Session L31: Drops: Dynamic Surface Interactions

Large-droplet deep-pool impact: larger isn't necessarily better

The shape of a droplet impact onto a liquid pool may greatly influence the dynamics of the cavity formation and splashing. Although droplet impact onto a liquid pool has been investigated for relatively small droplets, behaviors greatly change when the droplet is large enough that significant oscillation deformation occurs. We experimentally investigate large droplet impact (1 to 6 cm dia.) onto a deep pool of water. Droplet release height is selected to cover a larger range of Weber numbers from 10² to 10⁵. The droplets oscillate significantly when diameters are larger than the capillary length. The shape and dynamics of the cavity formed by the large droplets are significantly affected by the deformation of the droplet at impact. In general, three different shapes of the impacting droplets occur: prolate, oblate and circular. We show that, for a fixed liquid volume and a fixed Weber number of an impacting droplet, prolate shaped droplets produce the maximum cavity depth whereas an oblate shaped droplet results in a minimum cavity depth. The ratio of cavity width to maximum cavity depth (cavity aspect ratio) is a function of the horizontal diameter of the droplet at impact. Increased horizontal diameter results in a reduced impact energy flux leading to reduced cavity depth.   

Dimple dynamics beneath lubricated droplets

Liquid droplets on solid surfaces with a thin oily lubricant coating have recently emerged as a route for enhanced droplet mobility. Previous experiments have hinted that a dimple of lubricant may be trapped beneath such droplets. However, the details of the evolution of this dimple, including its lifetime and effect on the equilibrium and dynamics of the droplet, remain mysterious. Using a combination of experimental measurements and a simplified mathematical model, we will show that such dimples can be surprisingly long-lived and discuss their effect on the behaviour of droplets on lubricated surfaces.   

Silicone Oil Grafting Optimization for Robust Low Hysteresis Water Repellent Surfaces

Hydrophobic low adhesion surfaces have gained increasing attention due to their excellent self-cleaning and water repellent properties which find applications in many engineering and industrial fields. Several methods to create hydrophobic surfaces which include grafting silicone oil on solid substrates have been reported in literature. Grafting of silicone oil creates polydimethylsiloxane (PDMS) brushes attached to the original solid surface, in our case smooth silicon but other substrates such as copper, aluminium and glass could also be adopted. The surface wettability and adhesion can then be tuned by various parameters such as viscosity, volume and/or number of layers, and temperature of the grafted oil. These surfaces created using the optimized parameters exhibit favourable results in terms of wettability with high contact angles (up to 109^o) and low contact hysteresis (below 1^o) which is comparable to Slippery Omniphobic Covalently Attached Liquid (SOCAL) surfaces or Slippery Lubricant Infused Porous Surfaces (SLIPS). Furthermore, mechanical and chemical stability tests demonstrate the robustness of the grafted coating. These findings provide a basic framework to select appropriate parameters for fabricating low hysteresis hydrophobic surfaces by silicone oil grafting.   

Drop friction on state-of-the-art micro/nanoengineered oil-impregnated silicon micro-pillars and micro-holes

Recently discovered micro/nanoengineered oil-impregnated surfaces have shown great promise in improving the performance of numerous engineering systems ranging from liquid handling in lab‑on-a-chip microfluidics to hydrodynamic drag reduction and heat transfer because of the remarkable droplet mobility that arises from record low contact angle hysteresis (≤2-3°). Past studies on drop friction on hemi-solid and hemi-liquid composite oil-impregnated surfaces has been limited to well-defined silicon micro-pillars. In this work, using a custom-made submicronewton accuracy cantilever force sensor, we experimentally characterize and model drop friction on oil-impregnated micro-holes. The insights gained from this work provide new understanding of the mechanism of drop friction on textured oil-impregnated surfaces and aid the rational design of surfaces that can reduce hydrodynamic drag.   

Direct numerical simulations of vibration-induced drop atomization

Atomization is a fundamental multiphase phenomenon essential in various applications from spray cooling, ink-jet printing to biomedical and drug delivery methods. Vibration-induced drop atomization (VIDA) is one such method where a liquid droplet is placed on a vertically oscillating plate which results in interfacial bursting and spray formation. However, VIDA is sparsely studied because of various intricacies as follow: (1) the poloidal dependence of acceleration on the liquid-gas interface leading to a superposed effect of Faraday and Kelvin-Helmholtz waves due to oscillatory normal and tangential stresses; (2) the non-linear interaction of a such waves form chaotic patterns which characterizes the spray formation (drop sizes and distribution); and, (3) an accurate and robust fluid dynamic model for tracking fast time-scales of the superposed interfacial waves. Hence, in this work, a three-dimensional direct numerical simulation is utilized for the first time as a proof-of-concept of VIDA. It uses a highly parallelized hybrid front-tracking level-set interface capturing method that enables a realistic visualization of the transient interfacial behaviour. The effects of the Faraday and Kelvin Helmholtz waves is demonstrated to characterize the numerous droplet formation events and their velocity distribution.   

