Droplet interaction with soft substrates

• Main Author: Lin, Marcus Yiquan
• Other Authors: Tuan Tran
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/168540
• Institution: Nanyang Technological University

Droplet interactions with solid surface happen in daily life, both in nature and technological applications, raindrops falling on a leaf, industrial spray coating and inkjet printing. In recent years, there is a surge in interest for anti-fouling or self-cleaning surfaces. While many modified surfaces are available, there exists limitations such as durability, sustainability and inconsistency. An alternative is to use soft solids as a layer of coating, which exhibit wide range of elasticities and low surface energy. Fouling materials can therefore be remove by simple spraying of water, without the need of heavy mechanical cleaning. In this thesis, I study the interactions between droplets and soft solids on two different cases: (i) drop impact, and (ii) sessile drop on soft solids. This thesis begins by examining the impact behaviour of viscous droplets on soft substrate. By varying the viscosity of the working liquid, a phase diagram indicating the transition to wetting is obtained for each substrate elasticity. The transition to wetting is determined as the dependence of the Weber number (We), and Ohnesorge number (Oh). For rigid substrates, the transition to wetting trend could be well speculated. Intuitively, with increasing liquid viscosity, the spreading diameter would be smaller and traps less air. This reduction in air-cushioning effect would initiate wetting. However, when the substrate elasticity falls below a threshold value, the transition to wetting exhibits a counter intuitive non-linear behaviour; first increases then decreases with increasing Oh. I show that this link to the air cavity formed within the droplet upon impact. A scaling law is also obtained to relate the maximum allowable Weber number before wetting occurs, against the substrate elasticity. A bouncing or hovering droplet would eventually comes to a halt and wets the surface. In this thesis, I study the wetting ridge formation of a sessile droplet on a soft solid. The wetting ridge would have fully developed. The height of this wetting ridge depends on the elasticity of the substrate. On a stiff substrate, the ridge height is negligible. However, on a sufficiently soft substrate, the vertical component of the interfacial tension will be balanced by a relatively smaller substrate’s elasticity, which also approximates the height of wetting ridge. Over the past decades, there have been a surge in attention for this physical response of soft solid, with the development of several state-of-the-art optical imaging system to acquire direct visualisation the ridge; theories manifest strong quantitative agreement with the experimental results. However, due to the diffraction limit of these optical visualisation techniques, it possess great difficulty for researchers to have a direct visualisation of the wetting ridge in the certain regime. In fact, regime where droplets are smaller than the elastocapillary length of the substrate has never been experimentally resolved. This thesis investigates the wetting ridges formed by microdroplets that are smaller than the elastocapillary lengths, using atomic force microscopy (AFM) which eliminates the diffraction limit of optical imaging system. The substrates used were carefully selected to have contrasting Young’s modulus. This allows me to study the regime above and below the elastocapillary lengths of the substrates. For droplets that are well above the elastocapillary length, the elasticity theory display well quantitative agreement with the experimentally measured results.