Session L12: Drops: Jetting and Break-up

Vortex dynamics in an oscillating falling drop dripped from a faucet

The oscillations of a falling drop dripped from a faucet is investigated through experiment and direct numerical simulation. In the present study we consider the whole process of drop growth, detachment and oscillation. Excellent agreement between numerical and experimental results is achieved. The development of the drop before pinch-off is found to be well predicted by the theory of static pendant droplet. The influence of pinching to the high-order mode oscillations is investigated. The temporal evolution and spectra of the drop top and bottom radii show that Rayleigh's theory is valid in predicting the frequencies of high order modes. The interplay between capillary oscillation and the external flow induces a complex unsteady flow that is fundamentally different from the classic circulation in the form of Hill's vortex. The vortex dynamics and the resulting chaotic mixing within the drop are investigated by swirling strength (λci) criterion. 

Lattice Boltzmann simulations of droplet dynamics and break-up in generic time-dependent and turbulent flows

We study the behaviour of droplet break-up in generic time-dependent shear flows via a multicomponent Lattice Boltzmann algorithm. Our work can be seen as an extension to studies on the influence of inertia on droplet break-up (Y.Y. Renardy, V. Cristini, Phys. Fluids 13, 7 (2001)), whereas we deal with cases of time-dependent droplet break-up, which arise when the temporal rate of change of the shear intensity is of comparable size to the droplet relaxation time. This work is a continuation to a previous study on stable time-dependent droplet dynamics (F. Milan, M. Sbragaglia et al., Eur. Phys. J. E 41, 6 (2018)). Furthermore we investigate a more realistic model enabling the study of fully resolved dilute droplet suspension dynamics under the influence of turbulent fluctuations via multicomponent Lattice Boltzmann simulations, which we compare with previous studies containing non fully resolved droplets (L. Biferale, C. Meneveau, R. Verzicco, J. Fluid Mech. 754, 184 (2014)).
  

Internal versus External Droplet Breakup

A liquid droplet can be broken if exposed to external gaseous flows. If the gaseous flow is injected into the droplet the droplet will go through an internal breakup process. Whereas if the gaseous flow is injected over the droplet, it will go through an external breakup.  In this study, we have considered both types of droplet breakup and have compared their outcome with each other. The external droplet breakup is dominated by shear, whereas the internal droplet breakup is mainly pressure driven. Different mechanism of both internal and external breakups are categorized and reported based on jet and droplet Weber numbers.
  

Laser-induced cavitation and jetting in thin films

Laser-induced cavitation has a wide range of applications, ranging from needle-free injection to the 3D printing of flexible electronics and biomaterials. We investigate laser-induced cavitation in Newtonian films with a thickness on the order of the size of the cavitation bubble, where the bubble interacts with both the solid and the free surface. We experimentally study the dynamics of the bubble and the free surface, using high-speed imaging. In particular, we visualize the simultaneous interaction of the expanding and collapsing bubble with the free surface and the solid boundary. We further use boundary integral simulations to model the problem.   

Sub-threshold laser jetting via flow-focusing with transient meniscus formation

Increasing the printing resolution of drop-on-demand jet-based printing and deposition techniques is important for many industrial applications. In this study, we present two methods to minimize the jet size and reduce the laser threshold energy of a variant of the laser-induced forward transfer (LIFT) process called blister-actuated LIFT (BA-LIFT). In one method,  we use a magnetic shaker to introduce Faraday waves into the thin liquid film to be printed. At the resonance frequency, the acoustic waves lead to the formation of sinusoidal patterns and a transient meniscus on the liquid film surface.  In the other method, we use laser pulses to create capillary surface waves on the liquid film surface to form a transient meniscus. In both of these cases, a subsequent laser pulse is focused on the substrate at the right time to cause flow-focusing.  We demonstrate experimentally and computationally that transient meniscus formation enables jetting at lower laser pulse energies and leads to the ejection of smaller droplets.   

Surfactant-driven escape from endpinching in nearly-inviscid filaments

Routinely encountered in spraying and drop-on-demand applications, highly stretched liquid filaments are known to exhibit complex and counterintuitive dynamics as they recoil. Instead of simply retracting to a sphere, low-viscosity filaments of aspect ratio exceeding a critical value undergo localized pinch-off near their two ends, a phenomenon called endpinching. However, surfactant-free, Newtonian filaments of intermediate viscosity have been shown to escape endpinching even when in advanced stages of pinch-off.  It has been hypothesized that this unexpected behavior can be attributed to a viscous mechanism, and known to accompany the formation of a vortex ring inside the filament. Here we show that strikingly similar 'escape' events can also be induced in nearly-inviscid filaments when the interface is covered with surfactant. Simulations of the full 3D-axisymmetric equations reveal the complex sequence of processes that lead to the final escape and conclusively identify the dominant role of Marangoni stress. Further, we observe how the escape processes occurring in two starkly distinct physical systems, viscous surfactant-free filaments and inviscid surfactant-laden ones, are manifestations of the same physical phenomenon brought about by two distinct mechanisms.
  

Tip streaming of jets emanating from Rayleigh breakup of a charged viscous droplet

An isolated liquid drop charged beyond its Rayleigh limit becomes unstable and emits a fraction of charge and mass in the form of a jet which further disintegrates into progeny droplets. The formation of conical tips and the associated jet emission are the complex fluid dynamical phenomena and are still unclear. When a drop is modelled as a perfect conductor and solved using axisymmetric boundary element method (BEM) it is observed to form sharp conical ends where the numerical approach fails to predict the jet emission due to the occurrence of shape singularity. This indicates the dominance of normal electrical stresses over capillary stresses. To predict the jet formation in conducting drops, a modified electrostatic model is used in this work by including the surface charge dynamics. This accounts for the finite charge relaxation timescales over which the drop surface is charged as well as the convection of charges by the interfacial flow. The simulations show that it gives rise to the tangential electrical stresses which are expected to exert an axial momentum on the fluid and accelerate the proto-jet out of the tip of the drop. The jet grows progressively and forms progenies at the tip whose sizes are observed to vary inversely with the conductivity of the liquid drop.