Evaporation kinetics of pure water drops: Thermal patterns, Marangoni flow, and interfacial temperature difference

Abstract

We report a systematic study on the role of Marangoni convection on the evaporation kinetics of pure water drops, considering the influence of heating regime and surface wettability. The Marangoni flows were induced via heating under constant wall temperature (uniform heating) and constant heat flux (local heating) regimes below the drops. To visualize the thermal patterns/flows emerging within the water drops we employed infrared (IR) thermography and we captured the evolution of the drop profile with a CCD camera to follow the evaporation kinetics of each drop. We observed a strong correlation between the temperature difference within the drop and the evolution of drop shape during different modes of evaporation (i.e. constant radius, angle or stick-slip) resulting in different Marangoni flow patterns. Under uniform heating, stable recirculatory vortices due to Marangoni convection emerged at high temperature which faded at later stages of the evaporation process. On the other hand, in the localized heating case, the constant heat flux resulted in a rapid increase of the temperature difference within the drop capable of sustaining Marangoni flows throughout the evaporation. Surface wettability was found to also play a role in both the emergence of the Marangoni flows and the evaporation kinetics. In particular, recirculatory flows on hydrophobic surfaces were stronger when compared to hydrophilic for both uniform and local heating. To quantify the effect of heating mode and the importance of Marangoni flows, we calculated the evaporative flux for each case and found to it to be much higher in the localized heating case. Evaporative flux depends on both diffusion and natural convection of the vapor phase to the ambient. Hence, we estimated the Grashof number for each case and found a strong relation between natural convection in the vapor phase and heating regime or Marangoni convection in the liquid phase. Subsequently, we demonstrate the limitation of current diffusion-only models describing the evaporation of heated drops. 32 90°, the CCA mode of evaporation is reported and the 33 decrease in weight/volume is observed to be non-linear. 34 Further, the decrease in volume according to a power law 35 is reported for drops evaporating on hydrophobic and su-36 perhydrophobic surfaces [8, 9]. Apart from the extreme 37 modes of evaporation (CCR and CCA), a stick-slip mode 38 of evaporation with repetitive cycles of stick and slip of 39 the contact line is observed for pure fluids [10] and also 40 for colloidal suspensions [11]. The strong influence of 41 substrate wettability [8, 12], shape of the sessile drop 42 [13], ambient conditions [14-16] and substrate proper-43 ties [17, 18] on the evaporation process are extensively 44 reported. 45 For a sessile drop in contact with a solid substrate, 46 the evaporative flux at the liquid-vapor interface is non-47 uniform and depends on the drop shape [19]. The evap-48 orative flux is higher near the contact line for drops with 49 contact angles less than 90°, whereas for d...

Effect of Substrate Conductivity on the Transient Thermal Transport of Hygroscopic Droplets during Vapor Absorption

“…Sobac and Brutin [8] investigated the influence of substrate properties on the evaporation process in both hydrophilic and hydrophobic cases. Results highlight the need for more accurate models to account for the buoyant convection in vapor transport as well as the evaporative cooling and heat conduction between the droplet and the substrate [9]. Similar experiments were conducted by Talbot et al [10] on picoliter droplets, which suggest that the thermal effects on the evaporation rate are much stronger for droplets on low-thermal-conductivity substrates than those on high-thermal-conductivity substrates.…”

Cloaking Dynamics on Lubricant‐Infused Surfaces

“…To verify if cloaking affected droplet evaporation mass transfer, we conducted experiments using the traditional transient‐based evaporation technique on the same samples. [ 27 ] We deposited water droplets of identical size and allowed them to evaporate completely, measuring the droplet radius as a function of time. Furthermore, we measured the total time for complete droplet evaporation ( t e ).…”

Interface-mediated spontaneous symmetry breaking and mutual communication between drops containing chemically active particles

“…However, the particles inside the drop exhibit motility upon UV illumination; therefore, the differences cannot be attributed to a lack of chemical activity, but they correlate with the predicted absence of the Marangoni flow response from the interface. Furthermore, it is known that for a water drop in air the evaporation of water may give rise to Marangoni stress and flow within the drop 36 . Thus the absence of the collective motion in this latter experiment implies that the activity-induced nature of these stresses is essential for the emergence of the phenomenology of the collective effects reported here.…”