Droplet, Jets, and Leaky Surfaces

Abstract

Surface structuring on a micro-nano scale, combined with a low surface energy coating, leads to anti-wetting properties. Such surfaces also exhibit other properties such as self-cleaning, antifouling, bacteriostatic, drag reduction, and anti-icing. Hierarchical structures with dual-scale roughness provide the superhydrophobic surface with lower droplet adhesion and better protection against failure (i.e., Wenzel transition). This understanding has led to the study of nanostructured sieves, as the sieve wires (having diameters ranging from 10 to 100 microns) provide the higher-level roughness required in dual-scale surfaces. Thus, for sieves, a single nano-structuring step leads to dual-scale rough surfaces. Further, the pores in sieves provide an additional structural feature for enabling other applications such as oil-water separation. Hence, nanostructured sieves are being investigated today for novel applications.

Studying the impact of droplets on sieves with different wettability is fascinating as their porosity leads to several exciting scenarios that can be explored for potential use. This thesis investigates the different outcomes of droplet impact on sieves and explores new possibilities. The first part of the study explores droplet impact at the low Weber number regime. The formation of different cavities and their collapse have been studied. The focusing of kinetic energy in the cavity collapse process and the associated singularity leads to the generation of a single droplet. This work reports a new kind of cavity formation phenomenon unique to sieve configurations. In contrast to cavities observed for droplet impact on solid surfaces, this cavity is formed during the droplet impact's recoil phase. Hence, it is called the recoil cavity. The cavity formation and collapse are explained using experimental results and theoretical modeling.

The collapse of the recoil cavity leads to the generation of a satellite-free single droplet underneath the sieve. Essentially this phenomenon of ejecting a single drop opens up avenues for novel applications. This thesis explores the drop-on-demand technique for material jetting and printing. Interestingly, we found that using superhydrophobic sieves could eliminate a long-standing problem of clogging in printing. We explored the clogging issue in-depth and showed our technique's unique capability in printing high mass loading and large particle size.

We report printing of ink with mass loading as high as 71% using our technique. Further, the use of this printing technique has been demonstrated for various applications. Electronic circuits and devices have been printed on flexible substrates. 3D printing has also been demonstrated using high mass loading ink. Printing of live cells and bacteria has also been achieved using this technique.

This thesis explores droplet impact at a high Weber number regime in the second part. We developed double sieve-based air-transparent surfaces capable of repelling rain droplets impacting at terminal velocities. Such air-transparent surfaces will find use in roofs and windows of homes and public places. Current understanding would point towards the use of nanostructured superhydrophobic sieves. However, liquid leaks through such superhydrophobic sieves when the dynamic pressure of the impacting droplet is larger than the anti-penetration Laplace pressure. When the droplet penetrates through the sieve, it comes out in the form of jets. Due to Rayleigh-Plateau instability, the ejected jets break into smaller droplets. This jetting dictates the outcome of the impact. Contrary to the common understanding, we explain our experimental results, which show that the jet velocity can be larger than the impact velocity. This increase in velocity of the ejected droplet makes it difficult to stop the ejected jet using a second superhydrophobic sieve. We use a combination of superhydrophilic and superhydrophobic sieves to repel raindrops impacting at the terminal velocity.

Overall, this thesis deals with the droplet interaction with sieves of different wettability. The present work evolves to innovate interesting applications and solve significant problems.