Coalescence of sessile droplets is studied experimentally with water-glycerin mixtures of different viscosities. Effects of viscosity on the dimensionless spreading length (Psi) and the center-to-center distance (L) are investigated for two droplets; the first droplet (D-s) is stationary on a substrate and the second droplet (D-0) landing at a center-to-center distance L from the first droplet. For a low viscosity fluid, Psi is maximum when L approaches zero (or lambda -> 1, where lambda = 1 - L/D-s), which represents a head-on collision. For a high viscosity fluid, Psi is minimum when lambda -> 0.6. The effect of lambda on line printing for various viscosities is also examined by printing multiple droplets. We found that the larger the viscosity, the less the breakup between droplets; viscosities smaller than 60 wt% glycerin yielded line breakup. The overlap ratio of lambda > 0.3 produced not a line, but a bigger droplet or puddle because of coalescence. Data obtained in this work can provide insights for the fabrication of conductive microtracks or microinterconnects in printed-electronics applications where a line breakup between droplets would lead to an electrical circuit short. (C) 2013 Elsevier Ltd. All rights reserved.