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

Abstract

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.

Wetting and Drying of Colloidal Droplets: Physics and Pattern Formation

“…In this situation, especially for the droplets on heated substrates, another flow field, e.g., Rayleigh-Bénard convection, which is a thermo-gravitational flow, could replace the Bénard-Marangoni convection. For the sessile drop of colloidal suspension on the heated substrate, the temperature gradient induces a concentration gradient of solid particles between the top and the areas close to the three-phase contact line of the droplet [16]. The surface tension of the liquid at the top of the droplet is larger than the one of the areas close to the three-phase contact line, forming the surface tension gradient.…”

Improving heat and mass transfer rates through continuous drop-wise condensation

“…At equilibrium, the contact line force is balanced by the aerodynamics force generated by impinging jet that is trying to dislodge the droplet. The effects of velocity variation across the boundary layer can be ignored as the droplets are significantly larger than the boundary layer thickness (see supplementary material section S-5) 29 , 79 , 85 , 86 . Thus, the drag force, which is proportional to the stagnation pressure on the droplet, can be written as where is the drag coefficient of the droplet, is the surrounding gas density, is a shape factor of the projected area of the droplet in the flow direction (see supplementary material section S-5), and is the effective air velocity around the droplet.…”

A Novel Continuous Drop-Wise Condensation Technology for Improved Heat and Mass Transfer Efficiencies

“…At equilibrium, the contact line force is balanced by the aerodynamics force generated by impinging jet that is trying to dislodge the droplet. The effects of velocity variation across the boundary layer can be ignored as the droplets are significantly larger than the boundary layer thickness (see supplementary material section S-5) (27,75,81,82). Thus, the drag force, which is proportional to the stagnation pressure on the droplet, can be written as…”

“…This improvement is highly dependant on the frequency of droplet shedding on surfaces. Droplets shedding has been achieved primarily by gravity assistance (20)(21)(22), droplet jumping (14,(23)(24)(25), drag force (26)(27)(28)(29)(30), or by capillary driven movement (31,32). It has been widely accepted that droplets of diameters below 20 micron contribute about 80% of the total heat transfer to the surface (33).…”