Design Of Superhydrophobic Surfaces Research Articles

Preliminary study on the design of superhydrophobic surface by 3D inkjet printing of a sol-gel solution

Preliminary study on the design of superhydrophobic surface by 3D inkjet printing of a sol-gel solution

Long term antifouling performance of superhydrophobic surfaces in seawater environment: Effect of substrate material, hierarchical surface feature and surface chemistry

Biofouling is a serious concern for the materials that are in contact with seawater or brackish water. Superhydrophobic (SHP) surfaces have the potential to lower fouling. Although good progress is made on the design of SHP surfaces, the long-term antifouling performance of SHP surfaces is still a major concern. We report the effect of substrate material, hierarchical surface features, chemistry, and the nature of bacteria on the long-term antifouling properties of superhydrophobic for the first time. Three different superhydrophobic coatings on aluminum, Cr-Mo steel, and Ti substrates were studied in a seawater bacterial consortium. Silane based SHP Al with a water contact angle (WCA) ~170 ± 1.5°, cerium myristate based SHP Cr-Mo of WCA ~ 162 ± 2.4° and stearic acid based SHP Ti of WCA ~168 ± 3.3° were prepared by dip coating, electrodeposition, and rapid breakdown anodization, respectively. After 120 h of exposure in seawater bacterial consortium, the cell viability reductions were three, two and one order in SHP Al, SHP Cr-Mo, and SHP Ti, respectively. The SHP Al samples possessed both micron-sized structures (~2 µm) along with nano-sized spherical particles (~ 57 nm). The quat silane-like structures gave better antibacterial resistance to SHP Al. The predominant bacteria present on bare Al,Ti, Cr-Mo and SHP Al, and Ti surfaces after exposure were Bacillus and Pseudomonas sp., whereas cocci bacteria was seen on the SHP Cr-Mo. After 240 h of exposure, a thicker biofilm was seen on the SHP Cr Mo surface due to the densely populated nanostructures of sizes comparable to cocci diameter, which was favorable for the cocci bacterial attachment. Though the average WCA followed the trend SHP Ti> SHP Al>SHP Cr Mo, the bacterial adhesion trend was SHP Ti> SHP Cr Mo> SHP Al, after 120 h of exposure. This suggests that wettability is not the only parameter determining the antifouling performance of an SHP coating but is also influenced by the surface charge density, surface roughness, stiffness, topography, and environmental variables such as fluid dynamics, bacterial motility, and size and morphology of bacteria. Both the surface morphology and the chemical nature were responsible for this reduced cell viability of the SHP Al surface. These findings not only provide new insights into the understanding of long-term biofouling of SHP surfaces but also would provide new directions in the design of robust SHP surfaces.

A lattice Boltzmann simulation of coalescence-induced droplet jumping on superhydrophobic surfaces with randomly distributed structures

The motion of coalescence-induced condensate droplets on superhydrophobic surface (SHS) has attracted increasing attention in energy-related applications. Previous researches were focused on regularly rough surfaces. Here a new approach, a mesoscale lattice Boltzmann method (LBM), is proposed and used to model the dynamic behavior of coalescence-induced droplet jumping on SHS with randomly distributed rough structures. A Fast Fourier Transformation (FFT) method is used to generate non-Gaussian randomly distributed rough surfaces with the skewness (Sk), kurtosis (K) and root mean square (Rq) obtained from real surfaces. Three typical spreading states of coalesced droplets are observed through LBM modeling on various rough surfaces, which are found to significantly influence the jumping ability of coalesced droplet. The coalesced droplets spreading in Cassie state or in composite state will jump off the rough surfaces, while the ones spreading in Wenzel state would eventually remain on the rough surfaces. It is demonstrated that the rough surfaces with smaller Sks, larger Rqs and a K at 3.0 are beneficial to coalescence-induced droplet jumping. The new approach gives more detailed insights into the design of SHS.

An analytical model of nanopatterned superhydrophobic surfaces

Wetting behaviors on a hydrophobic nanopatterned surface, which are of primary consideration in the design of superhydrophobic surfaces, are studied on an analytic model that shows the hole-patterned substrate is more favorable for water droplets to nucleate on the inside of holes based on nucleation theory. After droplet nucleation, the transition from Wenzel state to Cassie–Baxter state may occur during the growth stage. The calculation of the free energy of droplets during the growth stage indicates that the wetting transitions depend on the volume of the droplets, and Wenzel state or Cassie–Baxter state depends on the absolute size and relative size of the geometrical morphology of the nanoholes.

