Symposium CP04: Interfacial Science and Engineering—Mechanics, Thermodynamics, Kinetics and Chemistry
Henri-Louis Girard, Massachusetts Institute of Technology
Waterbowls – Reducing Impacting Droplet Interactions by Momentum Reduction
Written by Aashutosh Mistry
Interactions of liquid drops and solid surfaces have been a classical problem in transport phenomena and extensively studied in various contexts such as heat, momentum, and mass exchanges. Depending on the application, one would want to either enhance the effective transport, for example, faster condensation on a colder solid (going from film-wise to drop-wise condensation) or attenuate the intrinsic characteristics of the solid–liquid pair, for example, icing of impacting drops on aircraft wings is detrimental and in turn reduced heat transfer is desired. The former problem of enhanced transport has received considerably more attention than the latter.
Henri-Louis Girard’s research focuses on the problem of attenuating the aforementioned transport modes. From a simplistic understanding, one could argue that liquid-repelling (e.g., hydrophobic for water-repelling) surfaces should provide reduced transport interactions. However, upon a closer look, Girard found that even if the hydrophobic surfaces do not let drops stick to the surface, the impact, oscillations, and rebound transients provide a thorough contact between the sold and flattened drop shape during the oscillation stage. In other words, both time of contact, as well as area of contact, contribute to effective transport. For a hydrophobic surface, the time of contact is smaller, but the contact area is fairly high which jointly leads to not so small interactions. Building upon this interpretation, the research group hypothesized that if the surface has circular ridges, the contact area can be reduced. Further experiments with varying circular ridge dimensions revealed three interaction regimes: (i) when the radius of the ring is smaller than that of the drop, upon drop impact it spreads over the ring and the ring cannot divert the momentum during spreading; (ii) when the radius of the ring is fairly higher than that of the drop (greater than three times), the effectiveness reduces as it does not divert the drop contact early on; and (iii) in between these two regimes, there is an interesting regime where the usefulness of the circular ridges deteriorates with an increase in their radii. At present, the researchers are analyzing the implications of an offset impact of the drop with such ridges in terms of momentum diversion and consequent attenuated transport interactions.