Investigation of the Wetting Phenomena on Liquid Infused Textured Surfaces


Student thesis: Doctoral Thesis

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  • Chonglei HAO


Awarding Institution
Award date6 Apr 2016


The development of bio-inspired interfacial materials with enhanced drop mobility that mimic the innate functionalities of nature will have significant impact on the energy, environment and global healthcare. Via emulating nature's time-tested patterns and strategies such as self-cleaning lotus leaves, together with recent advances in micro-/nano-fabrication and coating technologies, superhydrophobic surfaces (SHS) based on the air-impregnated lotus effect have been extensively studied for practical applications such as non-wetting textiles, anti-frost/icing material coatings and anti-biofouling medical devices. Recently, there is a dramatic surge in engineering slippery liquid-infused porous surfaces (SLIPS) based on the inspiration from the Nepenthes pitcher plant owing to many promising advantages. Different from droplet dynamics on the air-impregnated SHS, the droplet behaviors on the SLIPS is more complicated by the complex, mobile and soft nature of the solid-vapor-liquid interfaces. Despite numerous efforts, our understanding and the ability to control the droplet dynamics on the emerging SLIPS remains elusive. Thus, the aims of this thesis here are to advance our fundamental understanding of the relationship between droplet dynamics and basic constituent elements in SLIPS such as liquid lubricant viscosities, thickness and distribution, and explore novel means to actively tailor the droplet behaviors.
First, the droplet impingement dynamics on slippery thin liquid film was carefully examined. We discovered a new superhydrophobic-like bouncing regime on the slippery liquid interfaces, characterized by the contact time, the spreading dynamics, and the restitution coefficient independent of underlying liquid film. We also found that during the superhydrophobic-like regime, the droplet would shed off from SLIPS with the velocity at least two orders of magnitude higher than that under normal contact shedding mode from SLIPS. Through experimental exploration and theoretical analysis, we demonstrate that the manifestation of such a superhydrophobic-like bouncing necessitate an intricate interplay between the Weber number, the thickness and viscosity of liquid film. The coupling of robust superhydrophobic-like bouncing with inherent advantages of emerging slippery liquid interfaces will open up new avenues for a wide range of practical applications.
Second, the structure effect of asymmetric liquid-infused micro/nano structured surfaces on drop dynamics was studied. Above the critical micro-structure size, it was found that the droplet exhibits anisotropic sliding motion on this interface: it slid more easily along the triangular groove-stripe structures than across them. In contrast, below the critical structure size, the structure size effect of anisotropic wetting property is nearly negligible for the droplet dynamics. This anisotropic behavior is also closely dependent on the distribution of the liquid film.
Third, we also systematically investigated how to actively control the droplet dynamics on SLIPS by external electric filed. Leveraging on the negligible contact angle hysteresis and increased liquid-liquid viscous dissipation, a new scheme called electrowetting on liquid infused film (EWOLF) is developed, which outperformed the conventional electrowetting on dielectric (EWOD) with exceptional reversibility, stability and fast dynamic response. We also demonstrate the successful implementation of EWOLF in the liquid lens system with response speed 5 times faster than that by using EWOD. This new scheme signifies a new direction in the field of liquid lens imaging devices.
In summary, we systemically investigated the interfacial wetting phenomena on SLIPS. A new superhydrophobic-like bouncing regime on SLIPS is first reported, which bridged lotus effect and pitcher plant effect for our deep understanding of biomimicry study of liquid repellent surfaces. Moreover, the structure effect of anisotropic SLIPS on drop dynamics was explored in details.
We also demonstrated the successful implementation of EWOLF for optical imaging application. We believe that our work on the liquid repellent materials will provide important insights to our fundamental understanding of phase interactions and therefore spur a wide range of applications.