Wenzel Transition Research Articles

Repellency of the Lotus Leaf: Resistance to Water Intrusion under Hydrostatic Pressure

In an attempt to better understand the repellency of the lotus leaf, a model was constructed from hydrophobic hemispheres arranged on a hexagonal array. Two scenarios were considered. In the first, the hemispheres were smooth. In the second, the hemispheres had a secondary roughness. The model shows that, without the secondary structure, the repellency of this surface geometry is relatively poor. The secondary structure directs the surface tension upward, allowing much greater resistance to penetration of water and prevents the loss of repellency. From the proposed model, the maximum intrusion pressure (or so-called Cassie-Wenzel transition) of the lotus leaf is estimated to be 12-15 kPa. The predicted maximum pressure agrees well with reported values from experimental measurements.

Selective layer-by-layer self-assembly on patterned porous films modulated by Cassie–Wenzel transition

We describe a robust and facile approach to the selective modification of patterned porous films via layer-by-layer (LBL) self-assembly. Positively charged honeycomb-patterned films were prepared from polystyrene-block-poly(N,N-dimethyl-aminoethyl methacrylate) (PS-b-PDMAEMA) and a PS/PDMAEMA blend by the breath figure method followed by surface quaternization. Alginate and chitosan were alternately deposited on the films via LBL self-assembly. The assembly on the PS-b-PDMAEMA film exhibits two stages with different growth rates, as elucidated by water contact angles, fluorescence microscopy, and quartz crystal microbalance results. The assembly can be controlled on the top surface or across all surfaces of the film by changing the number of deposition cycles. We confirm that there exists a Cassie-Wenzel transition with an increase in deposition cycles, which is responsible for the tunable assembly. For the PS/PDMAEMA film, the pores can be completely wetted and the polyelectrolytes selectively assemble inside the pores, instead of on the top surface. The controllable selective assembly forms unique hierarchical structures and opens a new route for surface modification of patterned porous films.

Tunable Assembly of Nanoparticles on Patterned Porous Film

This paper describes an approach to fully selective assembly of nanoparticles on patterned porous surface. Copolymers of polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) synthesized by atom transfer radical polymerization were used to prepare honeycomb-patterned porous films by the breath figure method. The regularity and pore size of the films can be modulated by changing the polymer composition and casting conditions such as concentration and airflow speed. Positively charged films were fabricated directly from the quaternized copolymers or by surface quaternization. X-ray photoelectron spectroscopy and adsorption of negatively charged fluorescein sodium salt confirmed the quaternization. Then assembly of negatively charged silica nanoparticles from its aqueous dispersion was performed. Results indicate that they assemble on the external surface of patterned porous films that without prewetting. For prewetted films, the nanoparticles assemble both on the external surface and in the pores. Poly(acrylic acid) deposited from its aqueous solution can serve as an effective blocking layer, which directs the selective assembly of nanoparticles into the pores, instead of the external surface of the film. It is concluded that the Cassie-Wenzel transition is the key to the selective assembly on the highly porous films. The well-defined selective assembly forms unique hierarchical structures of nanoparticles and greatly enlarges the diversity of structures of nanoparticle aggregates. This general approach also opens a straightforward route to the selective modification of patterned porous films.

Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free Electrodeposition and Their Cassie−Wenzel Transition after Hydrophobization

We report a facile electrochemical route for the one-step fabrication of ZnO nanowire (NW) arrays. The method is seed-layer-free, and the NWs are directly attached to a fluorine-doped tin oxide (FTO) substrate. The effects of growth temperature, precursor concentration, substrate etching, and deposition time on the layer morphology and structure are analyzed. The ZnO NWs are vertically well-aligned and textured with the c-axis normal to the substrate. The growth of the more vertically oriented initial wires is favored by a self-alignment process, and the layer texturing with the c-axis oriented normal to the surface is increased upon deposition. NWs with aspect ratios higher than 30 have been synthesized. The as-grown layers were superhydrophilic, and they were converted to superhydrophobic by surface derivatization with stearic acid (SA). The surface could be commuted back from superhydrophobic to superhydrophilic by a simple acetone washing. The present work demonstrates the importance of oxide NW length to control the hydrophobic state of the surface. By increasing the NW length (and then the aspect ratio), a transition between a Wenzel state and a Cassie−Baxter state was found. For long and homogeneous ZnO NWs, the hysteresis is low (5.5°), and the advancing and receding contact angles are high (168.3°/162.8° max). The role of wire density is discussed. The superhydrophobic layers are of interest for self-cleaning surfaces, biological experiments, and nano/microfluidics.

