We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number \We and the roughness of the surface. At small \We, i.e. small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large \We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small \We $\apprle 120$ but an important one for large \We $\apprge 120$. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime \We $\apprle 150$
Phase change under reduced environmental pressures is key to understanding liquid discharge and propulsion processes for aerospace applications. A representative case is the sessile water droplets exposed to high vacuum, which experience complex phase change and transport phenomena that behave so differently than that under the atmosphere. Here, we demonstrate a previously unexplored aspect of the mechanism governing icing droplet self-launching from superhydrophobic surfaces when exposed to low pressures (~100 Pa). In contrast to the previously reported recalescence-induced local overpressure…
Unraveling the Interplay of Leaf Structure and Wettability: A Comparative Study on Superhydrophobic Leaves of Cassia tora, Adiantum capillus-veneris, and Bauhinia variegata
The Leidenfrost phenomenon entails the levitation of a liquid droplet over a superheated surface, cushioned by its vapor layer. For water, superhydrophobic surfaces are believed to suppress the Leidenfrost point ($\it{T}$$_{\rm L}$)-the temperature at which this phenomenon occurs. The vapor film obstructs boiling heat transfer in heat exchangers, thereby compromising energy efficiency and safety. Thus, it is desirable to realize superhydrophobicity without suppressing $\it{T}$$_{\rm L}$. Here we demonstrate that the $\it{T}$$_{\rm L}$ of water on microtextured superhydrophobic surfaces compris…
There is a huge interest in developing super-repellent surfaces for anti-fouling and heat transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be indirectly inferred from the Furmidge's relation. While easy to implement, contact angle measurements are semi-quantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an Atomic Force Microscope cantilever to directly measure adhesio…
Recent experimental and computational investigations have shown that trace amounts of surfactants, unavoidable in practice, can critically impair the drag reduction of superhydrophobic surfaces (SHSs), by inducing Marangoni stresses at the air-liquid interface. However, predictive models for realistic SHS geometries do not yet exist, which has limited the understanding and mitigation of these adverse surfactant effects. To address this issue, we derive a model for laminar, three-dimensional flow over SHS gratings as a function of geometry and soluble surfactant properties, which together encom…
We study here the microscopic deformations of elastic lamellae constituting a superhydrophobic substrate under different wetting conditions of a sessile droplet using electrowetting. The deformation profiles of the lamellae are experimentally evaluated using confocal microscopy. These experimental results are then explained using a variational principle formalism within the framework of linear elasticity. We show that the local deformation profile of a lamella is mainly controlled by the net horizontal component of the capillary forces acting on its top due to the pinned droplet contact line. …
Superrepellency is a favorable non-wetting situation featured by a dramatically reduced solid/liquid contact region with extremely low adhesion. However, drop impact often brings out a notable extension of the contact region associated with rather enhanced water affinity, such renders irreversible breakdowns of superhydrophobicity. Here, we report an alternative outcome, a repeated Cassie-Wenzel-Cassie (CWC) wetting state transition in the microscale occurs when a drop impacts a two-tier superhydrophobic surface, which exhibits a striking contrast to the conventional perspective. Influences of…
Rapid and sustained condensate droplet departure from a surface is key towards achieving high heat transfer rates in condensation, a physical process critical to a broad range of industrial and societal applications. Despite progress in enhancing condensation heat transfer through inducing its dropwise mode with hydrophobic materials, sophisticated surface engineering methods that can lead to further enhancement of heat transfer are still highly desirable. Here, by employing a three-dimensional, multiphase computational approach, we present an effective out-of-plane biphilic surface topography…
