Contact angle hysteresis on rough hydrophobic surfaces
ABSTRACT In this short note, we report a quantitative investigation of the hysteresis of the Cassie and Wenzel drops on a given rough surface. The Cassie drop shows much less hysteresis compared to a Wenzel drop and is therefore preferred in applications involving moving droplets. The experimental measurements are compared with the various theoretical models for the apparent contact angles and recommendations are made.
- SourceAvailable from: Rifat Bulut
Geo-Hubei 2014 International Conference on Sustainable Civil Infrastructure; 06/2014
- "If the energy required to form a unit area of solid-liquid interface is higher than the energy required to form the liquid-air interface, the drop will continue to spread on the solid surface (Cassie 1948). However, the roughness of the solid surface could affect the interaction of the liquid with the solid surface (He et al. 2004). Therefore, the spread of the liquid drop depends on the surface energy of the liquid and the solid surface as well as the surface roughness. "
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- "The rolling resistance torque on the particle arises from a combination of two distinct effects: (1) a difference in contact angles at the point where the film interface attaches to the particle front and rear due to contact angle hysteresis and (2) fluid transported from the front to the rear of the particle due to viscous shear associated with the rolling motion. Contact angle hysteresis, associated with a difference between the contact angles observed for advancing and receding contact lines (Lam et al. 2001), is thought to arise from a variety of effects, including surface roughness and chemical heterogeneity (Choi et al. 2009; He et al. 2004; Marmur 1994; Schwartz 1998) and evaporation/adsorption processes (Diaz et al. 2010; Extrand 1998; Extrand and Kumagai 1997; Schwartz 1980). The latter process is often referred to as ''intrinsic'' hysteresis to emphasize the fact that it does not require surface heterogeneities. "
ABSTRACT: A theoretical analysis was developed for the capillary torque acting on a spherical particle rolling on a flat surface in the presence of a thin liquid film. The capillary number (the ratio of viscous force to surface tension force) is assumed to be sufficiently small that the liquid bridge has a circular cross-section. The theory identifies two mechanisms for capillary torque. The first mechanism results from the rearward shift of the liquid bridge in the presence of particle rolling, which causes the line of action of the pressure force within the liquid bridge to be located behind the particle centroid, inducing a torque that resists particle rolling. The second mechanism results from the contact angle asymmetry on the advancing and receding sides of the rolling particle, which leads to a net torque on the particle arising from the tangential component of the surface tension force. Estimates for these two types of capillary torque are obtained using experimental data, and correlations for both torques are obtained in the form of power-law fits as functions of the capillary number. When combined with a standard expression for viscous torque on a rolling particle, the capillary torque expressions are found to yield predictions for particle terminal velocity that are in good agreement with experimental data for a particle rolling down an inclined surface.Chemical Engineering Science 04/2014; 108:87–93. DOI:10.1016/j.ces.2014.01.003 · 2.61 Impact Factor
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- ", is obtained assuming a trailing film remains on the pillar tops . As seen in Fig. 8a, this assumption, originally proposed for hydrophilic surfaces , overestimates hysteresis—providing an upper bound (for φ >0.1). "
ABSTRACT: This paper presents the principle, fabricated structure, characterization and experimental results obtained for a new class of surfaces—"hydrophobic non-fouling surfaces"—for droplet-based microfluidics. Building on the theory of wetting of rough surfaces, we have developed novel surfaces which are chemically hydrophilic, i.e., the droplet is in contact with a non-fouling hydrophilic material but has high contact angle as a result of thermodynamically stabilized air traps beneath the droplet. This paper also presents the experimental characterization of rough super- hydrophobic surfaces, dynamic measurements of sliding angles of water droplets, and a modeling approach to estimate bounds on contact angle hysteresis—a major dissipative mechanism in droplet based microfluidic systems. A comprehensive study of the dependence of hysteresis on texture parameters is presented to evaluate the current model, propose a modification, and show that the two models—original and modified—provide useful bounds on the hysteresis of the surface.