A fully penetrated rough-surface wetting model with , where is actual wetted solid area divided by projected area. Roughness amplifies the planar surface tendency: it lowers contact angle when .
The area ratio , not the projected fraction covered by particles. A hemispherical array of footprint coverage on an exposed flat substrate has .
A liquid spreads macroscopically over a solid with zero macroscopic equilibrium contact angle. In the simple interfacial-energy model the threshold is , or zero spreading coefficient; positive coefficient favors a spreading film. Extrapolating a concentration-dependent liquid-vapour surface tension to this threshold assumes the other interfacial energies remain fixed and no intervening adsorption or micellization behavior changes the trend.
The excess interfacial free energy released per newly wetted area when a liquid film replaces a solid-vapour interface is the displayed coefficient. Positive favors macroscopic spreading in the ideal equilibrium surface-energy model, whereas negative permits a finite contact angle obeying Young's equation. Surface preparation, chemical composition and adsorption can change all three interfacial energies.
A Cassie–Baxter state is a composite wetting state in which a drop contacts solid asperities over area fraction and trapped gas over the remainder. An ideal area-weighted contact-angle model gives the displayed apparent angle from the smooth-solid Young's equation angle . Reducing solid contact can strongly enhance apparent hydrophobicity. Such a state can be metastable and can collapse when liquid enters the texture; roughness alone does not guarantee it.
The line where the interfaces of three phases meet, for example where a liquid film meets a solid and the surrounding gas. Continuum assumptions at this line can differ from the bulk lubrication approximation.
A contact line is pinned when its position remains fixed while the surrounding drop shape or contact angle changes. Surface heterogeneity or deposited material can sustain such a state over a range of angles. Evaporation from a pinned drop then requires compensating liquid motion and can generate outward solute transport associated with the coffee-ring effect. A model with a freely receding line and fixed contact angle has a different boundary constraint and need not predict the same flow direction.
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Wetting refers to the ability of a liquid to maintain contact with a solid surface, resulting from adhesive forces between the liquid and the solid. This phenomenon is particularly important in various fields such as chemistry, materials science, and biology. When a liquid is poured onto a solid surface, the extent to which the liquid spreads out or forms droplets depends on the balance between cohesive forces (the forces holding the liquid molecules together) and adhesive forces (the forces between the liquid molecules and the surface).