Darong Chen

Criteria for entrapped gas under a drop on an ultrahydrophobic surface.

Ultrahydrophobicity of a rough surface is mainly attributed to the entrapped gas under a drop. Two criteria were proposed for the entrapped gas: an intruding angle criterion and an intruding depth criterion. These two criteria are that the intruding angle must be less than the maximum asperity slope angle and the intruding depth must be less than the height of the asperities. The intruding angle is determined by the true contact angle, the surface geometry, and the drop size. The intruding angle is directly proportional to the true contact angle, and it increases with an increase of the fractional area of the liquid-gas interface under the drop and with a decrease of the linear dimension of the three-phase contact line on the asperities. The effect of the drop size on the intruding angle is induced by Laplace and hydrostatic pressures. The intruding depth increases with an increase of the intruding angle and the distance between the asperities. The proposed criteria were evaluated using experiment data from the literature. Comparison between the experiment and calculation results showed that the experiment data supported the theory.

Superhydrophobic behavior achieved from hydrophilic surfaces

The superhydrophobic behavior of a surface can be generally attributed to the combination of its chemical composition and microscale texture. The surface can be both hydrophobic and rough, and the roughness enhances its hydrophobicity. For a natural or artificial surface, superhydrophobic behavior is generally induced by a structured hydrophobic surface. This paper proposes an alternative; that is the superhydrophobic behavior can be obtained from a structured hydrophilic surface. The superhydrophobic behavior of a T-shape micropillar surface coated with diamond-like carbon has been achieved, which experimentally proved the proposed hypothesis that superhydrophobicity can be created from a hydrophilic surface through surface microstructure modification.

Synthesis of Biomimetic Superhydrophobic Surface through Electrochemical Deposition on Porous Alumina

The superhydrophobicity of plant leaves is a benefit of the hierarchical structures of their surfaces. These structures have been imitated in the creation of synthetic surfaces. In this paper, a novel process for fabrication of biomimetic hierarchical structures by electrochemical deposition of a metal on porous alumina is described. An aluminum specimen was anodically oxidized to obtain a porous alumina template, which was used as an electrode to fabricate a surface with micro structures through electrochemical deposition of a metal such as nickel and copper after the enlargement of pores. Astonishingly, a hierarchical structure with nanometer pillars and micrometer clusters was synthesized in the pores of the template. The nanometer pillars were determined by the nanometer pores. The formation of micrometer clusters was related to the thin walls of the pores and the crystallization of the metal on a flat surface. From the as-prepared biomimetic surfaces, lotus-leaf-like superhydrophobic surfaces with nickel and copper deposition were achieved.

Bionic research on fish scales for drag reduction

To reduce friction drag with bionic method in a more feasible way, the surface microstructure of fish scales was analyzed attempting to reveal the biologic features responding to skin friction drag reduction. Then comparable bionic surface mimicking fish scales was fabricated through coating technology for drag reduction. The paint mixture was coated on a substrate through a self-developed spray-painting apparatus. The bionic surface with micron-scale caves formed spontaneously due to the interfacial convection and deformation driven by interfacial tension gradient in the presence of solvent evaporation. Comparative experiments between bionic surface and smooth surface were performed in a water tunnel to evaluate the effect of bionic surface on drag reduction, and visible drag reduction efficiency was obtained. Numerical simulation results show that gas phase develops in solid-liquid interface of bionic surface with the effect of surface topography and partially replaces the solid-liquid shear force with gas-liquid shear force, hence reducing the skin friction drag effectively. Therefore, with remarkable drag reduction performance and simple fabrication technology, the proposed drag reduction technique shows the promise for practical applications.

Molecular dynamics simulations of wetting behavior of water droplets on polytetrafluorethylene surfaces

Molecular dynamics simulations are performed to simulate the wetting behavior of nanosized water droplets on flat and pillar polytetrafluorethylene surfaces. The results show that the cutoff of the Lennard-Jones (LJ) potential has a large effect on the simulated value of the contact angle and some suggestions are given on how to choose an appropriate cutoff. On flat surfaces, the contact angle is independent of the size of the water droplet, which was determined by the energy parameters of the LJ potential. Furthermore, on pillar surfaces, two different equilibrium states are present: wetted contact and cross contact. For the wetted contact state, the contact angle increases with increasing droplet size and pillar size within a certain range. However, for the cross contact state, the contact angle and droplet size are uncorrelated, which results from the layering and structuring of molecules after their penetration into the hollows between pillars. However, additional simulations show that the final state depends on the initial geometry and the cross contact state is a metastable wetting state.

