Contact Angle Hysteresis Generated by Strong Dilute Defects (original) (raw)

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.

Contact angle hysteresis on superhydrophobic stripes

The Journal of chemical physics, 2014

We study experimentally and discuss quantitatively the contact angle hysteresis on striped superhydrophobic surfaces as a function of a solid fraction, ϕS. It is shown that the receding regime is determined by a longitudinal sliding motion of the deformed contact line. Despite an anisotropy of the texture the receding contact angle remains isotropic, i.e., is practically the same in the longitudinal and transverse directions. The cosine of the receding angle grows nonlinearly with ϕS. To interpret this we develop a theoretical model, which shows that the value of the receding angle depends both on weak defects at smooth solid areas and on the strong defects due to the elastic energy of the deformed contact line, which scales as ϕS(2)lnϕS. The advancing contact angle was found to be anisotropic, except in a dilute regime, and its value is shown to be determined by the rolling motion of the drop. The cosine of the longitudinal advancing angle depends linearly on ϕS, but a satisfactory...

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...

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.

Modeling contact angle hysteresis of a liquid droplet sitting on a cosine wave-like pattern surface

Journal of Colloid and Interface Science, 2012

A liquid droplet sitting on a hydrophobic surface with a cosine wave-like square-array pattern in the Wenzel state is simulated by using the Surface Evolver to determine the contact angle. For a fixed drop volume, multiple metastable states are obtained at two different surface roughnesses. Unusual and noncircular shape of the three-phase contact line of a liquid droplet sitting on the model surface is observed due to corrugation and distortion of the contact line by structure of the roughness. The contact angle varies along the contact line for each metastable state. The maximum and minimum contact angles among the multiple metastable states at a fixed viewing angle correspond to the advancing and the receding contact angles, respectively. It is interesting to observe that the advancing/receding contact angles (and contact angle hysteresis) are a function of viewing angle. In addition, the receding (or advancing) contact angles at different viewing angles are determined at different metastable states. The contact angle of minimum energy among the multiple metastable states is defined as the most stable (equilibrium) contact angle. The Wenzel model is not able to describe the contact angle along the three-phase contact line. The contact angle hysteresis at different drop volumes is determined. The number of the metastable states increases with increasing drop volume. Drop volume effect on the contact angles is also discussed.

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.

Trapping of Water Drops by Defects on Superhydrophobic Surfaces

In this work the effect of line-shaped defects on the motion of water drops on superhydrophobic surfaces have been investigated using high-speed video. The defects were introduced on superhydrophobic wax surfaces by a simple scratching method. It is shown that the motion of the drop in the vicinity of the defect can be approximated by a damped harmonic oscillator. Whether a drop gets trapped or not while traversing the defect is determined by the incident speed and the characteristics of the oscillator, more specifically by the damping ratio ζ and the nondimensional forcing constant â. We also show that it is possible to predict the trapping speed as well as the exit speed using a simple work-energy consideration in systems with negligible viscous dissipation.