Drops on a Superhydrophobic Hole Hanging On under Evaporation

Abstract

Drops with larger volumes placed over a superhydrophobic (SH) surface with a hole do not fall through unless they are evaporated to a size that is small enough. This feature offers the ability to preconcentrate samples for biochemical analysis. In this work, the influence of pinning on the behavior of drops placed on a 0.1 mm thick SH substrate with a 2 mm diameter hole as they evaporated was investigated. With 16 μL of water dispensed, the sessile drop component volume was initially higher than that of the overhanging drop component and maintained this until the later stages where almost identical shapes were attained and full evaporation was achieved without falling off the hole. With 15 μL of water dispensed, the volume of the sessile drop was initially higher than that of the overhanging drop component but the liquid body was able to squeeze through the hole after 180 s due to the contact line not having sufficient pinning strength when it encountered the edge of the hole. This resulted in the liquid body either falling through the hole or remaining pinned with an oval-like shape. When it did not fall-off, the liquid body had volume and contact angle characteristics for the sessile drop and overhanging drop components that were reversed. In the later stages, however, nearly identical shapes were again attained and full evaporation was achieved without falling off the hole. The effects of pinning, despite the substrate being SH, offer another path toward achieving practical outcomes with liquid bodies without the need for chemical surface functionalization. Similarities and differences could be seen in the behavior of a sessile drop on a SH plate that was inclined at 30° to the horizontal and evaporated.

Simultaneous Multidrop Creation with Superhydrophobic Wells for Field Environmental Sensing of Nanoparticles

“…21,22 That the same increased pinning effects are observed here using substrates that manifest predominant Cassie wetting correlates with recent findings of drops strongly retained on holes created on thin SH substrates. 23…”

Parallel Droplet Deposition via a Superhydrophobic Plate with Integrated Heater and Temperature Sensors

“…Usually, this is achieved by structuring the surface, so the droplets rest in a suspended Cassie-Baxter state [31]; the contact angle hysteresis of such surfaces is insignificant [32]. Droplets resting on a superhydrophobic hole were observed to evaporate in a mixed mode [33], but the superhydrophobicity is still required to avoid contact line pinning. The droplets dispensed from the needle array rest on the superhydrophobic hole plate and shrink in size due to evaporation (Figures 2c,d).…”

Evaporation-Based Low-Cost Method for the Detection of Adulterant in Milk

“…The evaporation dynamics of a liquid mixture involving a nonvolatile solute, colloids, and a volatile solvent is a complex phenomenon due to the coupled effects of hydrodynamics, heat and mass transfer, and contact-line dynamics. ,− The evaporative deposition pattern is highly sensitive to the surface tension, surface tension gradient of the liquid, wettability and roughness of the underlying substrate, and nature of the solute. − While there have been several works on the evaporative deposition of colloidal particles on different wettability substrates, the effect of dissolved solute on the deposition pattern is rather limited. ,, In aqueous saline drops, the salt crystallizes when the concentration exceeds the saturation concentration during evaporation. , The evaporative deposition is further affected by the presence of multiple components (evaporation of fuel, ouzo droplets, and surfactant in a binary mixture). Recently, the differences in drying patterns of sessile droplets of blood have been reported as a means to detect diseases. , Milk is a complex fluid that contains nonvolatile milk solids (fats, protein, salt) and volatile (water-enriched material, ketones, alcohols, hydrocarbons, esters, etc …”