Preventing the Cassie−Wenzel Transition Using Surfaces with Noncommunicating Roughness Elements

Control and switching of liquid droplet states on artificially structured surfaces have significant applications in the field of microfluidics. The present work introduces the concept of using structured surfaces consisting of noncommunicating roughness elements to prevent the transition of a droplet from the Cassie to the Wenzel state. The use of noncommunicating roughness elements leads to a confinement of the medium under the droplet in its Cassie state. Transition to the Wenzel state on such surfaces requires expulsion of this confined medium, which offers significantly increased resistance to the Wenzel transition unlike surfaces consisting of communicating roughness elements. This enhances the robustness of the Cassie state and significantly minimizes the possibility of the Cassie-Wenzel transition. In the present work, the resistance of a surface to the Wenzel transition is measured in terms of the electrowetting (EW) voltage required to trigger this transition. It is seen that surfaces with noncommunicating roughness elements (cratered surfaces) require significantly higher voltages to trigger the Wenzel transition than corresponding surfaces with communicating roughness elements. The findings from the present work also indicate that EW-induced droplet morphology control characteristics show a strong dependence on the nature of the roughness elements (communicating versus noncommunicating). Different aspects of droplet morphology and EW-induced state transition control on surfaces with noncommunicating roughness elements are analyzed; it is seen that such surfaces offer significant possibilities for the development of robust superhydrophobic surfaces.

Electrowetting-Based Control of Droplet Transition and Morphology on Artificially Microstructured Surfaces

Electrowetting (EW) has recently been demonstrated as a powerful tool for controlling droplet morphology on smooth and artificially structured surfaces. The present work involves a systematic experimental investigation of the influence of electrowetting in determining and altering the state of a static droplet resting on an artificially microstructured surface. Extensive experimentation is carried out to benchmark a previously developed energy-minimization-based model that analyzed the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The EW voltage required to trigger a transition from the Cassie state to the Wenzel state is experimentally determined for surfaces having a wide range of surface parameters (surface roughness and fraction of surface area covered with pillars). The reversibility of the Cassie-Wenzel transition upon the removal of the EW voltage is also quantified and analyzed. The experimental results from the present work form the basis for the design of surfaces that enable dynamic control of droplet morphology. A significant finding from the present work is that nonconservative dissipative forces have a significant influence in opposing fluid flow inside the microstructured surface that inhibits reversibility of the Cassie-Wenzel transition. The artificially structured surfaces considered in this work have microscale roughness feature sizes that permits direct visual observation of EW-induced Cassie-Wenzel droplet transition; this is the first reported visual confirmation of EW-induced droplet state transition.

Biomimetic Superhydrophobic Surfaces: Multiscale Approach

Micro- and macrodroplet evaporation and condensation upon micropatterned superhydrophobic surfaces built of flattop pillars are investigated with the use of an environmental scanning electron microscope. It is shown that the contact angle hysteresis depends upon both kinetic effects at the triple line and adhesion hysteresis (inherently present even at a smooth surface) and that the magnitude of the two contributions is comparable. The transition between the composite (Cassie) and wetted (Wenzel) states is a linear effect with the microdroplet radius proportional to the pitch over pillar diameter. It is shown that wetting of a superhydrophobic surface is a multiscale phenomenon that involves three scale lengths. Although the contact angle is the macroscale parameter, the contact angle hysteresis and the Cassie--Wenzel transition cannot be determined from the macroscale equations and are governed by micro- and nanoscale effects.

Why do pigeon feathers repel water? Hydrophobicity of pennae, Cassie–Baxter wetting hypothesis and Cassie–Wenzel capillarity-induced wetting transition

Wetting of pigeon feathers has been studied. It was demonstrated that the Cassie–Baxter wetting regime is inherent for pigeon pennae. The water drop, supported by network formed by barbs and barbules, sits partially on air pockets. Small static apparent angle hysteresis justifies the Cassie–Baxter wetting hypothesis. A twofold structure of a feather favors large contact angles and provides its water repellency. Cassie–Wenzel transition has been observed under drop evaporation, when drop radius becomes small enough for capillarity-induced water penetration into the protrusions, formed by barbules.