Visualization of contact line motion on hydrophobic textures

Contact angle hysteresis (CAH) has important implications in a variety of natural and industrial applications. The contact line motion of Cassie drops on microtextured surfaces is directly visualized using high-magnification imaging and fluorescence microscopy. The advancing contact line is observed to glide smoothly over the micro-posts, whereas the receding contact line exhibits significant stick-slip. Using simultaneous measurements of drop width and contact angle, the authors quantify stick-slip amplitude and define contact width hysteresis. A new scaling is proposed for drop roll-off angle α wherein sinα is linearly proportional to the practical work of adhesion (1 + cosθr). This study provides insight into the dynamic nature of CAH and the design of superhydrophobic surfaces with optimal drop shedding.

Simulation of droplet impact on super hydrophobic surface

Super hydrophobic surfaces find uses in many applications; therefore proper design of super hydrophobic surfaces is very crucial. A lot of work has already been done for static droplets and super hydrophobic surface interactions. There have also been some significant experiments carried out for dynamic droplet impact on super hydrophobic surfaces. The present work focuses on the super hydrophobic surface under dynamic conditions, with the study predominantly carried out through numerical simulation. Various parameters during impact and time variance after impact (typically up to 10 μs) were considered. The transition from water hammer pressure (order of ρCV) to flow pressure (order of 1/2 ρV2) is taken as the main parameter of analysis. During water hammer pressure domain, a strong tendency to cause wetting (Wenzel state) is seen. During flow pressure domain, wetting tendency is significantly reduced (Cassie-Baxter state). These states and the transition from one to the other are very crucial to the desig...

SURFACE EVOLVER SIMULATIONS OF DROPS ON MICROPOSTS

An important feature in the design of superhydrophobic surfaces is their robustness against collapse from the Cassie–Baxter configuration to the Wenzel state. Upon such a transition a surface loses its properties of low adhesion and friction. We describe how to adapt the Surface Evolver algorithm to predict the parameters and mechanism of the collapse transition on posts of arbitrary shape. In particular, contributions to the free energy evaluated over the solid–liquid surface are reduced to line integrals to give good convergence. The algorithm is validated for straight, vertical and inclined, posts. Numerical results for curved posts with a horizontal section at their ends show that these are more efficient in stabilizing the Cassie state than straight posts, and identify whether the interface first depins from the post sides or the post tips.

Design of superhydrophobic surfaces by synthesis of carbon nanotubes over Co–Mo nanocatalysts deposited under microwave irradiation on Ti-containing mesoporous silica thin films

The superhydrophobic surface has been designed by the synthesis of carbon nanotubes (CNTs) on Ti-containing mesoporous silica thin films (Ti-MSTFs) with Co-Mo binary nanocatalysts. The active Co-Mo catalysts have been successfully deposited on Ti-MSTFs under microwave irradiation. SEM and TEM observations after CNT synthesis revealed that surfaces of Ti-MSTFs were densely covered with CNTs having a diameter of 15 nm. Raman spectra indicated that the undesired structural defects in the carbon network of the synthesized CNTs, which would lead to the formation of hydroxyl groups, were scarce. Interestingly, hydrophobic properties of samples after CNT synthesis were enhanced with increasing titanium concentration of Ti-MSTF, and the water contact angle reached up to 165° on Ti-MSTF with a titanium concentration of 10 at%. The combination of dispersed titanium oxide moieties within the silica frameworks and the microwave irradiation made a great contribution to deposit active Co-Mo catalysts responsible for the formation of well-dense CNTs.

Optimizing Super-Hydrophobic Surfaces: Criteria for Comparison of Surface Topographies

Two useful criteria are employed for comparing the super-hydrophobicity of surface designs from a wetting point of view. The first is the apparent contact angle at the transition point between the homogeneous (Wenzel) wetting regime and the heterogeneous (Cassie–Baxter) wetting regime. The second criterion is the wetted area of the solid surface in the heterogeneous wetting regime. Four different types of model rough surfaces are compared using the above criteria, in order to demonstrate optimization considerations in the design of super-hydrophobic surfaces. These model surfaces consist of protrusions of cylinders, truncated cones, paraboloids, and hemispheres. It appears that protrusions of very small base radii are not desirable, since they must be very high in order to conform to the first criterion. For higher base radii it seems that rounded protrusions, such as the paraboloids, are advantageous. This is an interesting conclusion, since the protrusions of the Lotus leaf are of a similar geometry. Based on the case of hemispherical protrusions it also appears that more than one geometrical degree of freedom is necessary in order to optimize a super-hydrophobic surface.