Session Q30: Drops: Coalescence II

Do Surfactants Prevent the Partial Coalescence of Droplets?

When a droplet approaches its homophase, the two liquid bodies will coalesce upon contact. Under certain conditions, the droplet does not fully join the homophase and partial coalescence occurs, where a smaller droplet, forms. In this work, we investigate the partial coalescence of aqueous droplets with a liquid/liquid interface in the presence of surfactants. The experiments were carried out with 5 CST Silicone oil as organic liquid and pure water as the aqueous phase. Span80 was added to the oil with a mass ratio to silicon oil from 0 to 5e-5. It was found that for high surfactant concentrations over 5e-5, surfactants prevented partial coalescence by significantly deforming the interface. For low surfactant concentrations of 1e-5, partial coalescence occurs. At medium concentrations of 2e-5, however, a new phenomenon occurs, where multiple secondary droplets form during partial coalescence. The surfactants resulted in a thin liquid cylinder, where the Rayleigh-Plateau Instability occurred. Due to the instability, the cylinder broke into two or three droplets. Numerical simulations were also conducted to help analyze the mechanism driving the generation of multiple drops. 

    

Droplet-droplet interaction and coalescence mechanism of water droplets on textured oil-impregnated surfaces

Droplet-droplet interaction and coalescence mechanism on lotus leaf-inspired textured hydrophobic surfaces has been studied extensively for decades and the governing physics is well understood. The fundamental physics, however, changes on state-of-the-art micro/nanotextured oil-impregnated surfaces due to the presence of wetting ridge that forms due to the imbalance of forces at the contact line. In this study, using high-speed visualization, we show that water droplets coalesce in two stages: surface tension-mediated droplet-droplet interaction (pre-coalescence attraction) followed by coalescence. The two stages of coalescence appear as peaks in the velocity and acceleration of the droplets. We modeled the coalescence mechanism using the standard damped mass-spring system by treating the water droplets as squishy spheres. Our results show that the period of oscillation of the droplets scales with the droplet radius to the 3/2 power. We also modeled the time required to squeeze out the oil separating the droplets after the initial attraction, which scales with the oil thickness to the -3/2 power. This work provides new understanding and unveils the fundamental physics that governs droplet dynamics on textured oil-impregnated surfaces.

    

Numerical Investigation of the asymmetric coalescence of spreading dissimilar microscopic drops

Interaction of dissimilar drops is encountered across various fields of science with a wide range of applications including additive manufacturing, photonic crystals, lab-on-a-chip, etc. Despite the ubiquitous nature of such interactions, the dynamics of such multi-material coalescence of non-Newtonian polymeric drops is not well explored. Understanding such multi-material coalescence - which often involves spreading, coalescence, and mixing- is crucial in understanding the post-deposition kinetics of droplet-based multi-material direct ink writing processes. In this study, we probe the coalescence dynamics of two different polymeric, shear-thinning, power-law obeying micrometric drops using Direct Numerical Simulation (DNS). We observe non-monotonic migration of the liquid-liquid mixing front of the miscible coalescing drops over time which is influenced by the spreading, diffusive, and Marangoni effects. We quantify the growth of the mixing front and the mixing region which shows distinct regimes depending on whether the mixing is diffusion-limited or Marangoni convection-driven. These results would provide critical insights for the design of miscible multi-material systems.

    

Coalescence dynamics of ferrofluid droplets suspended in non-magnetic fluid medium

We experimentally reveal the coalescence dynamics of ferrofluid drops suspended on another fluid medium in the absence or presence of an external magnetic field. By tuning the initial droplet size, viscosity ratio between ferromagnetic droplet fluid medium and non-magnetic surrounding fluid medium, magnetic field intensity, we can alter the coalescence hydrodynamics of the droplets. High speed visualisation of the three step phenomena: droplet approach, film drainage and film rupture engendering coalescence reveal fascinating results. The influence of drop initial size, viscosity ratio between droplet fluid and surrounding medium fluid, and strength of applied magnetic field on the drop approach speed, droplet aspect ratio and drop fusion dynamics, are highlighted. The aforementioned parameters are elucidated in terms of non-dimensional numbers such as Weber number and magnetic bond number. The magnetohydrodynamic stresses generated due to the influence of magnetic field and the interfacial tension stress between drop phase and surrounding fluid phase governs the interface profile of droplets suspended in another medium with no fluid motion. The externally applied magnetic field deforms the droplet shape and the surface tension force resists the deformation. The drop deformation is dependent on applied magnetic field and in turn the magnetic field profile inside and around the droplet is dependent on the shape of the droplet. This two-way coupling of the interfacial drop shape and variation of magnetic field is crucial for analytical modelling of the hydrodynamics. The results obtained from the present study can be extended towards the understanding of several applications such as emulsion and oil recovery techniques, to name a few.   

