Controlled coalescence-induced droplet jumping on flexible superhydrophobic substrates

Sessile droplets coalescing on superhydrophobic substrates spontaneously jump from the surface. In this process, the excess surface energy available at the initiation of coalescence overcomes the minimal surface adhesion and manifests as sufficient kinetic energy to propel the droplets away from the substrate. Here, we show that the coalescence induced droplet jumping velocity is significantly curtailed if the superhydrophobic substrate is flexible in nature. Through detailed experimental measurements and numerical simulations, we demonstrate that the droplet jumping velocity and jumping height can be reduced by as much as 40 % and 64%, respectively, by synergistically tuning the substrate stiffness and substrate frequency. We show that this hitherto unexplored aspect of droplet coalescence jumping can be gainfully exploited in water harvesting from dew and fog harvesting. Additionally, through an exemplar butterfly wing substrate, we demonstrate that this effect is likely to manifest on many natural superhydrophobic substrates due to their inherent flexibility

Some observations of diatoms under turbulence

The effect of turbulence on several freshwater diatom taxa was investigated and our findings are described herein. We have compared diatom morphology in shallow natural systems that experience turbulence due to wind and in river/waterfall systems where turbulence is due to high flow rates. We have also introduced turbulence into diatom laboratory cultures by mechanical shaking and by forcing air into the media. In particular, we have studied diatoms in five independent environments or cultures: the freshwater diatoms Tabellaria and Eunotia in equatorial lakes experiencing extreme seasonal variability in depth; two freshwater diatom monocultures of Aulacoseira granulata var angustissima and Melosira varians in the laboratory; and a freshwater diatom community possessing equal amounts (by number) of elongated and non-elongated diatoms (mostly Nitzschia and mostly Cyclotella, respectively) in the laboratory. We have demonstrated the effect of turbulence on freshwater diatom frustule morphologies and, perhaps more importantly, the effect of turbulence on freshwater diatom species population after controlled perturbation of the organisms’ environment. It has been widely reported that symmetry is often preferred in biological evolution, however here we have observed a preference towards asymmetry for the survival of diatoms in the presence of environmental stress (in particular, turbulence). We also note that to date there have been no systematic attempts to manipulate diatom frustules using external stimuli. We therefore present a proof-of-concept study in order to demonstrate: (i) that diatom morphologies can be manipulated by controlled simple external triggers (chemical and physical) (ii) that population balance (i.e. natural selection) can be controlled via simple external triggers (chemical and physical). This approach could open up an entire new field of future studies wherein controlled environmental perturbations are used to manipulate the structure, form, growth and reproduction of biological species