Interaction between aqueous/ferrofluid droplets and superhydrophobic surfaces

Abstract

The interaction between droplets and solid superhydrophobic surfaces is ubiquitous in various domains, such as self-cleaning surfaces inspired by the lotus leaf or efficient transportation of test sample droplets in bioassays, etc. The droplets studied in the research can encompass a variety of substances, including water, ferrofluid, liquid metal, and more. Indeed, the interaction can be divided into two categories: in-plane motion, where droplets move along the surface without detaching, and out-of-plane motion, which includes falling, impacting, spreading, and rebounding on superhydrophobic surfaces. Although the interaction between droplets and superhydrophobic surfaces has been extensively studied, there are still some challenges that persist due to the diversity of the scenarios where the interaction occurs. The present work starts by investigating the dynamics of an aqueous droplet obliquely impacting a superhydrophobic solid surface. Numerical simulations using OpenFOAM are conducted to quantitatively analyze the oblique impacting process and energy transformation. The impact velocity, both the magnitude and direction, is varied to study its effects on spreading area, droplet kinetics, and energy distribution. It is found that the normal impact velocity is the main factor influencing the spreading area, and the impact and rebound angles show a linear correlation whose slope is determined by the normal Weber number.Next, the dynamics of a ferrofluid droplet impacting a superhydrophobic solid surface within a nonuniform magnetic field is experimentally investigated, including droplet stretching in the falling process, droplet spreading after impacting, and the final impact outcomes. A scaling relation involving the Weber and magnetic Bond numbers is derived to predict the maximum spreading diameter. Two types of fission, evenly- and unevenly-distributed daughter droplets, are identified, and their corresponding mechanisms are revealed. Lastly, a semi-empirical formula based on Rayleigh-Taylor instability is proposed to estimate the number of the daughter droplets in the regime of even distribution, which agrees well with the experimental data. This study can provide more insight into the large-scale droplet generation with mono-dispersive size by utilizing external stimuli. In the last part of the study, a liquid spring system is developed for fast and contactless manipulation of droplets and solid particles. The liquid spring is based on the actuation of a ferrofluid droplet in a uniform magnetic field, which resembles the release of a compressed metal spring. The actuation enables the spring to propel tiny objects, including liquid droplets and solid particles, with adjustable motion velocity. Furthermore, it is able to achieve the trade-off between the target droplet volume and the motion efficiency. With the aid of the liquid spring, the single or multiple droplets/solid particles advancing, on-demand droplets coalescence, and droplet out-of-plane motion are achievable, which paves the way to various applications, including biomedical assays and material sieving. In conclusion, the interaction between aqueous/ferrofluid droplets and superhydrophobic surfaces have been investigated in the present study. The investigation is not only conductive to our understanding of the natural phenomenon but also provides some guidelines for the engineering applications.