Manish K. Tiwari
Group Leader and Senior Scientist
Laboratory of Thermodynamics in Emerging Technologies, ETH Zurich
Droplets are fascinating objects for thermofluidic research. Understanding droplet formation and droplet-solid interactions is crucial to interpreting not only the natural phenomena such as rain drop size distribution or ice accumulation on mountains, but numerous technologically relevant applications starting from classic examples such as spray cooling, designing paints and tuning their adhesion, to some exciting emerging possibilities such as designing surfaces with extreme liquid affinity or repellency or surfaces for anti-icing and anti-fog applications.
In this course, I would like to introduce how recent advances in nanoengineering of solid surfaces promises to push the frontiers of research on droplet-solid interactions. We will start by looking into the fundamentals of droplet-solid interfacial interactions and the concept of surface energy balance. Thermodynamic principles will be employed to develop a basic understanding of contact angle that a small liquid droplet typically forms on a solid surface, which is a basic measure of the wettability of the solid with the liquid. In addition, adhesion of liquid droplets characterized by contact angle hysteresis, which is present under dynamic conditions of droplet impact and/or movement on a solid, will also be thoroughly discussed. Then we will see how manipulation of solid surface morphology can used to engender extremes in the wettability of surfaces: Ranging from superhydrophilic surfaces (i.e. surfaces with extreme affinity for water) on one hand to superhydrophobic (i.e. surfaces with extreme water repellency) and superoleophobic surfaces (i.e. surface with extreme repellency for low surface tension oils) on the other. Following this, we will see how and why some natural surfaces such as lotus leaves and butterfly wings have a hierarchical morphology with micro-to-nanoscale roughness to help them achieve high hydrophobicity. Recent prolific efforts and successes to mimic such extreme repellencies using artificially nanoengineered surfaces will then be introduced. Both static and dynamic considerations of droplet-solid interaction on such surfaces will be discussed. A brief introduction will be also provided into surface fabrication techniques.
Given their water repellency, one may intuitively expect superhydrophobic surfaces to also possess ice-repellency and low ice-adhesion. However, the results of freezing experiments on these surfaces will be shown to demonstrate that the actual behavior of these surfaces depends on a delicate balance of surface, thermal and environmental conditions. After introducing some basic concepts in designing icephobic surfaces, we would see that ice-repellency of a solid surface can be seriously impaired by minor changes in the environmental conditions such as air flow or humidity, making it a challenging research problem. A deeper thermal analysis will be presented to show how droplet freezing is quite dramatically affected by its simultaneous evaporation. Finally, some perspective will be provided on outstanding and exciting opportunities in this inherently multidisciplinary field of research on droplets interacting with nanoengineered surfaces.
Keywords: Nanostructuring, Thermofluidics, Superhydrophobicity, Superhydrophilicity, Anti-icing, Phase change