Dripping Dynamics of Suspensions

Abstract

The study of droplet formation dynamics of Newtonian fluids and non-colloidal suspensions has significant industrial applications. Experimental studies on the influence of fluid properties such as Newtonian and non-colloidal particle suspension on drop formation have resurged in the last decade, with growing interest in combustion, inkjet printing, and medical applications. The dynamics exhibited by the simple Newtonian liquid drop evolution show complex structures in both the dripping faucet and jetting regime. The characteristics of these regimes depend on the fluid properties, nozzle diameter, and flow rate. Recent research has shown that the presence of particles can significantly affect the droplet characteristics such as regime transition (dripping to jetting) that affects the droplet formation frequency and droplet size distribution. In the dripping regime, the droplet pinchoff occurs near the nozzle in the presence of gravity. This pinch-off occurs once the surface tension forces no longer balance the weight of the droplet. As the flow rate increases, pinchoffs of the droplet transit from the dripping faucet regime to the jetting regime. This transition from the dripping to the jetting regime occurs above a critical flow rate. The parameters that govern this transition are the liquid properties and the nozzle diameter. At the critical flow rate, the liquid jet evolves from a dripping to a jetting regime through a transition regime. Here, the temporal evolution of the liquid jet at the critical flow rate below which the dripping regime or droplet formation regime exists is studied. Shadowgraphy was used to show how the liquid jet grows through this transition regimes and different mechanisms, such as tip contraction and bulb formation. The escape of the pinch-off mechanism that causes the bulb to oscillate as the liquid jet tip moves in the transition regime. Such regime transitions were experimentally observed for wide ranges of liquid properties. The nonlinear calculations were used to show that the critical flow rate at which dripping-to-jetting transitions occur reduces for a given nozzle diameter as a function of fluid properties. The study also reveals that the temporally observed initial dripping regime disappeared to reach jetting at lower flow rates in viscous fluids. The study also extended to control the regimes at this critical flow rate. Interestingly, the results obtained allow us to conclude that, based on the velocity perturbation parameter combination, the pinch-off can be controlled to be in the dripping or jetting regime.

Before studying the dripping dynamics of suspensions where particles are homogeneously distributed in the fluid, we study dripping of the fluid over a granular chain. First, flow over a cylindrical fibre is studied as a function of nozzle diameter, fibre diameter and inlet flow rate. In this configuration, instead of pinchoff, a liquid film forms on the fibre along with the droplet formation. The liquid film flow is unstable and results in formation of droplets travelling downstream. Such flows are ubiquitous in industrial applications for coating and droplet capturing in the condensation process. Both the liquid jet from a nozzle and film on cylindrical fibre configurations are driven by the Rayleigh Plateau instability mechanism. We investigate the effects of fibre morphologies, such as single bead and granular chain with torus bead, on liquid film evolution using experimental and axi-symmetric numerical simulations with a one-fluid formulation. We introduce a non-dimensional parameter, ’bead ratio’, that is, the ratio of bead diameter to the film thickness. When both the bead ratio and its distance from the nozzle exceed a critical value, the selection mechanism for the mode of thin film flow leads to the development of ’dominating’ waves: from regularly spaced droplets to coarsening or droplet merging. The two mechanisms, influenced by the BR for a single bead fibre, contribute to droplet merging are: the formation of the downstream healing length, which influences the initial stage, and its oscillating behaviour resulting in droplet merging. When the bead position is away from the healing length, far from the nozzle, the transient simulations capture the behaviour similar to the finite amplitude perturbations at the inlet. However, when the bead is within the healing length, the film evolution has only a coarsening effect on the droplet spacing. It is significantly altered when the bead spacing on a granular chain is less than the droplet spacing of a Rayleigh-Plateau regime. Subsequently, we study the dripping dynamics of suspensions using particles of sizes 80 – 600 microns. The influence of the particle volume fraction on the droplet evolution in the dripping regime and dripping-jetting (i.e. both dripping to jetting and jetting to dripping) transition is investigated. First, the influence of the increase in particle volume fraction at a fixed nozzle-to-diameter ratio is studied near the dripping-jetting transition flow rates. Experiments were conducted by increasing (forward sweep) and decreasing (reverse sweep) of the flow rate. The study includes the capturing of successive droplet pinch-offs in a dripping regime, dripping to jetting, and jetting to dripping transitions. With an increase in the particle volume fraction, the transition from the dripping to the jetting regime occurs through a simple dripping regime. However, for Newtonian liquid (φ = 0), a chaotic dripping regime is observed before the jetting regime. The particle volume fraction also impacts the hysteresis during the transition. With an increase in the particle volume fraction, the jetting regime has occurred at early flow rates in the forward sweep. Similarly, in the reverse sweep of flow rate, the jetting to dripping transition occurred at a lower flow rate than the dripping to jetting transition. This transition results in the widening of the hysteresis loop of flow rate with the pinchoff length. The frequency of droplet pinchoff decreases with an increase in the particle volume fraction. The transition from dripping to jetting is observed to have the recurrent escape of pinch-off events as the jet length changes, influencing the droplet size distribution. As the particle volume fraction increases, the droplet size distribution between the dripping and the jetting regimes decreases. The findings of the thesis are of great relevance in industries such as cosmetics, pharmaceuticals and particle based 3D printing techniques.