Study of Drop Impact on Slippery Lubricant Infused Surfaces

Slippery lubricant infused surfaces (SLIPS) have gained interest because of their anti-icing capabilities. Experiments have shed light on the physical aspect of the water drop interacting with SLIPS, little is known about the dynamics of the drop and SLIPS interaction. Recent studies indicate the problem of cloaking, resulting from surface energy differences between the low energy lubricants and high energy drops. But the water drops impacting on lubricants have different outcomes in terms of spreading and retracting. It is critical to study the dynamic behavior of the cloaking problem since cloaking induces lubricant drainage and thus renders the anti-icing property of the surface. In this study, we report our drop dynamics study to understand the dynamic behavior and the mechanism of deformation of drops and lubricants. A very important aspect of cloaking behavior is captured by varying the interfacial tensions, where Neumann triangle is used at the junction of the three phases (water-lubricant-air) and the resulting force-balance calculation dictates the movement of the triple point. Commonly used lubricant from experimentally published results is used as a basis for our simulation, and such values are then systematically varied to understand the energy differences and spreading coefficients that indicates the cloaking phenomenon of an impacting water drop on a lubricant. Our simulation captures the cloaking phenomena with the same size droplet but with varying speed of the impact at varying interfacial tensions. Through our exploration of the drop impact on lubricants, we observe that varying interfacial tension between the water-lubricant stabilizes the droplet and SLIPS.

    

Measurements of Contact Angle Variations and Resultant Adhesion Force Acting on a Pining Water Droplet Exposed to a Boundary Layer Airflow over a Flat Plate

An experimental investigation was conducted to examine the variation characteristics of the contact angles along the entire contact line for a pining droplet exposed to a boundary airflow over a flat surface. A novel digital image project (DIP) technique was developed to achieve non-intrusive measurements of the three-dimensional (3D) shape of the pining droplet under different test conditions (i.e., by varying the droplet volume and incoming airflow speed). While the variations of the contact angles along the entire droplet contact line (i.e., as a function of the azimuthal angle on test plate) were determined quantitatively based on the DIP measurements, the adhesion force acting on pining droplet was estimated by integrating the surface tension forces along the droplet contact line on the test plate. The DIP measurement results were also used to validate/verify four different theoretic/empirical models available in literature in predicting the contact angle variations and the resultant adhesion forces acting on the pining droplets. This work provided here is believed to the first of its kind to achieve non-invasive measurements of the 3D shapes of wind-driven water droplets, thereby, quantifying the variations of the contact angles along the entire droplet contact lines, which is very essential in determining the adhesion forces acting on the wind-driven droplets over solid surfaces.

    

Hanging Droplets from Liquid Interfaces

The impact of a heavier droplet into a deep pool of lighter liquid is investigated using three-dimensional numerical simulations. Unprecedented to any numerical simulations, we demonstrate that the heavier droplets can hang from the surface of a lighter liquid using surface tension. The impact phenomenon and the evolution of the heavier droplet as a function of its size and release height are explored. A theoretical model is also formulated to understand the role of different forms of energies associated with the hanging droplet. We further solve the force balance equations for the hanging droplets analytically and demonstrate that the results obtained from our simulations match very well with the analytical solution. This research offers opportunities in many areas, including drug and gene delivery, encapsulation of biomolecules, microfluidics, soft robots, and remediation of oil spills.   

Morphing textures by dynamic polymorphic elastocapillarity

We present soft structures which switch their shapes or pattern textures via delicate external forces driven by hydrodynamics, capillarity, and elasticity. We fabricate an elastomeric fin between two parallel walls, and close to a drainage pore at the base on one side to break the symmetry. The drainage pore is used to secrete and drain liquid which interacts with the soft fin. We further break the symmetry of the device by adding an eccentricity such that the fin is closer to the wall on one side, that side without the drainage pore. Depending on the drain rates controlled by a syringe pump, the fins show bimorphic textures due to the interplay between hydrodynamics and capillarity. At the slow drainage, the flexible fin deforms to the narrow side due to the capillary pressure at the narrow spacing. However, the fin flaps to the wide side at fast drainage. This is mainly due to the suction pressure within the soft cell induced by drain dynamics We develop a theoretical model to provide a universal regime map for the texture style of the flapping fins. Further, we present a more complex polymorphism in the array of the flapping fins. This novel switchable texture system can pave the way for various practical implications such as reflective display and multimodal signaling system.

    

Self-lubricated silicones for enhanced atmospheric water vapour collection

Access to fresh water is of significant importance in the context of global warming. Polydimethylsiloxane (PDMS) has a high emissivity in the atmospheric window between 8 and 13 µm, which ensures efficient radiative sky cooling to possibly induce water condensation¹. Self-lubricated polydimethylsiloxane (iPDMS), crosslinked PDMS swollen in silicone oil, displays a self-healing oil layer with increasing thickness (in the µm range) due to surface energy minimization and residual elastomer crosslinking². During water vapour condensation on iPDMS, the interplay between water droplets and the lubricating and droplet-cloaking oil layer insures a rapid growth of the droplets via the synergetic effects of droplet mobility, oil-menisci mediated capillary attraction, and re-nucleation in cleared regions. With an increase of the oil layer thickness, up to an order of magnitude increase of the condensation rate is found in the early stage of condensation due to the continuous nucleation within the cloaking oil and in the cleared regions left by the long-range coalescence events. During gravity-assisted water vapour collection, an increase of the oil thickness is shown to decrease the latency time for shedding (from 1h to 5min) and to increase the collected water volumes (+50%, after 4h).

1. E. Lee and T.Luo, Solar Energy Materials and Solar Cells, 2019, 194, 222-228.

2. N. Lavielle, D. Asker, B. D. Hatton, Soft Matter, 2021, 17, 936-946.