The flatbed scanner as a scientific instrument: Spontaneous pattern formation statistics on large-area printed surfaces
(2019)Abstract
Viscous fingering is a long known pattern formation phenomenon1 that is ubiquitous in the ink splitting process in printing technology. We study the hydrodynamic instability of the retracting ink-air meniscus on a large scale using a calibrated flatbed scanner. Air penetrates into the ink, liquid bridges with dominant wavelength λ form, and create finger-like patterns which can effectively be digitized by the scanner (Fig. 1). Viscous fingering leads to surface inhomogeneity. This causes severe issues in e.g. printed electronics. In order to improve process control, we aim to understand the effect of printing velocity, ink viscosity, ink surface tension and printing form elasticity on viscous fingering. To gain statistically relevant data, we make use of a scanner as a large-throughput optical measuring device. We digitize large-area printouts with several hundreds of fingers on it. Byimage processing techniques we analyze characteristic pattern parameters e.g. the dominant pattern wavelength λ. This is typically in the range of a few hundred micrometers. When using elastic instead of rigid printing forms, statistically relevant measurements are laborious because λ changes only very slightly with varying printing velocity. In flexographic printing, which uses elastic printing forms, doubling the printing velocity leads to a decrease of the dominant pattern wavelength by only about 5 % according to a mathematical model of Sauer et al.(in gravure printing with rigid steel printing forms it is 30 %)2. In other words, differences in wavelength in the range of a few micrometers need to be detected. With our experimental results we can verify the model with high precision 3. We demonstrate how a flatbed scanner can be employed for this task.