Experimental methods in chemical engineering: Contact angles

Abstract

The contact angle (CA) formed at equilibrium at the three‐phase line of contact between a liquid, a solid, and a gas may be expressed as a function of both the interfacial and surface tensions. Young first derived this thermodynamic relationship in 1805. In practice, multiple CA values are observed due to kinetic phenomena induced by evaporation, vapour adsorption, or swelling, and thermodynamic ones induced by roughness and surface chemical heterogeneities, even at molecular‐scale. These non‐ideal conditions result into an hysteresis, i.e., a difference between wetting and dewetting behaviours, and Young's equation rarely applies. Three measuring methods stand out for their applicability and reliability. In the sessile drop method, a syringe deposits a liquid drop on a flat surface and the contact angle is measured through optical means based on the drop shape. In the Wilhelmy balance method, the force required to immerse a solid plate in a bath of liquid is indirectly related to the contact angle. In the Washburn capillary rise method, the contact angle is derived from the rate at which a liquid rises by capillarity through a packed bed of powder. Employing probe liquids of various polarities, the free surface energy of the solid may be estimated. Over the last two years, ∼8600 published articles mentioned “contact angle” in their topic. Their main focus was either to develop the fundamental understanding of wetting science, or to assess the success of surface modification methods for the production of novel surfaces, composites, and membranes with enhanced wetting, adhesive, and filtration properties.

Orienting Cellulose Nanocrystal Functionalities Tunes the Wettability of Water-Cast Films

“…A liquid droplet sitting on a surface has a three-phase boundary between the liquid, the solid, and the air, which is characterized by its contact angle (CA). 29 Ideal substrates are expected to be flat, rigid, chemically homogeneous, and unperturbed by the chemical interactions they may form with the liquid. 30 There, the Young contact angle, θ Y , is the single CA that may form at rest: it is a thermodynamic equilibrium that may be met independently of the sample's or the drop's history.…”

“…Irregularities impede the progression of the liquid upon wetting and prevent its receding upon dewetting. 29 In a dynamic experiment, the highest CA value is thus measured in advancing conditions: the advancing contact angle, θ a ; and the lowest CA in receding conditions: the receding contact angle, θ r . The contact angle hysteresis, H, is defined as H = θ a − θ r (H ≥ 0°).…”

AKD emulsions stabilized by guar gel: a highly efficient agent to improve cellulose paper hydrophobicity

“…Water resistance of cellulose paper sized by AKDEs Furthermore, the contact angle provided quantitative information on the physical phenomenon of sizing: the larger the initial angle is, the smaller the decrease rate of the contact angle over time, and the better the sizing performance of the slurry (Bruel et al 2018). The initial contact angles of all the paper samples were larger than 90°, indicating that the papers possessed a hydrophobic surface (Chen et al, 2019).…”

Issue Highlights

“…The variety of phenomena influencing CAM makes their interpretation tedious: here, we highlight their strengths, detection limits, sources of error, and applications. A 2016–2018 bibliometric keyword map identifies two main motives for performing CAM characterization: either developing the fundamental understanding of wetting science or assessing the success of surface modiication methods to produce materials with enhanced wetting, adhesive, or iltration properties …”