Exploitation of Super(de)wettability via Scalable Hierarchical Surface Texturing

Abstract

The field of wettability is an age-old topic that has been revitalized in the last two decades. Historically, the diverse physical phenomena of wetting has influenced the development of inventions that dates back to the paleolithic era (2,600,000 to 10,000 BC) in the form of charcoal and ochre -based cave paintings, or the mesolithic (10,000 to 5,000 BC) and neolithic (5,000 to 2,000 BC) periods as pottery and soaps. Since the end of the Stone Age, human civilizations and scientific discoveries have progressed by leaps and bounds. Despite the advances in metallurgy, optics, chemistry, mechanics, mathematics and electricity, our understanding of fluid-surface interactions remained stagnant until 1804. Between 1804 and 1805, Thomas Young described the concept of a wetting contact angle, which controls the equilibrium shape of a fluid droplet on a surface, thus making wettability a quantified branch of physics. The late entry of this scientific field is astounding, considering the ubiquitousness of water on Earth. Despite Young’s discoveries, the area remained largely unexplored. Work on wettability was intermittent, with Edward Washburn on capillary effects in 1921 and later on, Robert Wenzel and Cassie-Baxter in 1936 and 1944 on the wetting of rough interfaces.

In 1997, almost exactly 20 years ago, the field was rejuvenated by the corresponding discoveries of superhydrophilicity (water droplets spread into a sheet) and superhydrophobicity (water droplets ball up), by Wang et al. and Neinhuis et al. respectively. Since their work into these distinct super(de)wetting states, the field has grown exponentially. Today, its revival can be attributed to biomimetics (engineering mimicry / imitation of life) and a revolutionized understanding behind super(de)wetting mechanisms that are found in nature. The precise combination of hierarchical (multi-scale) texturing with select surface chemical composition is vital towards fabricating interfaces with specialized wetting properties. Knowledge behind the careful control of surface texturing holds immense potential for enabling a plethora of user-defined functional interfaces. As of the time of writing, the field of wettability encompasses multiple domains, such as superhydrophilicity (water-loving),[8] slippery superhydrophobicity (water-fearing), adhesive superhydrophobicity (an unintuitive love-fear relationship with water), superoleophobicity (oil-fearing), superamphiphobicity (water- and oil-fearing),[11] superomniphobicity (all-fearing) as well as a range of other important intermediary, cross-environment wetting states.

Methods employed for achieving super(de)wettability can be broadly classified under 2 sub-classes. The first relies on intricate top-down photolithography (-drawing with light) or templating-based designs while the other uses the realms of chaotic, but deterministic and scalable bottom-up self-assembly. Both routes are promising for the development of unique super(de)wetting states, albeit with considerable drawbacks on both fronts. For instance, while lithography and templating have demonstrated exemplary surface texturing precision and super(de)wetting performance, these methods remain limited by poor scalability, complexity and costs in instrumentation and operation. Alternatively, scalable and cheap bottom-up self-assembly methods can exist within complex electro-, hydro-, aero-, thermal- or thermo-dynamically varied regimes. Consequently, each system requires intense cross-optimization research efforts in determining niche operating parameters.

In this work, we explore a series of highly promising hierarchically structured material interfaces that were enabled by understanding, taming and controlling scalable but chaotic bottom-up methods. To this end, we demonstrate their potential within the entire super(de)wetting spectrum, showcased through a series of coatings and further exemplified by functional micro(fluid)mechanical systems (M-F-MS). Show more