Yuying Yan

Mimicking natural superhydrophobic surfaces and grasping the wetting process: a review on recent progress in preparing superhydrophobic surfaces.

A typical superhydrophobic (ultrahydrophobic) surface can repel water droplets from wetting itself, and the contact angle of a water droplet resting on a superhydrophobic surface is greater than 150°, which means extremely low wettability is achievable on superhydrophobic surfaces. Many superhydrophobic surfaces (both manmade and natural) normally exhibit micro- or nanosized roughness as well as hierarchical structure, which somehow can influence the surfaces water repellence. As the research into superhydrophobic surfaces goes deeper and wider, it is becoming more important to both academic fields and industrial applications. In this work, the most recent progress in preparing manmade superhydrophobic surfaces through a variety of methodologies, particularly within the past several years, and the fundamental theories of wetting phenomena related to superhydrophobic surfaces are reviewed. We also discuss the perspective of natural superhydrophobic surfaces utilized as mimicking models. The discussion focuses on how the superhydrophobic property is promoted on solid surfaces and emphasizes the effect of surface roughness and structure in particular. This review aims to enable researchers to perceive the inner principles of wetting phenomena and employ suitable methods for creation and modification of superhydrophobic surfaces.

A lattice Boltzmann method for incompressible two-phase flows on partial wetting surface with large density ratio

This paper reports a new numerical scheme of the lattice Boltzmann method for calculating liquid droplet behaviour on particle wetting surfaces typically for the system of liquid-gas of a large density ratio. The method combines the existing models of Inamuro et al. [T. Inamuro, T. Ogata, S. Tajima, N. Konishi, A lattice Boltzmann method for incompressible two-phase flows with large density differences, J. Comput. Phys. 198 (2004) 628-644] and Briant et al. [A.J. Briant, P. Papatzacos, J.M. Yeomans, Lattice Boltzmann simulations of contact line motion in a liquid-gas system, Philos. Trans. Roy. Soc. London A 360 (2002) 485-495; A.J. Briant, A.J. Wagner, J.M. Yeomans, Lattice Boltzmann simulations of contact line motion: I. Liquid-gas systems. Phys. Rev. E 69 (2004) 031602; A.J. Briant, J.M. Yeomans, Lattice Boltzmann simulations of contact line motion: II. Binary fluids, Phys. Rev. E 69 (2004) 031603] and has developed novel treatment for partial wetting boundaries which involve droplets spreading on a hydrophobic surface combined with the surface of relative low contact angles and strips of relative high contact angles. The interaction between the fluid-fluid interface and the partial wetting wall has been typically considered. Applying the current method, the dynamics of liquid drops on uniform and heterogeneous wetting walls are simulated numerically. The results of the simulation agree well with those of theoretical prediction and show that the present LBM can be used as a reliable way to study fluidic control on heterogeneous surfaces and other wetting related subjects.

Characterisation of surface wettability based on nanoparticles

Nanoparticles are becoming frequently used in the research area of creating functional surfaces because they can be more versatile than just making dimensions smaller. Particularly, a variety of nanoparticles have been applied for the construction of superhydrophobic and superhydrophilic surfaces with micro- and nano-scaled structures. As nanoparticles can also be fashioned and modified, their effects will be of great importance to the formed surface structures. In the present paper, we review the recent research progress in the utilization of nanoparticles to form extremely wettable/non-wettable surface structures and their influence on surface wettability. This report manifests an apparent inclination of nanoparticle structured surfaces using the multidisciplinary approaches, from the viewpoint of engineer/scientist. Therefore, the typical methodologies with regard to the use of nanoparticles, including the preparation and functionalisation processes, for the realization of surface wettabilities are discussed in this work. The discussions also represent some of the size-determined phenomena that are related to wettable/non-wettable surfaces. This Review thus provides an insight into the connection between nanoparticles and surface wettability.

NUMERICAL MODELING OF ELECTROHYDRODYNAMIC (EHD) EFFECT ON NATURAL CONVECTION IN AN ENCLOSURE

Mathematical and numerical modeling of electrohydrodynamic (EHD) enhancement of natural convection in enclosures is carried out. An electric current in dielectric liquid is modeled as a directed motion of electrically charged particles injected into a neutral fluid; the electric body force and Joule heat are added to the momentum and energy equations, respectively. Based on this, numerical studies are carried out for EHD effects on natural convection in enclosures. It is found that, at the same electric field intensity, the EHD enhancement of heat transfer is different for different electric density injections; applying a nonuniform electric field offers better EHD enhancement of heat transfer than applying a uniform electric field.

