Topography‐Directed Hot‐Water Super‐Repellent Surfaces

Abstract

Natural and artificial super‐repellent surfaces are frequently textured with pillar‐based discrete structures rather than hole‐based continuous ones because the former exhibits lower adhesion from the reduced length of the three‐phase contact line. Counterintuitively, here, the unusual topographic effects are discovered on hot‐water super‐repellency where the continuous microcavity surface outperforms the discrete microneedle/micropillar surface. This anomaly arises from the different dependencies of liquid‐repellency stability on the surface structure and water temperature in the two topographies. The unexpected wetting dynamics are interpreted by determining timescales for droplet evaporation, vapor condensation, and droplet bouncing. The associated heat transfer process is unique to the wetting states and remarkably distinct from each other in the two topographies. It is envisioned that hot‐water super‐repellent microcavity surfaces will be advantageous for a variety of applications, especially when both self‐cleaning and thermal insulation are imperative, such as clothing for scald protection and digital microfluidics for exothermic reactions.

Tunable Wettability Pattern Transfer Photothermally Achieved on Zinc with Microholes Fabricated by Femtosecond Laser

“…The mechanism of the wettability transformation can be expressed by the following equations [ 3 , 31 ]: Γ(T ) = 75.714 − 0.1414 × T − 2.5399 × 10 −4 × T 2 (20 °C ≤ T ≤ 90 °C) P = − l × Γ × cos θ / A (0° ≤ T ≤ 180°) TCP = l × Δ Γ / A where Γ is the surface tension of the liquid–vapor interface, T is the temperature of water, P is the hydrostatic pressure, l is the circumference of the microholes, θ is the CA, A is the cross-sectional area of the microhole, and TCP is the temperature conducting pressure.…”

Droplet Bouncing: Fundamentals, Regulations, and Applications

“…To maintain repellency of the surface to the hot water, work is performed by topographically designing micro-cavity structure (typical size ≈25 μm) to inhibit the connection between the hot droplet (as high as 90 °C) and the surface. [114] Solely tuning the pillar height of the micropillar surface (< ≈100 nm or > ≈10 μm) can also achieve the repellency of the hot water (≈40 °C). [110] Overlayer: Besides the mentioned deposition methods that use designing surface structure or varying droplet components by adding additives, the droplet shape by external field and droplet temperature by heating, the bouncing suppression can also be achieved by introducing liquid overlayer with low surface tension and high viscosity.…”

Behavior of Sliding Angle as Function of Temperature Difference between Droplet and Superhydrophobic Coating for Aircraft Ice Protection Systems

“…The evaporation rate of the droplet, dm/dt, depends on the temperature difference and is described as follows, if convective and conductive heat transfer between the droplet and the air is neglected [37]:…”