Drop formation from axi-symmetric fluid jets

In DoD inkjet printing, an ink jet is ejected from a nozzle, which forms a liquid filament after breaking up from the nozzle. The stability of this filament must be controlled for optimal print quality. This stability is the focus of the research comprised in this thesis. We start the investigation by constructing a model, which accurately describes the dynamics of a liquid filament (chapter 2). For the model that describes the stability of the liquid filament, we apply 4 assumptions: a) The filament is an axially symmetric body of Newtonian fluid. b) The dynamics of the filament can be described in the slender jet approximation. c) The viscosity and the surface tension of the fluid are constant over time and space. d) We use the sharp interface approximation for the fluid-air interface. The sharp interface approximation leads to a singularity at pinchoff. The pinchoff singularity is regularized using a modification of the surface tension. This modification conserves mass and momentum. With this regularization, the model can simulate jet breakup beyond the pinch-off. The presented numerical model is validated by analytical, experimental and numerical results of the Rayleigh-Plateau instability in chapter 2. In chapter 3 and 4 the contraction and stability of the liquid filament are studied. In chapter 5 an experimental method to measure the ink velocity inside the filament optically is presented. This experimental method is validated using the numerical model presented in chapter 2. Finally in chapter 6, we demonstrate how to obtain a stream of widely-spaced droplets by imposing a superposition of two Rayleigh-Plateau instabilities. The growth of the perturbations and the resulting drop formation are studied with linear theory and a fully nonlinear model, resulting in a deep insight in the jet breakup.