Coalescence dynamics of PEDOT:PSS droplets impacting at offset on substrates for inkjet printing

Abstract

The dynamics of coalescence and consequent spreading of conducting polymer droplets on a solid substrate impacting at an offset are crucial in understanding the stability of inkjet printed patterns, which find application in organic flexible electronic devices. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) dispersion in water is a widely used commercial conducting polymer for the fabrication of electron devices. The effects of droplet spacing, impact velocity, substrate hydrophilicity, polymer concentration, and charges on the coalescence of two sessile droplets have been experimentally investigated, and the characteristics of dynamic spreading during the coalescence process are determined through image processing. The equilibrium spreading length of the coalesced droplets decreases with concentration and spacing of the droplets, revealing the necessity of optimum fluid properties (viscosity and surface tension) for the stability of the desired pattern. The droplet's impact energy governs the maximum extent of spreading and receding dynamics, as the velocity gradients developed in polymer droplets during coalescence are a function of the inertia of the fluid elements. Hydrophilicity affects the maximum spreading extent but it has no influence on the equilibrium droplet diameter. The spreading length dynamics of charge-neutralized PEDOT:PSS is found similar to the charged droplets, which show that the charged nature of the polymer does not affect the coalescence behavior. Furthermore, different spreading regimes are identified and the governing forces in each regime are described using a semianalytical formulation derived for the coalescence of two droplets. The model has been found to accurately provide insight into the various mechanisms that play a role during the complex spreading event.