Impact Dynamics of Compound Droplets Encapsulating Bubbles

Abstract

The impact of compound droplets on the surface plays a crucial role in various applications, including surface coating, spray cooling, and ink-jet printing. Specifically, gas-liquid compound droplets, characterized by their dual-phase nature comprising both liquid and gas components, have garnered significant attention in recent years. This interest stems from their utility as ideal model systems for numerous practical processes, such as the design of thermal barrier coatings and the 3D printing of foam materials. Motivated by the need to understand the fundamental behaviors of air-encapsulated droplets upon impact, this thesis investigates the dynamics of compound droplet impacts with a specific focus on gas-liquid systems, encompassing single/double bubbles and foam-structured configurations. Here, we systematically investigated the effects of gas phases on the droplet impact behavior and how to dynamically control the droplet behaviors by simply altering the gas phase characteristics--quantity, size, and spatial distributions.

The first part of this work examines the dynamic behavior of droplets containing a single bubble (hollow droplets, HDs) impacting non-wetting surfaces. Experimental and theoretical analyses reveal that HDs exhibit significantly reduced rebound coefficients compared to conventional droplets. The capillary flows induced by the deformed inner bubble surface counterbalance those driven by the outer droplet surface, resulting in a reduction of effective take-off momentum. We propose a double-spring system model with reduced effective elasticity for hollow droplets, wherein the competing springs exhibit distinct behavior from the classical single-spring model used for single-phase droplets. This rebound suppression can be achieved solely by modifying droplet inner phase parameters without altering surface or liquid properties, and the rebound height can be controlled by regulating the bubble volume fractions and impact velocities.

In the second part, we introduce a pair of bubbles into a liquid droplet (bubble-inclusion droplets, BIDs) to investigate the impacting behavior on non-wetting surfaces. BIDs exhibit concurrent spreading and retraction during impact, adopting a cross shape rather than a circular one. The contact time of the BID impact is tunable by adjusting the bubble volume fraction and impact velocity. The early onset of retraction enhances upward liquid momentum, facilitating prompt droplet takeoff and significantly reducing both the contact area (up to 50%) and contact time (up to 60%). Considering the anisotropic capillary effects, we propose an anisotropic spring model that effectively describes and predicts the observed contact time dynamics.

In the third part of this thesis, we extend the scenario to foam droplets that contain multiple microbubbles. The foam droplets show the ability to spontaneously jump on hydrophilic surfaces at 120°C, activating the Leidenfrost state at temperatures significantly below conventional transition points. The experimental observation suggests that the spontaneous jump is triggered by the explosive rupture of collective microbubbles at the droplet-substrate interface. Systematic experiments confirm that the initiation temperature is tunable by controlling volume fraction and individual bubble size.

From rebound suppression in single-bubble systems to symmetry-breaking regimes in dual-bubble configurations, as well as the self-initiated vapor sustainment of foam droplets, the demonstrated ability to tune contact times, adhesion forces, and momentum redistribution realized by air-liquid compound droplets advances our fundamental understanding of multiphase impact physics while providing a blueprint for designing droplet-based systems with on-demand performance. These findings can extend the applications of shock-absorbing hollow systems, targeted thermal management systems, and other broader horizons.

Date of Award	3 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pingan ZHU (Supervisor)