Water–surface interactions and definitions for hydrophilicity, hydrophobicity and superhydrophobicity

Abstract

AbstractHydrophilicity and hydrophobicity are among the most important concepts in surface chemistry. Samuel and co-workers reported the measure of interactive forces between water and 20 different surfaces using the microbalance technique. Results showed that the wetting force correlates well to the advancing contact angle (θA), the larger the θA the lower the surface wettability. The adhesion force, measured when the water and surface first separates, correlates well to the receding contact angle (θR), the larger the θR the smaller the surface adhesion. The data also reveals that small residual water droplets are observed after the water droplet and the surface separate for surfaces with θR < 90°. This indicates high water affinity for these surfaces. No residual water droplet is observed for surfaces with θR > 90°. From the basic meaning of philicity-phobicity, θR∼90° is proposed as the new cut-off between hydrophilicity and hydrophobicity. The main driver for hydrophobicity is attributed to the high water surface tension. The merit of this proposed definition is discussed. Since wetting interaction becomes zero at θA ≥ 145°, surfaces with θR > 90° and θA ≥ 145° can further be defined as superhydrophobic. The extension of this approach to define oleophilicity/phobicity and superoleophobicity with hexadecane is discussed.