Efficient Self-Propelling of Small-Scale Condensed Microdrops by Closely Packed ZnO Nanoneedles

Abstract

Realizing the efficient self-propelling of small-scale condensed microdrops is very challenging but extremely important to design and develop advanced condensation heat transfer nanomaterials and devices, for example, for power generation and thermal management. Here, we present the efficient self-propelling of small-scale condensed microdrops on the surface of closely packed ZnO nanoneedles, as-synthesized by facile, rapid, and inexpensive wet chemical crystal growth followed by hydrophobic modification. Compared with flat surfaces, the nanostructured surfaces with the same low-surface-energy chemistry possess far higher time-averaged density of condensed droplets at the microscale, among which those with diameters below 10 μm occupy more than 80% of the total drop number of residual condensates. Theoretical analyses clearly reveal that this remarkable property should be ascribed to the extremely low solid-liquid adhesion of the surface nanostructure, where excess surface energy released from the coalescence of smaller condensed microdrops can be sufficient to ensure the self-propelled jumping of merged microdrops.

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

“…However, the experimental velocities of the self-jumped microdrops are apparently smaller than their theoretical values. Later, researchers noted that the energy dissipation caused by viscous flow (E vis ) and interface adhesion (E int ) must be considered, [31,[56][57][58][59][60] while gravi tational potential energy may be neglected due to microdrop sizes that are far smaller than the capillary length. Very recently, our group further found that, besides the solid-liquid van der Waals attraction, nanoscale line tension at three-phase contact lines cannot be neglected in calculating adhesioninduced dissipation.…”

“…Very recently, our group further found that, besides the solid-liquid van der Waals attraction, nanoscale line tension at three-phase contact lines cannot be neglected in calculating adhesioninduced dissipation. [31] As shown in a simplified model that only considers the merging of two microdrops with identical diameters (Figure 2a-d), excess energy can be generated as the released surface energy deducts the viscous-flow-induced energy dissipation. In principle, the merged microdrop can selfremove as long as the excess surface energy is greater than the energy dissipation caused by interface adhesion.…”

“…[66][67][68][69] Second, their tip size should be as small as possible to minimize the interface adhesion. [31,50,51,[66][67][68][69] Third, the building blocks must have a certain height or depth to avoid the collapse of the suspended water bridges. [68] Following these design principles and inspired by nature, our group proposed and demonstrated that efficient self-propelling of small-scale condensed microdrops can be [5] Copyright 2007, Wiley-VCH.…”

“…With the as-synthesized closely packed zinc oxide nanoneedles as an example (Figure 2f), with ≈60 nm interspace, ≈20 nm top dia meter, and ≈3 µm height, our experiments have demonstrated that the departure sizes of condensate drops can be controlled at the microscale, where more than 80% of the drops are smaller than 10 µm (Figure 2g). [31] In sharp contrast, condensate drops on the flat hydrophobic surface can mutually coalesce but cannot self-remove. They are firmly pinned on the surface, where the drop diameters vary from tens of micrometers to hundreds of micrometers.…”

“…The latest research has indicated that these small-scale condensate microdrops may self-remove via mutual coalescence as long as the coalescence-released excess surface energy is larger than the interface adhesion-induced energy dissipation. [31] Much effort has been devoted to exploring the utilization of knowledge of bionic low-adhesivity superhydrophobicity to develop innovative condensate microdrop self-propelling (CMDSP) surfaces. Different from traditional superhydrophobic surfaces, which are characterized by the bouncing or rolling off of deposited millimeter-size large drops, [32,33] CMDSP surfaces support the self-removal capability of smallscale condensate microdrops.…”