Session G11: Drops II

Droplet impact onto finite-depth pools

Droplet impacts onto finite-depth liquid layers can be seen throughout industry and nature, from raindrops impinging onto puddles to pesticide sprays coating plants. We present an integrated high-speed imaging and numerical investigation of droplets impacting perpendicularly onto finite-depth pools and thin films, elucidating the post-impact dynamics and secondary droplet formation in this regime. In particular, we explore the rich behaviour and transitions that arise when varying the pool depth and liquid fluid properties, from inherently 3D-effects including "crown collapse", to variations in the generation of an axisymmetric Worthington jet. The complementary strengths of experiments and numerical simulation are exploited throughout in order to constructively explore the physical mechanisms underpinning the exciting dynamics observed.   

Droplet Viscoelastic Splash on a Soft Micropillared Surface

Liquid droplet impact on soft substrate decorated with micropillar array is simulated by finite volume method. Volume of fraction method is coupled with Navier-Stokes solver to rebuild the interfaces between two immiscible fluids. Standard linear solid (SLS) model for viscoelastic material is adopted in the modeling. Moreover, viscoelasticity is implemented on the substrate whose elasticity and viscosity are functions of the micropillar density, which are represented by the Maxwell series of the SLS model. As substrate deformation velocity is treated as Dirichlet boundary condition, the coupling between substrate motion and flow field is critical across different modeling domains. First order Euler time integration is used to obtain substrate deformation velocity. With diverse viscosity and elasticity applied, the deformation of various composite substrate is obtained. Furthermore, splashing threshold is explored by considering Weber number, ambient pressure and other factors. It turns out with Weber number and internal pressure raising, splashing magnitude are enhanced as impact time is shortened.   

Substrate curvature affects droplet splashing thresholds

Understanding the conditions under which droplets splash when impacting dry substrates is crucial in a variety of natural and industrial situations (e.g. coating and inkjet printing), usually to prevent the formation of smaller droplets with indiscriminate trajectories and unwanted aerosolisation of potentially-hazerdous fluids. Whilst splashing propensity is known to depend on various parameters, including droplet fluid properties, the surrounding gas dynamics, and substrate wettability, the influence of substrate geometry is not well understood. In our high-speed imaging study, we precisely delineate splashing thresholds for axisymmetric impacts across a wide range of dry curved substrates and impact conditions. By quantitatively assessing the post-impact spreading dynamics, including extracting dynamic contact angles, we propose a physical mechanism to underpin the curvature-induced modification of the splashing threshold seen. Under the framework of our proposed physical mechanism, we present a consistent parameterization of the threshold observed across the whole range of concave, flat, and convex substrates studied.   

Impact Stability of Particle Coated Droplets

The intactness and mechanical stability of liquid droplets are of utmost importance because of their applicability in various chemical and bioreactors. The powder-coated droplets (Liquid Marbles) can provide a way to increase robustness for such applications. Here, we report the deformation and stability of such liquid marbles after impacting them onto non-wetting surfaces. Not only do liquid marbles show higher robustness at lower impact velocity, but they also show exceptional stability at higher velocity impact where droplet fragmentation is unavoidable. The reason behind the robustness at lower velocities is found to be the lesser adhesion between liquid marble and surface. The lower adhesion helps in early lift-off from the surface; thus, liquid marble cannot stretch beyond the critical ratio specified by Rayleigh-plateau criteria, and fragmentation is delayed. In contrast, the higher velocity intactness was due to the early fingering instability (Liquid Flower). We found the active role of jamming and particle distribution in such early liquid flower formation, responsible for higher viscous losses and mechanical stability.   

Droplet impact on thin fiber veils

Drops impacting on fibrous materials are ubiquitous phenomena that are interesting for a wide range of applications, ranging from bloodstain pattern analysis to coating of fibrous materials. While current literature extensively covers the capture phenomenon of a drop impacting a single fiber, the capture of drops by thin fibrous media has received less attention. We study experimentally the impact of droplets on thin fibrous veils. We consider both non-woven fibrous textiles and woven grids of controlled geometry. We look for a criterion for droplet capture as a function of the drop radius and velocity, as well as the characteristics of the textile (fibre radii and densities). The observed dynamics and the final wet diameter are a combination of spreading at the surface of the veil, imbibition within the fibrous matrix, and flow through the thin porous textile.   

