The fracture of yield stress fluid jet in air and in viscous fluids

One of the largest family of complex materials are represented by fluids with yield stress. In this category are included creams, pastes, greases, gels, in general materials which start to flow at a certain value of the imposed shear stress. A common characteristic of yield stress materials is the association of material instability with the onset of the fluid behavior. The present paper is concerned with the experimental investigations and numerical modeling of the dynamics of a yield stress fluid jet in air or immersed in viscous/viscoelastic liquids. A cream jet at constant flow rate is generated through a capillary with a syringe pump and the visualizations of its motion in a vessel filled with an immiscible fluid is performed with normal and high-speed cameras. One goal of the study is to test the capability of the VoF code to simulate the jetting phenomena, the rheology of the sample being modeled with the Carreau equation and the Herschel-Bulkley relation, respectively. The visualizations show the specific yield stress instabilities from jetting to coiling, buckling and fracture.