Drop Shedding by Shear Flow for Hydrophilic to Superhydrophobic Surfaces

Abstract

A balance of surface science and aerodynamic knowledge is brought to bear to elucidate the fundamental parameters determining the incipient motion (runback) for a drop exposed to shearing airflow. It was found that wetting parameters such as contact angle are very influential in determining the minimum required air velocity for drop shedding. On the basis of experimental results for drops of water and hexadecane (0.5-100 microL) on PMMA, Teflon, and a superhydrophobic aluminum surface, an exponential function is proposed that relates the critical air velocity for shedding to the ratio of drop base length to projected area. The results for all of the water systems can be collapsed to self-similar curves by normalization. Results from other researchers also conform to the exponential self-similar functional form proposed. It was shown that the data for hexadecane drops can be matched relatively well to those for water drops by means of a corrective factor based on fluid properties and contact angles. Also, the critical air velocity for shedding from the superhydrophobic surface is seen to be more constant over a range of volumes than for the other surfaces. Finally, contact angle measurements from airflow shedding experiments are compared to measurements made by tilted plate and quasi-static advancing and receding tests. The observed differences between contact angles from different measurement methods show that the transfer of contact angle data among various applications must be done with care.

Mechanism of supercooled droplet freezing on surfaces

“…Therefore, U crit for droplet roll-off/sliding on superhydrophobic surfaces was lower than for smooth, hydrophobic surfaces with lower contact angle, higher CAH and smaller frontal area (for example, surface 3 and 4 in Table 1) 15 . For suitably designed superhydrophobic surfaces, air pockets can be trapped below the water drop (Cassie state), which results in reduced CAH and less sticking 16 .…”

“…The difference between the advancing and the receding contact angles is referred to as contact angle hysteresis (CAH) 15,16 . The critical shear flow velocity, U crit , for the onset of droplet roll-off/sliding depends on factors such as the drop shape, properties and substrate wettability.…”

Optimal Design of Permeable Fiber Network Structures for Fog Harvesting

“…As the small deposited water droplets coalesce, the growing droplets are influenced by the competition between aerodynamic drag forces ( F drag ) and surface adhesion forces ( F adhesion ). [28][29][30] When the drag force overwhelms the adhesion force, the droplets are re-entrained in the fog flow, leading to a decrease in the fog collection efficiency (Figure 3a). …”

Supercooled Water Droplet Impacting Superhydrophobic Surfaces in the Presence of Cold Air Flow

“…It was reported that droplet contact time can be reduced by increasing the receding contact angle [25] or by switching recoiling dynamics from an axisymmetric retraction to a nonaxisymmetric one [23] up to about 50% (based on inertia-capillary time scale [26]: ρD 3 o /8σ 1/2 ) through breaking up the droplets and also through the pancake bouncing mechanism [27]. Although the aforementioned techniques were used to reduces droplet adhesion with a surface by minimizing droplet contact time or by shedding the droplet by airflow [16,28], the effect of stagnation air flow on an impacting water drop [29] whether in room or super cooling conditions has not been fully addressed yet. In the present study, the influence of stagnation flow on a spreading water droplet is evaluated through classical Homann flow approach [30].…”