Airflow-Assisted Impact of Drops of Various Viscosities: The Role of Viscous Dissipation, Normal Imposed Pressure, and Shear Flow of Air.

The role of liquid viscosity on the spreading for an airflow-assisted impact of drops on a surface is investigated. The spreading diameter is found to increase with the Reynolds number of the airflow (Reair) for a given viscosity and impact Weber number (We) compared to the still air. The increment is higher at a low We for viscous drops, whereas the effect of Reair dominates at the intermediate We as the viscosity decreases. Two extra forces, the normal imposed pressure and shear force of air, act on the drop and influence the spreading along with the viscous dissipation. The drop's curvature decreases depending on the viscosity and impact velocity while spreading. Large-scale eddies near the drop-surface region are observed due to the separation of the incident airflow. The formation of eddies signifies low-pressure zones, which extract the trapped air, causing the spreading diameter of the viscous drop to increase at a low We. With the increase in the We, the lamella thickness of low-viscosity drops decreases and is pushed out by the air shear causing the spreading factor to increase. The boundary layer thickness is estimated using the energy balance method to predict the maximum spreading factor. The prediction compares well with the experimental one for higher viscosities. The accuracy improves when the effect of low pressure is incorporated. To confirm, the experimental spreading is compared with that obtained from three existing models, and one, which considers the influence, is observed to provide a better prediction.

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 (U(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 (L(b)) are found to be the controlling factors for adhesion. A correlation was also developed to deduce the drag coefficient, C(D) for the drop. It was found that C(D) can decrease under icing conditions. In general, a lower C(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 C(D) values, at all temperatures tested.

Dislodging a sessile drop by a high-Reynolds-number shear flow at subfreezing temperatures.

The drop, exposed to an air flow parallel to the substrate, starts to dislodge when the air velocity reaches some threshold value, which depends on the substrate wetting properties and drop volume. In this study the critical air velocity is measured for different drop volumes, on substrates of various wettabilities. The substrate initial temperatures varied between the normal room temperature (24.5∘C) and subfreezing temperatures (-5∘C and -1∘C). The physics of the drop did not change at the subfreezing temperatures of the substrates, which clearly indicates that the drop does not freeze and remains liquid for a relatively long time. During this time solidification is not initiated, neither by the air flow nor by mechanical disturbances. An approximate theoretical model is proposed that allows estimation of the aerodynamic forces acting on the sessile drop. The model is valid for the case when the drop height is of the same order as the thickness of the viscous boundary in the airflow, but the inertial effects are still dominant. Such a situation, relevant to many practical applications, was never modeled before. The theoretical predictions for the critical velocity of drop dislodging agree well with the experimental data for both room temperature and lower temperatures of the substrates.

Evidence of oscillatory convection inside an evaporating multicomponent droplet in a closed chamber.

Visualization of an evaporating binary (ethanol-water) droplet reveals presence of oscillatory internal circulation. The visualization is done by using a laser scattering technique. The oscillatory circulation possibly results from the opposing effect of solutal and thermal Marangoni convection as proposed in some earlier theoretical works. The frequency of this oscillation is measured and the variation of this frequency with the initial concentration of the volatile component (ethanol) is reported.