Possible, Impossible and Expected Diameters and Flow Rates of Droplets in Aerosols and Sprays

In our recent work we demonstrated a novel liquid atomization technique to produce micro-sprays which combine into an aerosol of submicron-diameter droplets for pure solvents, suspensions, and solutions. Our droplet generation process is based on disintegration by gas jets of thin liquid films formed as bubbles on a liquid surface. For this system and, in general, for any other droplet aerosol or spray generation system, one of the main questions is how to theoretically predict possible, impossible and expected droplet diameters and flow rates which the system will produce at different regimes. In this work we show that the diameters and flow rates of the produced droplets are governed by the interplay of process timescales including capillary breakup, liquid viscosity, and atomizing gas pressure. Timescale ratios can be converted into the ratios of specific energies and into the ratios of specific energy rates provided by the gas and dissipated by the atomized liquid. Using those ratios, we developed a new theoretical approach to determine the lower and the upper limits and the expected values of droplet diameters and flow rates in a droplet generation process. We introduced atomization diagrams to graphically visualize the respective regions for liquid atomization. Theoretically predicted and measured droplet diameters and droplet flow rates for various liquids (water, gasoline, diesel, and solutions of sodium alginate and sodium benzoate) were in a good agreement for aerosols of submicron-diameter droplets generated by our liquid atomization process, as well as for sprays of regular-size droplets produced by conventional pressure nozzle. Finally, we demonstrate that our approach can serve as a theoretical framework for comparison between different aerosol and spray production techniques.   

Breakup dynamics of a liquid droplet through high-speed airflows

The atomization of liquid droplets through the interaction of aerodynamic forces has been widely exploited in the fields of liquid jet atomization, agricultural spraying, combustion engines, and aerospace applications.  The dynamics of droplet-breakup process is classified into two phases: i) initial shock interaction ii) droplet breakup regimes. Phase I presents the initial impact of shock-wave with the droplet and formation of reflected, transmitted, and refracted waves adjacent to the droplet surface. Phase II includes the deformation and breakup of the droplet process. In phase I, the droplet holds in its original shape, where the changes in surrounding flow conditions were influenced by the external shock-induced air-flows the shear-induced breakup process. In the present study, droplet deformation, break-up patterns (such as multi-bag, SIE) were investigated using visualization techniques of shadowgraph and schlieren. The range of We numbers was applicable in the above-mentioned applications for convenient atomization of the liquid droplets. This study idealizes the case of complex methodologies involved in the manufacture of metal powder through closed coupled atomization.    

Rectifying jet breakup by electric forcing

Droplet microfluidics is a technology used in a wide range of applications relying on the production of droplets uniform in size. Monodisperse droplets are produced by a flow focusing junction in the dripping regime. Increasing the throughput of production by increasing the flow rate eventually leads to a transition from the dripping to the jetting regime. A fluid jet is unstable to external perturbations resulting in a polydisperse emulsion. Imposing external perturbations on a jet can alter the break-up dynamics resulting in a more monodisperse emulsion. Electric fields have been shown to be a method which offers a fast way to affect the behaviour at the flow focusing junction in the dripping regime.

We experimentally investigate how an amplitude modulated electric field imposes a local perturbation on a hydrodynamic jet in a flow focusing junction. We manufactured four electrodes symmetrically around the flow focusing junction. By modulating the electric field applied to the upstream electrodes, we impose an interfacial perturbations, controlled externaly. We investigate the effect of the modulation frequency and amplitude on the break-up frequency of the jet and find that the monodispersity of the produced droplets is increased when the modulation frequency is close to the natural break-up frequency of the jet. Further we find that this increase relates directly to the amplitude of the electric field. We are able to decrease the standard deviation of the distrubtion of the droplet diameter a 3.5-fold as compared to unperturbed jet.

The results indicate that perturbing a jet by amplitude modulated electric fields is an efficient method to increase the monodispersity of droplets produced in the jetting regime. This method could be further integrated in high-throughput droplet production systems.   

Synchronization of interacting waves in microfluidic oil-water interfaces

Two oscillating fluid-fluid interfaces can interact hydrodynamically and serve as a model system to study synchronization behavior in fluids. Previously we have discovered the simultaneous breakup of droplets (in-phase synchronization) from two interacting oil-water interfaces in a microfluidics double T-junction, which transits from the alternating breakup mode (anti-phase synchronization) depending on the relative distance between the droplet protrusions. In this work, we investigate the intermediate regime between droplet generation and continuous jetting. In this regime, the oscillating interfaces of wavy shapes also exhibit hydrodynamic interaction depending on the distance between the interfaces. When the interfaces are close, we observe an anti-phase synchronization of the waves similar to G. I. Taylor's synchronous infinite sheets. However, when the distance between the interfacial waves increases, the state of in-phase synchronization can be stable, reminiscing biflagellate cells' motion. We study the relations of hydrodynamic synchronization modes of interfaces to parameters such as the flow rates, viscosity of fluids, and channel dimensions, as well as the shapes of the waves.