Drop-on-demand inkjet drop formation of dilute polymer solutions
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The research discussed in this dissertation was conducted to understand drop formation of inkjet printing with inks containing polymer. Solutions containing a water soluble polymer, poly ethylene oxide (PEO), with different molecular weights and polydispersities were used as inks. A flash photographic technique was used to visualize the whole process of DOD drop formation of dilute polymer solutions. The effects of driving signal, frequency and liquid properties on drop speed, drop size, breakup time and the formation of satellites were studied in detail. The addition of PEO increases the shear viscosity at all molecular weights, but the change is small for dilute solutions. However, the addition of a small amount of PEO can have a significant effect on the DOD drop formation process, increasing breakup time, decreasing primary drop speed and decreasing the number of satellites in some cases. The effects depend on both molecular weight and concentration. At lower molecular weights (14k and 35k g/mol), the effect of PEO was small when the drop formation process for the dilute solution was compared with that of a Newtonian liquid having similar shear viscosity, and the effect of PEO was small even at concentrations large enough that the solution does not fall in the dilute regime. As molecular weight is increased, the effects of PEO on DOD drop formation increase significantly, and the effects of concentration become important. These effects are explained by the fluid elasticity which increases with increasing in molecular weight and concentration. When the liquid jets out of the nozzle, the polymer chains are stretched, and thus depart from their ideal coiled state. As a result, an elastic stress develops in the liquid column and resists capillarity-driven pinch off from the nozzle and is responsible for the decrease in drop speed and longer breakup time. DOD drop formation data were shown to correlate closely with effective relaxation time, proposed by Tirtaatmadja based on Rouse-Zimm theory. When driving voltage amplitude is 44.2 V, two important parameters (breakup time and primary drop speed) in DOD drop formation for solutions containing monodispersed PEO and aqueous solutions containing mixtures of monodispersed PEO were closely predicted by correlation equations involving effective relaxation time . A mixture rule was developed to calculate the relaxation time for mixtures of monodispersed PEO. However, for polydispersed PEO, effective relaxation time was based on viscous molecular weight since the molecular weight distributions of the polydispersed PEO were unknown. When breakup time was plotted versus effective relaxation time for 1000k g/mol PEO, the data did not lie on the same line as that for the 100k and 300k g/mol PEO. This is believed to be due to the molecular weight distributions of the polydispersed PEO. When more than one species are present, viscous average molecular weight does not adequately account for the long chain species making up the polymer sample. DOD drop formation dynamics is highly affected by the actuating waveform, including the driving voltage, waveform shape, and frequency. The effects of parameters (jetting frequency, voltage amplitude and the shape of waveform) characterizing the signal were investigated. The open time and first drop problem were also studied. Research in this dissertation gives a better understanding of DOD drop formation process of polymer solutions, which may lead to improvement of inkjet printing quality for a variety of industry inks and polymer micro scale deposition and patterning in large areas.