Formation of beads-on-a-string structures during break-up of viscoelastic filaments

Break-up of viscoelastic filaments is pervasive in both nature and technology. If a filament is formed by placing a drop of saliva between a thumb and forefinger and is stretched, the filament’s morphology close to break-up corresponds to beads of several sizes interconnected by slender threads. Although there is general agreement that formation of such beads-on-a-string (BOAS) structures occurs only for viscoelastic fluids, the underlying physics remains unclear and controversial. The physics leading to the formation of BOAS structures is probed by numerical simulation. Computations reveal that viscoelasticity alone does not give rise to a small, satellite bead between two much larger main beads but that inertia is required for its formation. Viscoelasticity, however, enhances the growth of the bead and delays pinch-off, which leads to a relatively long-lived beaded structure. We also show for the first time theoretically that yet smaller, sub-satellite beads can also form as seen in experiments.National Science Foundation (U.S.). ERC-SOPS (EEC-0540855)Nanoscale Interdisciplinary Research Thrust on 'Directed Self-assembly of Suspended Polymer Fibers' (NSF-DMS0506941

Breakup and coalescence of liquid drops

Free-surface flows, and in particular, their tendency to break into smaller drops, or merge to form larger drops are common aspects in several industrial contexts, like printing applications, coating flows, sintering processes, electronics and drug delivery, cell and tissue engineering, and multi-phase flows. Apart from such obvious industrial significance, they are familiar to anyone who has witnessed a dripping faucet, or raindrops falling on the windshield of an automobile. Underneath this veil of inherent familiarity are a whole slew of unexpected dynamics that characterize these processes. Coalescence and breakup, are two prominent examples of finite time singularities which occur owing to the dramatic changes in topology – two initially disconnected masses merge and become one, in the former case; and a contiguous mass of liquid disrupts to form two or more daughter droplets, in the latter. The general scope of my dissertation study is to understand these processes – especially, the rich nonlinear behavior in the vicinity of the singularity, at extremely small length scales that lie in the limit of the continuum approximation. For this study, powerful and well-benchmarked Arbitrary Lagrangian-Eulerian (ALE) algorithms based on the Galerkin/Finite Element Method (G/FEM) to numerically solve either the 1–D slender jet or the 3–D axisymmetric Navier-Stokes equations are developed. Some of the most significant results in this dissertation include the answer to the formation of beads–on–a–string structures in the breakup, and an estimation of the extensional viscosity, of a thinning viscoelastic filament. Another significant result is the discovery of a universal asymptotic initial regime of drop coalescence, where contrary to common knowledge, all three forces – inertial, viscous and capillary – are important. The true significance of this discovery is the similarity in the vicinity of the singularity, of drop breakup and coalescence

Prediction of Interfacial Area Transport in a Coupled Two-Fluid Model Computation

A computer code has been written to predict interfacial area transport within the framework of the two-fluid model. The suitability of various constitutive models was evaluated from a scientific and numerical standpoint, and selected models were used to close the two-fluid model. The resulting system was then used to optimize the empirical constants in the interfacial area transport equation for large diameter pipes. The optimized model was evaluated based on comparison with the data of Shen et al. and Schlegel et al. The optimization shows agreement with previous research conducted by Dave et al. and Talley et al. using TRACE-T, and reduced the RMS error in the interfacial area concentration prediction for the large diameter pipe data from 52.3% to 34.9%. The results also highlight a need for additional high-resolution data at multiple axial locations to provide a more detailed picture of the axial development of the flow. The results also indicate a need for improved modeling of the interfacial drag, especially for Taylor cap bubbles under relatively low void fraction conditions