Analysis of colloidal pigment aggregation and ink media interactions in porous media
Thirukonda Viswanath, Karthik
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The commercialization of a modern inkjet printing system is a complex undertaking that requires an interdisciplinary team to address numerous critical and fundamental issues encountered throughout the product development cycle. An inkjet development effort spans a broad range of diverse yet coupled activities that include the rational design of ink formulations, design and optimization of a droplet generator, precision control of a high-throughput deposition of droplets onto a moving media and modeling and characterization of various ink media interactions, e.g. spreading, absorption and coalescence of droplets, to achieve high image quality. The focus of this dissertation is on ink media interactions, which directly impact inkjet print quality and speed. We present and demonstrate two models for predicting ink media interactions, a CFD-based approach for predicting the absorption and spreading of droplets upon impact with porous media, and a population balance approach for predicting the flocculation of colloidal ink pigments during the ink deposition process. The CFD analysis takes into account the physical properties of the media such as porosity, permeability and contact, and critical properties of the ink including surface tension and viscosity, as well as the size and velocity of the incident droplet. The population balance model, which is based on Smoluchowski kinetics and DLVO theory, is used to predict the flocculation of ink pigments taking into account the initial size distribution and surface potential of the particles and the ionic strength of the ink. Theoretical and experimental results are presented to quantify these processes. The challenges associated with the quantifying ink media interactions are also discussed. A study is presented of various effects involved in the interactions of pigmented ink with porous media in conventional inkjet printing applications. The study involves the use of two different numerical models, one for predicting the absorption of ink in porous media and another for predicting the flocculation of pigment during absorption. Specifically, computational fluid dynamic (CFD) analysis is used to predict the absorption of an impacting droplet on a porous substrate taking into account the velocity, viscosity and surface tension of the droplet and the porosity and permeability of the media. A population balance model is used to study flocculation of pigments. The models are demonstrated via application to conventional inks and media.