Stochastic Modelling of Drop Coalescence on Non-Porous Substrates for Inkjet Applications

Coalescence between two drops on a substrate is one of the important factors that can affect print quality in inkjet applications. Two stochastic models (constant contant angle mode and constant contact area mode) that consider drop placement error, drop impact, and drop evaporation are proposed for determining the probability of coalescence between adjacently printed drops on nonporous substrates. Experiments are conducted to measure the probability of coalescence with respect to deposition time difference between adjacently printed drops and compared to the predictions of the models. The measured coalescence follows the constant contact angle mode evaporation model during the initial phase of the life of the first drop, which is followed by a mix between the constant contact angle mode and the constant contact area mode models for the remainder of the life of the first drop. This study shows that for probabilities of coalescence between 10% and 80% the constant contact angle mode model can be used to determine deposition time difference threshold values for adjacent drops in applications promoting drop coalescence while the constant contact area mode model can be used for applications avoiding drop coalescence. Further efforts are needed to capture the dynamics of the mixed-model evaporation and to more accurately predict larger (greater than 80%) and smaller (less than 10%) occurrences of coalescence.