Droplet mobility and impacting dynamics on superhydrophobic surfaces

Abstract

Droplet wetting phenomenon is crucial in various applications, such as antiicing and self-cleaning surfaces, heat transfer, water harvesting, and so on. The desired wettability can be achieved by a combination of physical structuring of the surface and chemical coatings. In particular, superhydrophobic surfaces with low adhesion can be accomplished by a two-tier roughness structure, which results in good water-repellent properties as well. However, such surfaces lose their super-repellency or high droplet mobility under certain conditions, such as low temperature or high humidity environments. Maintaining the surface superhydrophobocity and water-repellency is challenging in these cases. In this thesis, superhydrophobic surfaces based on micro-pyramids and nanowires as a second-tier roughness are fabricated on silicon wafers using wet etching techniques. The surfaces follow the same geometrical characteristics of conical structures, which have previously shown potential as good water repellent surfaces, but now using a much simpler and faster fabrication method. In order to investigate the superhydrophobic and repellency properties of the pyramids with nanowires surfaces, their wetting properties are characterized in terms of static contact angle and contact angle hysteresis to address droplet mobility. This is done both at dry and wet conditions, where the surfaces are already wet before depositing a new droplet. Droplet impact experiments are performed so as to evaluate the effect of varying contact angle hysteresis during the droplet motion on the surface. The surfaces with pyramids and nanowires are superhydrophobic (the contact angle larger than 150◦) and show high droplet mobility with low surface adhe sion, both at dry and wet conditions. It is shown that only micro-pyramids or only nanowires cannot maintain droplets in Cassie wetting state and high droplet mobility, but that it is the combination of the two types of structures that achieves the good surface performance. The pointed tops of the pyramids show less surface adhesion for Cassie state droplets than similar pillar-like structures. High contact angles and low adhesion on the pyramids with nanowires was also observed for liquid of different viscosities (DI water and glycerol) and surface tension (DI water and ethanol). During droplet impact, where droplet mobility is of relevance, the effect of contact angle hysteresis appears to be significant even for superhydrophobic surfaces. For surfaces with non-negligible contact angle hysteresis, a relaxation phase during the droplet retraction is observed. The time spent in this relaxation phase can be comparable to the spreading and retracting time, thus playing an important role when thinking on the total contact time of the droplet with the surface. Our results indicate that both the contact angle hysteresis and the capillary forces play a major role in defining the relaxation time and that the relaxation time scales with the inertial-capillary time when using the droplet relative deformation as the characteristic length scale instead of the often used initial droplet diameter. This work not only shows the excellent anti-wetting properties of surfaces with pyramids and nanowires, but it also shows how these structures and their geometry can help to maintain high droplet mobility under dry and wet conditions and how they can help in minimizing droplet contact time during droplet impact. The results from this work provide guidelines for the design of super-repellent surfaces for practical applications.