JET INSTABILITY AND DROPLET FORMATION FOR BIO-PRINTING APPLICATIONS

The research conducted in this dissertation was done to obtain a fundamental understanding of the formation of droplets in a drop-on-demand (DOD) inkjetting setup when the ink contains a polymer additive. The objective is to precisely control and even predict the forming dynamics and final droplet characteristics (velocity, size and satellite elimination). A polymeric fluid called sodium alginate has been gaining attention due to its highly bio-compatible nature. Solutions of sodium alginate were used as the ink material and in some cases compared with an equivalent Newtonian fluid (same zero-shear viscosity). The droplets were inspected in one of two ways: 1) allow the alginate droplets to fall into a bath of calcium chloride causing gelation, then observing the gelled droplets (\u27microspheres\u27) with use of a microscope camera; or 2) visualizing the dynamics of droplet formation with a stroboscopic flash photographic system. The DOD waveform and material property effects on the final droplet characteristics are extensively studied. The DOD waveform parameters significantly affected the droplet formation. Since there are many input waveform parameters that can be adjusted, it is a great challenge to fundamentally understand and to optimize the printing process in terms of the DOD process. First, all of the parameters are studied (voltage amplitudes, dwell times, pulse times and frequency) with a statistical method do determine those that are most influential for droplet formation using two different types of input: bipolar and tripolar waveforms. From this it was found that the bipolar waveform is more robust than the tripolar and that the voltage amplitude and the dwell time are the most significant waveform conditions. Then the DOD process was characterized in a single parameter called the ejection velocity (Dong et al. 2005). The velocity of the ejecting fluid during the very first moments of ejection is a function of only the waveform and the material property influence can be neglected. This is true for Newtonian fluids and was found to be true for dilute solutions of alginate. Using this ejection velocity the DOD process can be represented with the Weber (We) number. In micro-scale DOD inkjet printing, there are also limitations for the ink material to be printed in order to obtain successful droplet formation (uniform, single droplets). Since sodium alginate is a polymeric fluid it has a chain-like molecular architecture, with liquid and solid-like properties which classifies it as a viscoelastic fluid. In this work, sodium alginate was characterized in terms of its relevant physical properties. Understanding the ink behavior in terms of its viscoelastic properties will give insight to the fluid limitations for good printability (i.e. printing conditions that lead to successful droplet cases). The alginate solutions were found to lie in one of three regimes according to its concentration: dilute, semi-dilute unentangled and semi-dilute entangled. The alginate was compared to a Newtonian ink to observe the viscoelastic behavior during inkjetting compared to the well-understood Newtonian behavior. The polymer relaxation time was deduced for the alginate solutions and found to be dependent on the concentration and determined with the method used by Haward et al. (2012) for semi-dilute solutions. The viscoelastic properties were summarized with two dimensionless numbers: Ohnesorge (Oh) and Deborah (De) numbers that represent the viscous and elastic effects respectively. With the three dimensionless numbers, a map was constructed as a guide for DOD printing of viscoelastic fluids. The map shows ranges of Oh, De and We where successful droplet cases were obtained. The map applies to other fluids and DOD setups as long as the material properties and the ejection velocity are known. The experimental results are summarized in this map. The works detailed in this dissertation resulted in the following contributions: (1) understanding of bipolar and tripolar waveform performance in microsphere formation, (2) the characterization of a typical bio-ink (sodium alginate), identification of dimensionless quantities that capture the DOD inkjetting process of a viscoelastic ink and creation of a new phase-diagram for droplet formability, and (3) the construction and implementation of a numerical method for modeling DOD viscoelastic jet break-up. The research covered in this dissertation will offer a better understanding of printing a polymeric ink with a DOD inkjetting device