Spatial Control of Condensation and Freezing on Superhydrophobic Surfaces with Hydrophilic Patches

Abstract

Certain natural organisms use micro‐patterned surface chemistry, or ice‐nucleating species, to control water condensation and ice nucleation for survival under extreme conditions. As an analogy to these biological approaches, it is shown that functionalized, hydrophilic polymers and particles deposited on the tips of superhydrophobic posts induce precise topographical control over water condensation and freezing at the micrometer scale. A bottom‐up deposition process is used to take advantage of the limited contact area of a non‐wetting aqueous solution on a superhydrophobic surface. Hydrophilic polymer deposition on the tips of these geometrical structures allows spatial control over the nucleation, growth, and coalescence of micrometer‐scale water droplets. The hydrophilic tips nucleate water droplets with extremely uniform nucleation and growth rates, uniform sizes, an increased stability against coalescence, and asymmetric droplet morphologies. Control of freezing behavior is also demonstrated via deposition of ice‐nucleating AgI nanoparticles on the tips of these structures. This combination of the hydrophilic polymer and AgI particles on the tips was used to achieve templating of ice nucleation at the micrometer scale. Preliminary results indicate that control over ice crystal size, spatial symmetry, and position might be possible with this method. This type of approach can serve as a platform for systematically analyzing micrometer‐scale condensation and freezing phenomena, and as a model for natural systems.

Self-Organization of Microscale Condensate for Delayed Flooding of Nanostructured Superhydrophobic Surfaces

“…It can be seen that more than 90% of the nucleating droplets are formed on the engineered sites. This shows the ability of MW surfaces to promote spatial order by preferential nucleation at low supersaturations, which has been studied and reported by numerous researchers. ,− The effect of MW on nucleation at higher supersaturations, however, is shown in Figure b for S = 1.54, where the portion of the surface that is shown has been freshly exposed due to the removal of a large droplet. It is seen that new droplets nucleate in a random fashion with only ∼56% of the droplets located on the engineered sites.…”

On the Formation and Dynamics of Micro Dew Droplets on Grass: the Role of Epicuticular Wax

“…The enhanced surface wettability in regions with low wax coverage suggests a biphilic composition as a result of the hydrophilic leaf surface decorated with superficial hydrophobic wax platelets, as illustrated in the top‐view SEM images and the schematic in Figure 3c . [ 88 ] Optical microscopy images in Figure 3c show contact angle measurements on the bare leaf (≈55 ± 5°) after wax removal and flat coatings made up of 1‐octacosanol (≈115 ± 2°), corroborating the biphilic properties of the surface of the wheatgrass leaf. Materials with such biphilic wetting behavior often perform efficiently as dew collectors in natural and engineered solutions.…”

Nanoscale Drag Reduction Effect Enables Efficient Fog Harvesting

“…According to statistics, more than two‐thirds of the world experienced annual average humidity levels exceeding 50%. [ 8b ] Despite frequent encounters with foggy weather ( Figure a), its potential has not been fully realized. In this study, a fiber‐based harp with upgraded Janus mode (PAN‐TO harp) was developed to passively capture fresh water during foggy conditions (Figure 1b).…”

“…As shown in Figure 1c and Table S1 (Supporting Information), the PAN‐TO harp exhibited a superior WCR of ≈4035 mg cm −2 h −1 , surpassing many materials in this aspect. [ 3a–d,4c,6,8b,10 ]…”