Session: 5.1.1 - Advanced thermal-flow diagnostic techniques - I

Paper Number: 158596

158596 - Scaling Law-Based Analysis of Coalescence Dynamics of Colloidal Droplets on Substrates During the Freezing Process 

Abstract: 

Inkjet-based three-dimensional (3D) printing technology is widely used in additive manufacturing processes. Printing materials, such as colloidal particles, are suspended in carrier fluids to create inkjet solutions. A series of ink droplets are released from a nozzle to form desired patterns on substrates through programmed nozzle motions. After the carrier fluids are removed via a drying process, the colloidal particles remain on the substrate surface, forming the intended patterns. By repeating this process layer by layer, complex 3D structures can be printed for the rapid fabrication of prototypes or non-standard parts in various research and industrial applications.

In previous studies, we proposed a novel freezing sublimation-based 3D printing technology that improves the printing quality of patterns on substrates. The concept has been experimentally validated using single droplets on substrates as follows: Droplets of inkjet solution were deposited onto precooled substrates from a nozzle. The droplet froze rapidly on the substrate through a heat transfer process, and then was transferred to a freeze dryer for sublimation-based drying under low temperature and pressure. During the drying process, the frozen carrier fluids transitioned directly from the solid phase to the gaseous phase. Internal flows within the droplet were minimized, significantly reducing the possibility of colloidal particle relocation during the drying process of 3D printing. As a result, compared to the traditional evaporation-based drying process, the uniformity of colloidal particle distribution in printed patterns on substrates was significantly improved using the proposed novel method.

However, the interaction between neighboring droplets during the freezing and sublimation processes in this proposed method has not been investigated, specifically the coalescence process between two droplets on substrates. Experimental observations show that when a droplet impinges on a substrate, it spreads out and forms a disc-like shape in a short period. When a second droplet spreads next to the original droplet, it is very likely that a liquid bridge may form between the two droplets if they make physical contact. Due to the high curvature at the neck of the liquid bridge, a surface tension-driven fluid motion occurs, causing the diameter of the liquid bridge to grow rapidly in the lateral direction of coalescence. At room temperature, these two droplets merge into a dumbbell-shaped droplet, and the growth dynamics of the liquid bridge are determined by the combination of inertia, viscous, and surface tension forces acting on the liquid bridge between these droplets. When the coalescence process occurs at low temperature during the rapid freezing of droplets, the growth dynamics of the liquid bridge become more complex, requiring more rigorous study.

In this study, we experimentally investigated the coalescence of colloidal droplets on substrates during the freezing process using high-speed imaging technology and image processing methods. The effects of substrate surface temperature on the growth dynamics of the liquid bridge between impinging droplets were examined to provide a scaling law for predicting the coalescence pattern of two droplets in the freezing sublimation-based 3D printing method. The distribution of colloidal particles in printed patterns was also summarized. Our aim is to provide a reference for optimizing printing patterns on substrates by minimizing void regions between neighboring droplets in the inkjet-based 3D printing process.

Presenting Author: Haipeng Zhang City College of New York

Presenting Author Biography: