Surface declination governed asymmetric sessile droplet evaporation

Abstract

The article reports droplet evaporation kinetics on inclined substrates. Comprehensive experimental and theoretical analyses of the droplet evaporation behaviour for different substrate declination, wettability and temperatures have been presented. Sessile droplets with substrate declination exhibit distorted shape and evaporate at different rates compared to droplets on the same horizontal substrate and is characterized by more often changes in regimes of evaporation. The slip-stick and jump-stick modes are prominent during evaporation. For droplets on inclined substrates, the evaporative flux is also asymmetric and governed by the initial contact angle dissimilarity. Due to smaller contact angle at the rear contact line, it is the zone of a higher evaporative flux. Particle image velocimetry shows the increased internal circulation velocity within the inclined droplets. Asymmetry in the evaporative flux leads to higher temperature gradients, which ultimately enhances the thermal Marangoni circulation near the rear of the droplet where the evaporative flux is highest. A model is adopted to predict the thermal Marangoni advection velocity, and good match is obtained. The declination angle and imposed thermal conditions interplay and lead to morphed evaporation kinetics than droplets on horizontal heated surfaces. Even weak movements of the TL alter the evaporation dynamics significantly, by changing the shape of the droplet from ideally elliptical to almost spherical cap, which ultimately reduces the evaporative flux. The life time of the droplet is modelled by modifying available models for non-heated substrate, to account for the shape asymmetry. The present findings may find strong implications towards microscale thermo-hydrodynamics.

Inclined colloidal drops: Evaporation kinetics and pattern formation

“…At α = 20 °, the minimum value of J(θa) J(θ0) ~0.951 appears at r R ~− 0.467, whereas at α = 40 °, the minimum value of J(θa) J(θ0) ~0.735 at the front edge, r R ~0.999. This indicates that the minimum at the apex of the symmetric drop shifts due to the asymmetrical shape of the inclined drops, in agreement with the literature (Dhar et al, 2020). The depression of the evaporative flux is stronger in the 40 °case, starting at r R ~0.572 and continues to the edge r R ~0.999 whereas in the 20 °case, the depression occurs over an almost three times smaller distance from r R 0.865 to the edge r R ~0.999.…”

“…These differences in the evaporation kinetics may be attributed to the different and asymmetrical distribution of evaporation flux along the contact line between the front and rear edges. For an evaporating drop in a quasi-steady state, the evaporative flux is given (Dhar et al, 2020):…”

“…In the last few years, the topic of the evaporation of inclined drops started attracting scientific attention. Increasing drop inclination to 90 °was reported to lead to faster evaporation kinetics (Kim et al, 2017;Dhar et al, 2020;Tredenick et al, 2021) and irregular coffee-ring patterns (Mondal et al, 2018;Gopu et al, 2020;Logesh Kumar et al, 2021) due to the opposing flows driven by capillary and sedimentation. Nonetheless, the exact pattern formation mechanism remains unclear with parameters such as evaporation rate and drop volume yielding contradictory results (Du and Deegan, 2015;Kim et al, 2021).…”

Droplet evaporation on curved surfaces: transients of internal and external transport phenomena

“…This pinning behaviour of the contact line becomes more prominent on concave substrates, while the de-pinning tendency is frequent on convex substrates [18]. This stick-slip behaviour is also significantly noticeable with increased inclination of the substrates [19]. Thus, we infer that the geometrical characteristics of the droplet-substrate system can appreciably modulate the rate and contact line dynamics of sessile droplet evaporation.…”

Local evaporation flux of deformed liquid drops

“…As reviewed on the drop deformation due to the gravity and pinning process [38], it is agreed that the surprisingly less attention was attracted for either the drops evaporation on non-horizontal substrates or deformed drops. Practically, the drop on nonhorizontal plates is critically important, particularly in the applications for the 3D printing technology where fresh solution jet is always evaporating on its own deposit with highly curved facets [39][40][41]. Espin and Kumar built a model system of drop evaporation on a slope and used the one-side model to estimate the evaporation flux then combine the lubrication approximation with the convection-diffusion of solute inside the drop towards the deposit patterns [36].…”