Impact Dynamics of Solid-Liquid Compound Droplets

Abstract

The impact dynamics of droplets on solid surfaces lie at the nexus of fluid mechanics and underpins a wide range of natural and industrial processes, including raindrop erosion, heat and mass transfer, inkjet printing, agricultural spraying, self-cleaning, and energy conversion. The prevailing paradigms for manipulating droplet impact predominantly rely on substrate engineering or external conditions, often constrained by limited adaptability in practical applications. This thesis advances an alternative fluid-side route by integrating solid particles into liquid droplets to form heterogeneous core-shell droplets encapsulating particles (DEPs). Through systematic investigation, this thesis consolidates a unified framework of particle-mediated impact regulation, enabling the tailoring of impact behavior on demand. Three distinct outcomes are successfully demonstrated: breaking the contact time limit (ultrafast bouncing), overcoming wetting hysteresis (hydrophilic rebound), and suppressing vapor levitation (high-temperature immobilization).

First, ultrafast rebound was achieved for DEPs, yielding unprecedentedly short contact times of 0.3 ms and 0.05 ms using polydimethylsiloxane and glass particles, respectively. We observed that this minimization occurs at relatively low impact velocities, where the rebound dynamics are governed predominantly by the encapsulated particle. As a result, the contact time is instead controlled by the impact velocity, enabling the droplet to transcend the conventional inertial-capillary limit. With increasing velocity, a transitional regime emerges in which the particle and liquid shell synergistically govern rebound process, leading to a linear increase of contact time with impact velocity. Once the second critical threshold is exceeded, the influence of inner particle weakens and the contact time converges to a capillary-dominated constant. These findings offer a new insight into the impact dynamics of solid-liquid coupled systems, particularly relevant to the understanding of icing droplet impact and the mitigation of icing-related challenges.

Second, to overcome intrinsic wetting pinning on hydrophilic surfaces, this thesis addresses the challenge of high-speed impact, a regime where strong interfacial adhesion typically precludes droplet detachment. Leveraging the rapid rebound capability of DEP, anomalous rebound was achieved on hydrophilic substrates by modulating the elasticity of the encapsulated particle. Distinct from conventional liquid droplets, the impact behavior is dictated by the local dynamics of the solid core: its elastic recoil imparts upward momentum to the liquid shell, entraining the entire droplet to detach against surface adhesion. Furthermore, this particle-mediated detachment was further exploited to develop a novel self-cleaning strategy, demonstrating the ability to capture contaminants and remove them from hydrophilic surfaces without residue.

Third, this thesis demonstrates that particle-mediated impact regulation can be strategically inverted, shifting from promoting detachment to enforcing pinning in high-temperature environments. On superheated surfaces, an insulating vapor cushion typically forms beneath an impacting droplet, inducing erratic levitation and poor heat transfer associated with the Leidenfrost effect. To overcome this thermal barrier, embedded hydrogel particles were used to puncture the vapor layer and establish solid contact bridges with the substrate. This mechanism firmly anchors the droplet, effectively arresting levitation and chaotic motion over a broad temperature range above the Leidenfrost point. This approach opens a novel route for inhibiting Leidenfrost-induced rebound and substantially enhances heat-transfer efficiency in the Leidenfrost regime.

Overall, this thesis demonstrates that solid-liquid compound droplets provide a versatile pathway for manipulating impact behavior across extremes of wettability and temperature, and establishes a theoretical and technical basis for anti-icing, self-cleaning, and enhanced heat transfer.

Date of Award	18 Aug 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pingan ZHU (Supervisor)