Understanding the Edge Effect in Wetting: A Thermodynamic Approach

Edge effect is known to hinder spreading of a sessile drop. However, the underlying thermodynamic mechanisms responsible for the edge effect still is not well-understood. In this study, a free energy model has been developed to investigate the energetic state of drops on a single pillar (from upright frustum to inverted frustum geometries). An analysis of drop free energy levels before and after crossing the edge allows us to understand the thermodynamic origin of the edge effect. In particular, four wetting cases for a drop on a single pillar with different edge angles have been determined by understanding the characteristics of FE plots. A wetting map describing the four wetting cases is given in terms of edge angle and intrinsic contact angle. The results show that the free energy barrier observed near the edge plays an important role in determining the drop states, i.e., (1) stable or metastable drop states at the pillar’s edge, and (2) drop collapse by liquid spilling over the edge completely or staying at an intermediate sidewall position of the pillar. This thermodynamic model presents an energetic framework to describe the functioning of the so-called “re-entrant” structures. Results show good consistency with the literature and expand the current understanding of Gibbs’ inequality condition

Modeling Liquid Bridge between Surfaces with Contact Angle Hysteresis

This paper presents the behaviors of a liquid bridge when being compressed and stretched in a quasi-static fashion between two solid surfaces that have contact angle hysteresis (CAH). A theoretical model is developed to obtain the profiles of the liquid bridge given a specific separation between the surfaces. Different from previous models, both contact lines in the upper and lower surfaces were allowed to move when the contact angles reach their advancing or receding values. When the contact angles are between their advancing and receding values, the contact lines are pinned while the contact angles adjust to accommodate the changes in separation. Effects of CAH on both asymmetric and symmetric liquid bridges were analyzed. The model was shown to be able to correctly predict the behavior of the liquid bridge during a quasi-static compression/stretching loading cycle in experiments. Because of CAH, the liquid bridge can have two different profiles at the same separation during one loading and unloading cycle, and more profiles can be obtained during multiple cycles. The maximum adhesion force generated by the liquid bridge is found to be influenced by the CAH of surfaces. CAH also leads to energy cost during a loading cycle of the liquid bridge. In addition, the minimum separation between the two solid surfaces is shown to affect how the contact radii and angles change on the two surfaces as the liquid bridge is stretched

Fast Liquid Transfer between Surfaces: Breakup of Stretched Liquid Bridges

In this work, a systematic experimental study was performed to understand the fast liquid transfer process between two surfaces. According to the value of the Reynolds number (<i>Re</i>), the fast transfer is divided into two different scenarios, one with negligible inertia effects (<i>Re</i> ≪ 1) and the other with significant inertia effects (<i>Re</i> > 1). For <i>Re</i> ≪ 1, the influences of the capillary number (<i>Ca</i>) and the dimensionless minimum separation (<i>H</i>_min* = <i>H</i>_min/<i>V</i>^1/3, where <i>H</i>_min is the minimum separation between two surfaces and <i>V</i> is the volume of liquid) on the transfer ratio (α, the volume of liquid transferred to the acceptor surface over the total liquid volume) are discussed. On the basis of the roles of each physical parameter, an empirical equation is presented to predict the transfer ratio, α = <i>f</i>(<i>Ca</i>). This equation involves two coefficients which are affected only by the surface contact angles and <i>H</i>_min* but not by the liquid viscosity or surface tension. When <i>Re</i> > 1, it is shown for the first time that the transfer ratio does not converge to 0.5 with the increase in the stretching speed

Shedding of Water Drops from a Surface under Icing Conditions

A sessile water drop exposed to an air flow will shed if the adhesion is overcome by the external aerodynamic forces on the drop. In this study, shedding of water drops were investigated under icing conditions, on surfaces with different wettabilities, from hydrophilic to superhydrophobic. A wind tunnel was used for experiments in a temperature range between −8 and 24.5 °C. Results indicate that the temperature has a major influence on the incipient motion of drop shedding. The critical air velocity (<i>U</i>_c) at which a drop first starts to shed generally increases under icing conditions, indicating an increase in the adhesion force. The contact angle hysteresis (CAH) and the drop base length (<i>L</i>_b) are found to be the controlling factors for adhesion. A correlation was also developed to deduce the drag coefficient, <i>C</i>_D for the drop. It was found that <i>C</i>_D can decrease under icing conditions. In general, a lower <i>C</i>_D and higher adhesion together lead to a higher critical air velocity. However, there are systems such as water on Teflon for which the critical air velocity remains practically unaffected by temperature because of similar adhesion and <i>C</i>_D values, at all temperatures tested

Drop Rebound after Impact: The Role of the Receding Contact Angle

Data from the literature suggest that the rebound of a drop from a surface can be achieved when the wettability is low, i.e., when contact angles, measured at the triple line (solid–liquid–air), are high. However, no clear criterion exists to predict when a drop will rebound from a surface and which is the key wetting parameter to govern drop rebound (e.g., the “equilibrium” contact angle, θ_eq, the advancing and the receding contact angles, θ_A and θ_R, respectively, the contact angle hysteresis, Δθ, or any combination of these parameters). To clarify the conditions for drop rebound, we conducted experimental tests on different dry solid surfaces with variable wettability, from hydrophobic to superhydrophobic surfaces, with advancing contact angles 108° < θ_A < 169° and receding contact angles 89° < θ_R < 161°. It was found that the receding contact angle is the key wetting parameter that influences drop rebound, along with surface hydrophobicity: for the investigated impact conditions (drop diameter 2.4 < <i>D</i>₀ < 2.6 mm, impact speed 0.8 < <i>V</i> < 4.1 m/s, Weber number 25 < <i>We</i> < 585), rebound was observed only on surfaces with receding contact angles higher than 100°. Also, the drop rebound time decreased by increasing the receding contact angle. It was also shown that in general care must be taken when using statically defined wetting parameters (such as advancing and receding contact angles) to predict the dynamic behavior of a liquid on a solid surface because the dynamics of the phenomenon may affect surface wetting close to the impact point (e.g., as a result of the transition from the Cassie–Baxter to Wenzel state in the case of the so-called superhydrophobic surfaces) and thus affect the drop rebound