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.
"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. "
"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. "
[Show abstract][Hide abstract] 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 · 2.34 Impact Factor
" The effect of morphology on wettability of rough surfaces has been studied through a variety of surfaces with different morphologies.    For example, surfaces with microscale square pillars are made to prove that water drops can be at different state on the same surface,  it is reported that the parallel grooves on the solid surface have great influence on the anisotropic wetting behavior ,   and circular pillars of 5–15 mm diameter are also fabricated on the solid surface, which have proved to be of great importance on the evaporation mechanism of water drops.  The controlled patterning of hydrophilic areas on superhydrophobic substrates could also be used in the expanding area of microfluidics , generating open structures on which liquids could be guided through channels by surface tension. "
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