Contact-Angle Hysteresis on Super-Hydrophobic Surfaces (original) (raw)
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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.
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.
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.
Relationship between Work of Adhesion and Contact Angle Hysteresis on Superhydrophobic Surfaces
The Journal of Physical Chemistry C, 2008
Low contact angle hysteresis is an important characteristic of superhydrophobic surfaces for nonstick applications involving the exposure of these surfaces to water or dust particles. In this article, a relationship is derived between the surface work of adhesion and the dynamic contact angle hysteresis, and the resulting predictions are compared with experimental data. Superhydrophobic surfaces with different contact angles and contact angle hysteresis were prepared by generating silicon pillars with varying pillar size and pitch. Surfaces were subsequently treated with fluoroalkyl silanes to modify further the hydrophobicity. The threephase contact line established for such systems was related to the Laplace pressure needed to maintain a stable superhydrophobic state.
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 Compression and Relaxation by a Superhydrophobic Surface: Contact Angle Hysteresis
Langmuir : the ACS journal of surfaces and colloids, 2012
In this article, the contact angle hysteresis (CAH) of acrylic glass is experimentally and theoretically studied through the compression-relaxation process of droplets by using a superhydrophobic surface with negligible CAH effect. In contrast to the existing technique in which the volume of the droplet changes during the measurement of CAH, this procedure is carried out at a constant volume of the droplet. By observing the base diameter (BD) and the contact angle (CA) of the droplet during the compression-relaxation process, the wetting behavior of the droplet can be divided into two regimes, the contact line withdrawal and the contact line pinning regimes, depending on the gap thickness (H) at the end of the compression process. During the compression process, both regimes possess similar droplet behavior; the contact line will move outward and the BD will expand while the CA remains at the advancing angle. During the relaxation process, the two regimes are significantly different...
Journal of Colloid and Interface Science, 2009
The Cassie-Baxter model is widely used to predict the apparent contact angles obtained on composite (solid-liquid-air) superhydrophobic interfaces. However, the validity of this model has been repeatedly challenged by various research groups because of its inherent inability to predict contact angle hysteresis. In our recent work, we have developed robust omniphobic surfaces which repel a wide range of liquids. An interesting corollary of constructing such surfaces is that it becomes possible to directly image the solid-liquid-air triple-phase contact line on a composite interface, using an electron microscope with non-volatile organic liquids or curable polymers. Here, we fabricate a range of model superoleophobic surfaces with controlled surface topography in order to correlate the details of the local texture with the experimentally observed apparent contact angles. Based on these experiments, in conjunction with numerical simulations, we modify the classical Cassie-Baxter relation to include a local differential texture parameter which enables us to quantitatively predict the apparent advancing and receding contact angles, as well as contact angle hysteresis. This quantitative prediction also allows us to provide an a priori estimation of roll-off angles for a given textured substrate. Using this understanding we design model substrates that display extremely small or extremely large roll-off angles, as well as surfaces that demonstrate direction-dependent wettability, through a systematic control of surface topography and connectivity.
From Sticky to Slippery Droplets: Dynamics of Contact Line Depinning on Superhydrophobic Surfaces
Physical Review Letters, 2012
This study explores how surface morphology affects the dynamics of contact line depinning of an evaporating sessile droplet on micro-pillared superhydrophobic surfaces. The result shows that neither a liquid-solid contact area nor an apparent contact line is a critical physical parameter to determine the de-pinning force. The configuration of a contact line on a superhydrophobic surface is multi-modal, comprised of both two-phase (liquid-air) and threephase (liquid-solid-air). The multi-modal state is dynamically altered when a droplet recedes. The maximal three-phase contact line attainable along the actual droplet boundary is found a direct and linear parameter that decides the de-pinning force on the superhydrophobic surface.