A modified Cassie–Baxter relationship to explain contact angle hysteresis and anisotropy on non-wetting textured surfaces (original) (raw)

Contact Angle Hysteresis Characterization of Textured Super-Hydrophobic Surfaces

TRANSDUCERS 2007 - 2007 International Solid-State Sensors, Actuators and Microsystems Conference, 2007

This paper presents the fabrication of rough super-hydrophobic surfaces, dynamic high-speed measurements of sliding angles of water droplets, and develops a mechanistic understanding of contact angle hysteresis-the major dissipative mechanism in droplet based microfluidic systems. We investigate texture-dependence of hysteresis, evaluate the current model, propose a modification, and observe that the two models-current and proposed-are useful bounds on hysteresis of the surface except in ultrahydrophobic regime where observed hysteresis is significantly higher than predictions of either model.

Contact-Angle Hysteresis on Super-Hydrophobic Surfaces

Langmuir, 2004

The relationship between perturbations to contact angles on a rough or textured surface and the superhydrophobic enhancement of the equilibrium contact angle is discussed theoretically. Two models are considered. In the first (Wenzel) case, the super-hydrophobic surface has a very high contact angle and the droplet completely contacts the surface upon which it rests. In the second (Cassie-Baxter) case, the super-hydrophobic surface has a very high contact angle, but the droplet bridges across surface protrusions. The theoretical treatment emphasizes the concept of contact-angle amplification or attenuation and distinguishes between the increases in contact angles due to roughening or texturing surfaces and perturbations to the resulting contact angles. The theory is applied to predicting contact-angle hysteresis on rough surfaces from the hysteresis observable on smooth surfaces and is therefore relevant to predicting roll-off angles for droplets on tilted surfaces. The theory quantitatively predicts a "sticky" surface for Wenzel-type surfaces and a "slippy" surface for Cassie-Baxter-type surfaces.

Bounds on Contact Angle Hysteresis of Textured Super-Hydrophobic Surfaces

This paper presents the fabrication of rough super-hydrophobic surfaces, dynamic measurements of sliding angles of water droplets, and a modeling approach to estimate bounds on contact angle hysteresis-the major dissipative mechanism in droplet based microfluidic systems. We investigate the dependence of hysteresis on texture parameters, evaluate the current model, propose a modification, and show that the two models-current and proposed-are useful bounds on the hysteresis of the surface.

Advanced understanding of stickiness on superhydrophobic surfaces

Scientific Reports, 2013

This study explores how contact angle hysteresis and titling angle relate with stickiness on superhydrophobic surfaces. The result indicates that contact angle hysteresis could not be mentioned as a proper factor to evaluate the surface stickiness. By analyzing the system pinning force of droplet placed on a titled surface, we concluded that both solid fraction and surface geometric factor are the critical factors determining the surface stickiness. O ver last decades, some special surfaces in nature feature high water contact angle (.150u) attracted enormous interests for both fundamental research and practical application 1-8. Some of these surfaces, lotus leaves for instance, not only exhibit high water contact angle, but also have extremely low sliding angle, which cause water droplets to bead and roll off from the surface by slightly titling it 1. This behavior provides extreme water repellency and self-cleaning characteristics 2-5. Meanwhile, some other surfaces, i.e. rose-petal surface, with large water droplet sliding angles have the ability to make water droplet pinned on the surfaces at any tilting angle (TA) 6. Such high-adhesion surfaces also possess many potential applications, no-loss fluid transportation for example 7,8. According to the different values of TA, the above mentioned surfaces fall into two categories, which are ''slippery'' and ''sticky'' ones, featuring low TA and high TA respectively, all these surfaces are uniformly named as ''superhydrophobic'' surfaces in litheture though. Meanwhile, people found that there exists a wide range of ''metastable'' contact angles when a liquid meniscus scans the solid surface 9. Because there are free energy barriers which exist between these metastable states, a true ''equilibrium'' contact angle is almost impossible to measure in real time. The famous Wenzel's and Cassie's theories are therefore only valuable in predicting the thermodynamically stable contact angles in theory 10,11. Therefore, contact angle hysteresis (CAH) defined as the difference of advancing angle and receding angle is usually measured to fully characterize any surface 12. Recently, however, it seems CAH has a new function and already somehow been treated as a criterion to evaluate the stickiness of superhydrophobic surfaces by researchers and students 13-16. People believe that increase in stickiness will absolutely result in a corresponding increase in CAH. As mentioned in the previous section, TA value has widely been used to describe the surface stickiness. Since both these two parameters have been mentioned in publications to describe the same physical phenomena, it is also naturally concluded by researchers that TA and CAH share the same changing tendency with the stickiness 15. However, we achieved an opposite conclusion according to our recent experimental investigation. We fabricated several posts arrays with different solid fractions. The TA increases with the solid fraction, which proves that the stickiness/pinning force increases with solid fraction 17-19. Nevertheless, the CAH shows a decreasing tendency while the solid fraction increases. This result is of particular interest from theory point of view. It reveals that CAH may not be the proper factor to describe the surface stickiness, and it is not necessary to take extra effects to make low CAH surface to achieve good slip performance.

Droplet Impingement and Wetting Hysteresis on Textured Hydrophobic Surfaces

2010 14th International Heat Transfer Conference, Volume 3, 2010

We study the wetting energetics and wetting hysteresis of sessile and impacting water droplets on superhydrophobic surfaces as a function of surface texture and surface energy. Detailed experiments tracking contact line motion simultaneously with contact angle provides new insights on the wetting hysteresis, stick-slip behavior and dependence on contact line velocity. For sessile drops, we find three wetting regimes on these surfaces: equilibrium Cassie at small feature spacing, equilibrium Wenzel at large feature spacing, and an intermediate state at medium feature spacing. We observe minimum wetting hysteresis not on surfaces that exhibit Cassie wetting but rather on surfaces in the intermediate regime. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the surface. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop to the capillary pressure associated with the wetting-energy barrier of the textured surface. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets show very good agreement with this simple pressure-balance model.

Advances in Contact Angle, Wettability and Adhesion

2013

Micro-or nano-textured hydrophobic surfaces have attracted considerable interest due to their highly water-repellent property, and are called superhydrophobic. Although such superhydrophobic surfaces typically exhibit high contact angles for water droplets, their adhesion and frictional properties such as contact angle hysteresis are signifi cantly affected by the dynamics of contact line pinning at the droplet boundary. However, a clear correlation between the contact line pinning and the contact angle hysteresis has not been revealed yet. In this paper, we review the literature reporting on their correlation, both for chemically and physically patterned heterogeneous surfaces, including our recent discovery on superhydrophobic surfaces. Then, we propose and discuss an appropriate new physical parameter that shows close and consistent correlation between the dynamics of contact line pinning and the contact angle hysteresis.

Mapping micron-scale wetting properties of superhydrophobic surfaces

arXiv: Soft Condensed Matter, 2019

There is a huge interest in developing super-repellent surfaces for anti-fouling and heat transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be indirectly inferred from the Furmidge's relation. While easy to implement, contact angle measurements are semi-quantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an Atomic Force Microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micron-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning-depinning dynamics as a droplet detaches from or moves across the surface.