The air entrapment under a drop impacting on a nano-rough surface

Abstract

We study the impact of drops onto a flat surface with a nano-particle-based superhydrophobic coating, focusing on the earliest contact using 200 ns time-resolution. A central air-disc is entrapped when the drop impacts the surface, and when the roughness is appropriately accounted for, the height and radial extent of the air-disc follow the scaling laws established for impacts onto smooth surfaces. The roughness also modifies the first contact of the drop around the central air-disc, producing a thick band of micro-bubbles. The initial bubbles within this band coalesce and grow in size. We also infer the presence of an air-film residing inside the microstructure, at radial distances outside the central air-disc. This is manifest by the sudden appearance of microbubbles within a few microseconds after impact. The central air-disc remains pinned on the roughness, unless it is chemically altered to make it superhydrophilic.

Enhanced flux in direct contact membrane distillation using superhydrophobic PVDF nanofibre membranes embedded with organically modified SiO2 nanoparticles

“…Rough membranes allow air entrapment in the membrane voids, which causes high repulsive forces between the membrane and water, thus inducing a higher hydrophobicity. Furthermore, membranes with nonuniform morphology and nondistributed surface roughness are likely to cause an uneven distribution of the contact angles within the same membrane . Detailed information regarding these observations is outlined in the next section.…”

“…Furthermore, membranes with nonuniform morphology and nondistributed surface roughness are likely to cause an uneven distribution of the contact angles within the same membrane. 42 Detailed information regarding these observations is outlined in the next section.…”

Droplet impacts onto soft solids entrap more air

“…This method has been used in many prior studies with much success. 9,13,[22][23][24] In a few cases, we also employ a transmission interferometry scheme, similar to our previous studies, 9,11,19,25 where the laser is positioned near the nozzle. The light then shines vertically downward, focusing through the drop and reflecting off of a 451 mirror to the camera.…”

“…Li, Vakarelski and Thoroddsen 18 found that with a surface roughness of only 2 nm RMS a faint ring of microbubbles was entrapped marking the location of the initial contact. Langley et al 19 found that by increasing the roughness to between 70 and 130 nm RMS a significant band of microbubbles was formed and the radius of the central air disc was decreased in comparison to a smooth surface.…”

A structure-free digital microfluidic platform for detection of influenza a virus by using magnetic beads and electromagnetic forces

“…The super-hydrophobic layer on the glass substrate was fabricated via a spin-coating process 28,29 (Glaco mirror coat "zero", Soft99 Co., Japan) with a spin-coating speed at 300 rpm for 30 s and then dried at room temperature for 15 min. After the super-hydrophobic spin-coating process, two kinds of PDMS reagents (Sylgad 184A/B, Dow Corning Corp., USA) was pre-mixed at a mass ratio of 10 : 1, degassed and stamped on the super-hydrophobic layer for the pattern of droplet SETs by using a polymethylmethacrylate (PMMA)-fabricated stamp.…”