Arvind Pattamatta

We investigate the evolution of the thermal field during evaporation, a fundamental aspect of evaporating sessile drops. With numerous reports in the literature investigating the contact line dynamics, we aspire to identify generalized features in the evolution of the thermal field and ultimately correlate these with the contact line dynamics. Considering a broad range of experimental parameters such as substrate wettability, substrate temperature, initial volume of the drop, and ambient relative humidity results in a wide range of evaporation rates, in turn affecting the strength of internal convective flows. Infrared thermography is utilized to extract the thermal field at the liquid–vapor interface, and optical imaging is used to record the evolution of drop shape during evaporation. We observe that the onset and presence of a convective cell as a cold spot at the interface highlights a non-axisymmetry in the thermal field. In consequence, a hitherto unreported asymmetry in the internal flow field is observed, as evidenced by the particle image velocimetry. Among the multitude of experiments conducted, we report four distinct trends in the evolution of interfacial temperature difference depending on the presence and duration of the presence of the convective cell, which are elucidated by discussing the evolution of maximum and minimum temperatures at the interface. The interplay between heat conducted into the drop and heat released due to evaporation can result in a momentary decrease in temperature of the drop, which is not reported previously. Lastly, a theoretical estimate for the temperature difference within the drop is extracted using vapor diffusion model and energy balance during evaporation. Comparison of this theoretical temperature difference with experimental observations highlights the influence of internal convective flows in homogenizing the thermal field within the drop.

We report a systematic study of the role of Marangoni convection in the evaporation kinetics of pure water drops, considering the influence of the heating regime and surface wettability. 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 emerging during the evaporation, we employed infrared 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 the 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, but they 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 in the temperature difference within the drop capable of sustaining Marangoni flows throughout the evaporation. Surface wettability was found also to play a role in both the emergence of the Marangoni flows and the evaporation kinetics. In particular, recirculatory flows in drops on hydrophobic surfaces were stronger when compared to flows on hydrophilic surfaces for both uniform and local heating. To quantify the effect of the heating mode and the importance of Marangoni flows, we calculated the evaporative flux for each case and found 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 the previously reported diffusion-only model in describing the evaporation of heated drops.

Abstract: The internal flow field in evaporating sessile water drops is experimentally investigated in the present work. The interdependency in the prevailing thermal field and the internal flow field is analyzed by simultaneous utilization of infrared thermography and particle image velocimetry. Experiments are conducted on a hydrophobic substrate while varying the substrate temperature between 25 and 60 ∘C, resulting in a significant variation in the strength of internal convection. For the case of a non-heated substrate, a monotonic variation in temperature along the liquid–vapor interface results in an axisymmetric flow field inside the drop. For heated substrates, the presence of a cold spot at the liquid–vapor interface due to the dominance of the Marangoni flow results in a non-axisymmetric flow field. In such a situation, two counter-rotating vortices inside the drop are visualized. Here, the velocities inside the drop are ∼ O(mm/s), where velocities of ∼ O(μm/s) are previously reported for buoyancy-dominated flows. Qualitative features in the internal flow field, such as the duration of the presence of the non-axisymmetric flow and the shift in the center of vortices, highlight more vigorous Marangoni convection in drops evaporating on substrates maintained at a higher temperature. Quantitative analysis of the flow field is presented in terms of the spatiotemporal evolution of velocity and vorticity inside the drop, which are further correlated to the evolution of the thermal field by analyzing the interfacial temperature difference. Further, by observing the deposition pattern of tracer particles formed after the evaporation of drops, the effect of variations in the internal flow field on deposition patterns is deduced. Graphical abstract: [Figure not available: see fulltext.].

Understanding the internal flow in evaporating sessile drops is of paramount importance in a myriad of applications such as ink-jet printing, surface patterning, and medical diagnostics. Marangoni flow driven by a gradient in surface tension is an essential internal flow mechanism, whose characteristics in evaporating water drops remain elusive in the literature. Here, by employing infrared thermography and particle image velocimetry, we show that the manifestation of Marangoni flow as a convective cell at the liquid-vapor interface results in a nonaxisymmetric internal flow field. Eventually, during evaporation, the flow transitions to a buoyancy-dominated regime, where an axisymmetric toroidal flow is observed. This transition marks a reversal in the flow along with an order of magnitude decrease in velocity. We corroborate this experimentally observed transition using previously reported analytical and scaling frameworks. Finally, we present hitherto unreported features correlating the three aspects of evaporating water drops, viz., contact line dynamics, thermal field, and internal flow field, which are generally investigated independently.

We report an experimental investigation on contact line dynamics, thermal patterns, and internal fluid flow during the evaporation of inverted sessile drops of pure water. This configuration of sessile drop when placed on a heated substrate should lead to thermal stratification and any internal convective flow will be governed by surface tension driven Marangoni flow. First, we report contact line dynamics and thermal patterns recorded using an optical camera and infrared camera, respectively. An interesting outcome from the present study is the resemblance observed between the evolution of contact angle and interfacial temperature difference during evaporation. By performing Particle Image Velocimetry to delineate the internal flow characteristics, we report an axisymmetric counter-rotating flow inside the drop. This flow is directed towards the substrate from the apex at the centerline of the drop. In literature, a similar directional flow is reported to be due to Marangoni flow albeit for a normal sessile drop. Further, by extracting the magnitude of velocity, we report a maximum velocity in the flow occurring at the center of drop which in turn increases with substrate temperature. The results reported in the present study shed light on the presence of Marangoni flow in pure water drops whose understanding is of paramount importance in many academic and industrial applications.

In an era defined by an insatiable thirst for sustainable energy solutions, responsible water management, and cutting-edge lab-on-a-chip diagnostics, surface wettability plays a pivotal role in these fields. The seamless integration of fundamental research and the following demonstration of applications on these groundbreaking technologies hinges on manipulating fluid through surface wettability, significantly optimizing performance, enhancing efficiency, and advancing overall sustainability. This Review explores the behavior of liquids when they engage with engineered surfaces, delving into the far-reaching implications of these interactions in various applications. Specifically, we explore surface wetting, dissecting it into three distinctive facets. First, we delve into the fundamental principles that underpin surface wetting. Next, we navigate the intricate liquid-surface interactions, unraveling the complex interplay of various fluid dynamics, as well as heat- and mass-transport mechanisms. Finally, we report on the practical realm, where we scrutinize the myriad applications of these principles in everyday processes and real-world scenarios.