Model and experimental studies for contact angles of surfactant solutions on rough and smooth hydrophobic surfaces

Abstract

Despite the practical need, no models exist to predict contact angles or wetting mode of surfactant solutions on rough hydrophobic or superhydrophobic surfaces. Using Gibbs' adsorption equation and a literature isotherm, a new model is constructed based on the Wenzel and Cassie equations. Experimental data for aqueous solutions of sodium dodecyl sulfate (SDS) contact angles on smooth Teflon surfaces are fit to estimate values for the adsorption coefficients in the model. Using these coefficients, model predictions for contact angles as a function of topological f (Cassie) and r (Wenzel) factors and SDS concentration are made for different intrinsic contact angles. The model is also used to design/tune surface responses. It is found that: (1) predictions compare favorably to data for SDS solutions on five superhydrophobic surfaces. Further, the model predictions can determine which wetting mode (Wenzel or Cassie) occurred in each experiment. The unpenetrated or partially penetrated Cassie mode was the most common, suggesting that surfactants inhibit the penetration of liquids into rough hydrophobic surfaces. (2) The Wenzel roughness factor, r, amplifies the effect of surfactant adsorption, leading to larger changes in contact angles and promoting total wetting. (3) The Cassie solid area fraction, f, attenuates the lowering of contact angles on rough surfaces. (4) The amplification/attenuation is understood to be due to increased/decreased solid-liquid contact-area.

Superhydrophobicity: Cavity growth and wetting transition

“…[53][54][55] The role of wetting agents and specifically surfactants at superhydrophobic surfaces has been studied in detail. 10,[56][57][58] We report long-range (>1 µm) normal forces between superhydrophobic surfaces in aqueous solution measured by AFM colloidal probe microscopy and show that superhydrophobicity displays as a series of events which warrants discussion of each event together with its statistical occurrence. We hereby extend work by Singh et al 30 increase the attachment of the smaller particles to the larger probes.…”

Behavior of Sliding Angle as Function of Temperature Difference between Droplet and Superhydrophobic Coating for Aircraft Ice Protection Systems

“…In fact, only for ∆T > 0, where the temperature of the droplet was higher than that of the superhydrophobic coating surface, was a significant increase in sliding angle observed, as shown in Figure 4. The significant increase in the sliding angles compared with the cases for ∆T ≤ 0 implied the change of wetting mode from the Cassie-Baxter mode with lower adhesion to the Wenzel mode with higher adhesion [25]. Furthermore, a greater temperature difference resulted in a higher sliding angle, as shown in Figure 4.…”

“…To enable a hybrid anti-/de-icing system, the superhydrophobic coating surface must promote this ease of droplet removal for different temperature conditions. A droplet's mobility over a superhydrophobic coating is evaluated by measuring the sliding angle of the water droplet placed on the coating surface [23][24][25][26]. Here, the scientific question arises whether the temperatures of the droplet and the surface or the temperature difference between the two dominates the droplet's mobility.…”

Recent Development on Self‐Cleaning Cementitious Coatings

“…Oils have relatively low surface tension and are, therefore, more difficult to suspend in a bridging state than water. Likewise, the addition of surfactants to water can reduce the surface tension and therefore the pressure required to penetrate between the features of the roughness, and many surfactants and oils have a high vapour pressure so will not evaporate under normal conditions, making them very difficult to remove [35,36].…”