloidal solutions are the mixture of small insoluble solid particles with a fluid, which have been widely used in inkjet-based three-dimensional (3D) printing technology. In detail, droplets of colloidal solution are injected onto a substrate from a printing nozzle to form desired printing patterns. Then the liquid phase of the solution is removed via subsequent drying processes. Solid particles, that were uniformly suspended in the liquid, remain on the substrate. By repeating this working process, a 3D structure is printed layer-by-layer. By following this procedure, it is possible to provide fast and efficient multi-material-based non-contact manufacturing. Thus, inkjet-based 3D printing has collected much attention in the last decades. 

However, internal flows might be induced in a droplet to affect the distribution of solid particles during the drying process. One example is the Deegan flow during the evaporation-based drying process: the region near the contact line of a droplet on substrates has a higher evaporation rate because of the profile with high curvature. An internal flow is induced from the center towards the contact line of the droplet during evaporation, which aims to refill the loss of water near the edge of the droplet. This flow could bring suspending particles to the edge of the droplet and form a coffee ring-like nonuniform printing pattern on the substrate along the contact line of the droplet. Finally, the quality and the mechanical properties of a printed 3D product using this method will be significantly affected because of the nonuniform particle distribution in it.

In our previous study, aiming to improve the quality of printing, we developed a freezing sublimation-based printing method that can minimize internal flow-related particle motions during the drying process. The concept has been experimentally proved by printing 0.5% (w/v) Pearlitol 100 SD water solution droplets on aluminum substrates with different surface wettability. An experimental platform was developed to mimic the inkjet-based 3D printing method as follows: a droplet of ink solution was injected through a needle suspended above a flat substrate using a syringe pump. The substrate was precooled using a temperature controller. After impingement, the droplet was rapidly frozen on the substrate. After that, the frozen droplet was sublimated in a freeze dryer. Through the sublimation process, only colloidal particles in the droplet remained on the substrate, and the particle distribution was uniform. 

According to our previous work, solid particles in a droplet formed a 3D porous structure on substrates by supporting each other after water was sublimated. In this study, we focused on the formation of porous structures using colloidal solutions via sublimation. By adjusting the setting of the commercial freeze dryer, we experimentally tested the sublimation process of Pearlitol 100 SD water solution droplets under different pressure and temperature conditions. The sublimation process of a frozen droplet was recorded using digital cameras and analyzed using image processing techniques. In addition, the porosity of formed solid structures after sublimation was visualized using microscopy imaging technology. In the end, the results of this study showed that the formation of holes in the porous structure was directly related to the escaping gas phase during the sublimation process. A relationship between the porosity of the formed solid structure and the condition of droplet sublimation was also revealed.

Based on the results obtained from this research, an ideal working condition of sublimation for the freezing sublimation-based inkjet 3D printing method was suggested. The suggested condition can avoid undesired consequences, such as a scattered structure between solid particles with low adhesivity or the rapture of formed 3D solid structures during sublimation. Conclusions from this research not only provide a useful reference for the improvement of inkjet-based 3D printing technology, but also broaden the horizon for the manufacturing of 3D porous structure materials used in chemical, biochemical, and biomechanical fields.

Presenting Author: Haipeng Zhang The City College of New York