Study on effect of microparticle’s size on cavitation erosion in solid-liquid system

Five different solutions containing microparticles in different sizes were tested in a vibration cavitation erosion experiment. After the experiment, the number of erosion pits on sample surfaces, free radicals HO∙ in solutions, and mass loss all show that the cavitation erosion strength is strongly related to the particle size, and 500nm particles cause more severe cavitation erosion than other smaller or larger particles do. A model is presented to explain such result considering both nucleation and bubble-particle collision effects. Particle of a proper size will increase the number of heterogeneous nucleation and at the same time reduce the number of bubble-particle combinations, which results in more free bubbles in the solution to generate stronger cavitation erosion.

Underwater locomotion strategy by a benthic pennate diatom Navicula sp.

The mechanism of diatom locomotion has been widely researched but still remains a hypothesis. There are several questionable points on the prevailing model proposed by Edgar, and some of the observed phenomena cannot be completely explained by this model. In this paper, we undertook detailed investigations of cell structures, locomotion, secreted mucilage, and bending deformation for a benthic pennate diatom Navicula species. According to these broad evidences, an updated locomotion model is proposed. For Navicula sp., locomotion is realized via two or more pseudopods or stalks protruded out of the frustules. The adhesion can be produced due to the pull-off of one pseudopod or stalk from the substratum through extracellular polymeric substances. And the positive pressure is generated to balance the adhesion because of the push-down of another pseudopod or stalk onto the substratum. Because of the positive pressure, friction is generated, acting as a driving force of locomotion, and the other pseudopod or stalk can detach from the substratum, resulting in the locomotion. Furthermore, this model is validated by the force evaluation and can better explain observed phenomena. This updated model would provide a novel aspect on underwater locomotion strategy, hence can be useful in terms of artificial underwater locomotion devices.

Influence of Illumination on Settlement of Diatom Navicula sp.

AbstractDiatoms are responsible for biofouling, which causes many problems in various marine industries. This study examined the effects of different light conditions (intensity, incident direction, time of illumination) on the settling behavior of the marine diatom Navicula sp. on glass surfaces. The density of this diatom’s settlement on glass was strongly influenced by light conditions. Moreover, very weak light emitted on the bottom of the culture dish could also rapidly inhibit diatom settlement. These phenomena were explained by spatial interference between chloroplast and holdfast-like structures inside the thecae. The holdfast-like structure is observed to be responsible for diatom locomotion and hence the settlement behavior. It was proposed that the interrelation of illumination and attachment of diatoms allowed them to better adapt to the habitat with higher efficiency of attachment and successive reproduction.

Investigations on dynamics of interacting cavitation bubbles in strong acoustic fields

Given its importance to the dynamics of cavitation bubbles, the mutual interaction between bubbles was carefully investigated in this work. The cavitation noises emitted in different sonication conditions were recorded to study the dynamical behavior of the bubbles. The frequency spectra of the noises suggest that the dispersing state of the bubbles severely influence the oscillations of bubbles, and that the nonlinear feature of the dynamics of cavitation bubbles, imposed by the mutual bubble-bubble interaction, gradually develops with the decrease of the dispersing height. Theoretical analysis shows that the size difference between the interacting bubbles should be responsible for the increase of nonlinearity of the oscillation, and that the decrease of the distance between them could effectively enhance the nonlinear feature of the oscillation of the bubble, both of which agree well with the experimental observation.

Spontaneous transition of a water droplet from the Wenzel state to the Cassie state: a molecular dynamics simulation study

It is widely accepted that the superhydrophobic state is attributed to the formation of the Cassie state. The Cassie state is mostly metastable, which can be turned into the Wenzel state. Therefore, the superhydrophobic state is generally considered to be unstable. In this study, the wetting behaviors of a water droplet on different pillar surfaces are simulated. The spontaneous transition from the Wenzel state to the Cassie state is achieved, which is significant for the stable existence of superhydrophobicity. The transition process is analyzed in detail and can be chronologically divided into two stages: the contact area decreases and the water droplet rises. Moreover, the transition mechanism is studied, which is due to the combined effect of the surrounding pillars and the central pillar. The surrounding pillars form a no-wetting gap under the droplet, and the central pillar forces the droplet to move upward. Furthermore, three parameters that may influence the transition are studied: the pillar height, the droplet size and the hollow size.