A simple and fast microfluidic control of two Hele-Shaw droplets towards coalescence

We present a simple and fast strategy to steer two channel-spanning (Hele-Shaw) soft particles along independent and arbitrary paths by utilizing the hydrodynamic force in a six-port, two stagnation point microfluidic device. Using the conformal mapping technique, the analytical solution of the flow field that accounts for the perturbation of the flow due to the hydrodynamic interactions between the particles was developed and implemented in the control loop to calculate the flow rates. In this method, the particles move along the straight line connecting their instantaneous position to the final position, at the velocity prescribed by the user through a single control parameter. The control parameter that is the ratio of the time required for fluid motion to the time scale of the control can also adjust the number and location of the stagnation points. These features were exploited to design and perform controlled head-on and glancing collisions for ~100 μm radius Hele-Shaw perfluorodecalin drops moving at O(100 μm/s) in silicone oil. The coalescence time between two Hele-Shaw drops undergoing a head-on collision in a dimpled mode was found to be independent of the strain rate of the hydrodynamic flow. For Hele-Shaw drops undergoing glancing collision on the other hand, the dimensionless coalescence time increases with the capillary number (ratio of the viscous to interfacial force) and reduces with the initial offset.

    

Coalescence of Polymeric Droplets

Droplet coalescence is an energy minimization process ubiquitous in nature and industry. For Newtonian droplets, the dynamic equilibrium is regime-wise universal with power law exponent b=1 in the viscous regime and b=0.5 in the inertial regime. The neck evolution is governed by the balance of capillary, inertial, and viscous forces. However, our recent study on polymeric droplets has reported the process as regime-dependent. These regimes classified based on concentration ratio c/c* are namely the inertioelastic(IE) c/c*<c_e/c*, viscoelastic(VE) c_e/c*<c/c*<20, and elasticity-dominated(ED) c/c*>20. Our results report the neck evolution radius R to scale as R~t^b, where b is constant in the IE and VD regimes and has a monotonic decrease in the ED regime. Based on this regime-dependent behavior of exponent b, our work proposes a possible novel method named Rheocoalescence to find the characteristic relaxation time λ of polymeric solutions.   

Droplets coalescences induced by an impact on a breath figure.

A breath figure describes the droplet pattern formed when a vapor condenses onto a surface. When vapor is constantly provided, the average droplets radius increase. This evolution has been actively studied in the last decades, to develop anti-dew surfaces and dew recovery systems.

We present here the effect of a mechanical impact on a solid substrate supporting a breath figure. A falling projectile impacts the top of the plate, the droplets being on its bottom part. We changed the drop height of the projectile and we observed at different locations the evolution of the breath figure (the droplets size distribution) with time and with a parameter we introduced: the droplet number reduction. We show that, for a given mean radius of the droplets, when the acceleration of the substrate exceeds a threshold, the final number of droplets starts to decrease and keeps on decreasing as acceleration is increased. We interpret this result knowing that droplets vibrate, their contact line unpin above a threshold in acceleration, which make them contact and coalescing with neighbours giving birth to a liquids networks, eventually fully connected. For a better understanding, we then focus on the coalescence dynamics between only two water droplets partially wetting under vibration.   

Directional Transport of Coalesced Droplets on Superhydrophilic Wires

For various applications, including water harvesting, anti-icing, and condensation heat transfer, it is essential to achieve directional droplet transport on solid surfaces. A vast number of prior studies have reported that a smaller droplet moves towards a larger droplet when they coalesce into a single droplet on non-wetting flat surfaces. Our recent findings reveal the opposite-directional droplet transport following coalescence on superhydrophilic, prewetted wires. When the droplets contact each other on the wire, the merged droplet moves to the direction where the small droplet was placed. This counter-intuitive droplet transport is attributed to the difference between the shear stress in the two parent droplets. The shear stress difference is generated from the liquid flow due to the local pressure difference between each droplet and the liquid bridge linking the two droplets. As such stress is inversely correlated to the thickness of two parent droplets, the smaller droplet remains relatively pinned during the coalescence while the larger one easily migrates. On vertically positioned superhydrophilic wires, merged droplets can move upward based on the mechanism, overcoming the gravitational force when the Bond number is smaller than unity.