Modeling Superhydrophobic Contact Angles and Wetting Transition

It is well known that surface roughness has a very important effect on superhydrophobicity. The Wenzel and Cassie-Baxter models, which correspond to the homogeneous and heterogeneous wetting respectively, are currently primary instructions for designing superhydrophobic surfaces. However, the particular drop shape that a drop exhibits might depend on how it is formed. A water drop can occupy multiple equilibrium states, which relate to different local minimal energy. In some cases, both equilibrium states can even co-exist on a same substrate. Thus the apparent contact angles may vary and have different values. We discuss how the Wenzel and Cassie-Baxter equations determine the homogeneous and heterogeneous wetting theoretically. Contact angle analysis on hierarchical surface structure and contact angle hysteresis has been put specific attention. In particular, we study the energy barrier of transition from Cassie-Baxter state to Wenzel state, based on existing achievement by previous researchers, to determine the possibility of the transition and how it can be interpreted. It has been demonstrated that surface roughness and geometry will influence the energy required for a drop to get into equilibrium, no matter it is homogeneous or heterogeneous wetting.

Wetting Behaviours of a Single Droplet on Biomimetic Micro Structured Surfaces

Natural surfaces with super hydrophobic properties often have micro or hierarchical structures. In this paper, the wetting behaviours of a single droplet on biomimetic micro structured surfaces with different roughness parameters are investigated. A theoretical model is proposed to study wetting transitions. The results of theoretical analysis are compared with those of experiment indicating that the proposed model can effectively predict the wetting transition. Furthermore, a numerical simulation based on the meso scale Lattice Boltzmann Method (LBM) is performed to study dynamic contact angles, contact lines, and local velocity fields for the case that a droplet displays on the micro structured surface. A spherical water droplet with rs = 15 µm falls down to a biomimetic square-post patterned surface under the force of gravity with an initial velocity of 0.01 m·s−1 and an initial vertical distance of 20 µm from droplet centre to the top of pots. In spite of a higher initial velocity, the droplet can still stay in a Cassie state; moreover, it reaches an equilibrium state at t ≈ 17.5 ms, when contact angle is 153.16° which is slightly lower than the prediction of Cassie-Baxter’s equation which gives θCB = 154.40°.

CFD Simulation of Fish-like Body Moving in Viscous Liquid

The study of fish-like bodies moving in liquid is an interesting and challenging research subject in the fields of biolocomotion and biomimetics. Typically the effect of tail oscillation on fluid flow around such a body is highly unsteady, generating vortices and requiring detailed analysis of fluid-structure interactions. An understanding of the complexities of such flows is of interest not only to biologists but also to engineers interested in developing vehicles capable of emulating the high performance of fish propulsion and manoeuvring. In the present study, a computational fluid dynamic (CFD) simulation of a three-dimensional biomimetic fish-like body has been developed to investigate the fluid flows around this body when moving in a viscous liquid. A parametric analysis of the variables that affect the flow surrounding the body is presented, along with flow visualisations, in an attempt to quantify and qualify the effect that these variables have on the performance of the body. The analysis provided by the unsteady transient simulation of a fish-like body has allowed the flow surrounding a fish-like body undergoing periodic oscillations to be studied. The simulation produces a motion of the tail in the (x, y) plane, with the tail oscillating as a rigid body in the form of a sinusoidal wave.

Superhydrophobic Composite Films Based on THS and Nanoparticles

The present paper reports a facile and direct method to render superhydrophobicity onto substrate surfaces. SiO2 nanoparticles of various sizes are added into trimethoxyhexadecylsilane (THS) solutions to prepare superhydrophobic composite films, which are formed on test substrates. The formed composite films, with different nanoparticle concentrations and sizes, exhibit hierarchical structures in micro- and nano-scale that are positively important for superhydrophobicity. For the sake of comparison, the composite films of polydimethylsiloxane (PDMS) and SiO2 nanoparticles are also prepared and investigated. The contact angles of water droplets are measured and their change with nanoparticle concentrations and sizes are discussed. Typical structures of those formed surface are observed by using Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM). Based on the observation and measurement, we investigate how the pattern of superhydrophobicity changes with the concentration and size of nanoparticles. Crucial theories involved and related to the phenomena are also discussed.

A Monte Carlo (MC) method applied to the medium with nongray absorbing-emitting-anisotropic scattering particles and gray approximation

A Monte Carlo (MC) method is applied to calculate radiative transfer in a nongray medium using spectral radiative exchange factor RD ij u . The creditability of the present MC model has been validated by comparing it with the results using other research methods. Meanwhile, the radiative transfer in an isothermal and nonisothermal medium with nongray absorbing-emitting-anisotropic ash particles is calculated by a nongray model and several gray approximation methods. A simplification from a nongray problem to a gray one by Rosselands mean extinction coefficient, mean albedo y bar 2 , and Planck mean phase function is suggested.

Numerical Simulation of Electroosmotic Flow near Earthworm Surface

The electroosmotic flow near an earthworm surface is simulated numerically to further understand the anti soil adhesion mechanism of earthworm. A lattice Poisson method is employed to solve electric potential and charge distributions in the electric double layer along the earthworm surface. The external electric field is obtained by solving a Laplace equation. The electroosmotic flow controlled by the Navier-Stokes equations with external body force is simulated by the lattice Boltzmann method. A benchmark test shows that accurate electric potential distributions can be obtained by the LPM. The simulation shows that the moving vortices, which probably contribute to anti soil adhesion, are formed near earthworm body surface by the nonuniform and variational electrical force.