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.

The thermal performance and the internal flow regimes of a closed-loop flat plate pulsating heat pipe (FPPHP) are experimentally investigated. Reports on the FPPHP using the ethanol-water mixtures as working fluids are scarce in the literature. The binary mixtures with different boiling point components are suitable for a wide range of heat fluxes. Therefore, the results are reported for the ethanol-water mixtures of ratios 3:1, 1:1, and 1:3, and the corresponding pure liquids filled in the range of [40–80] % with power inputs given from 40 to 200 W. The effect of different condenser cooling modes, such as forced convective water cooling, forced convective air cooling, and natural convective air cooling on the thermal performance of the FPPHP, is also reported. With the increase in the power input, the observed flow characteristics in the FPPHP channels are: no oscillations, slug-plug oscillations, droplet oscillations, and the evaporator dry out. The binary mixtures with increased ethanol content give better slug-plug flow oscillations with smaller thermal resistances and fewer evaporator drying out instances than the pure working fluids. For power inputs of less than 120 W, the ethanol:water mixture ratio of 3:1 at all filling ratios gives a larger slug departure frequency in the evaporator. The smallest thermal resistance measured is 0.1 K/W, a decrease of 27% over pure ethanol. For power inputs greater than 120 W, the mixture ratio of 1:1 at all filling ratios performs better with continuous droplet oscillations. The smallest thermal resistance measured at the 80% filling ratio is 0.12 K/W, a decrease of 22% over pure ethanol. When the condenser cooling mode is changed to air cooling, the evaporator temperatures reach around 100 °C for power inputs greater than 40 W and 80 W for natural and forced air convection. Thus, the FPPHP filled with ethanol-water mixtures with the water-cooled condenser gives a stable flow regime and better thermal performance for a long-range of power inputs.

Wettability patterning of a surface is a passive method to manipulate the flow and heat transport mechanism in many physical processes and industrial applications. This paper proposes a rational wettability pattern comprised of multiple superhydrophilic wedges on a superhydrophobic background, which can continuously remove the impacted spray droplets from the horizontal surface. We observed that the spray droplets falling on the superhydrophilic wedge region spread and form a thin liquid film, which is passively transported away from the surface. However, most of the droplets falling on the superhydrophobic region move towards the wedge without any flooding. The physics of the passive transport of the liquid film on a wedge is also delved into using numerical modelling. In particular, we elucidate the different modes of droplet transport in the superhydrophobic region and the interaction of multiple droplets. The observed droplet dynamics could have profound implications in spray cooling systems and passive removal of liquid from a horizontal surface. This study’s findings will be beneficial for the optimization of efficient wettability patterned surfaces for spray cooling application.

Pumpless transport of droplets on open surfaces has gained significant attention because of its applications starting from vapor condensation to Lab-on-a-Chip systems. Mixing two droplets on open surfaces can be carried out quickly by using wettability patterning. However, it is quite challenging to split a droplet in the absence of external stimuli because of the interfacial energy of the droplet. Here, we demonstrate a standalone power-free technique for transport and splitting of droplets on open surfaces using continuous wettability gradients. A droplet moves continuously from a low to a high wettability region on the wettability-gradient surface. A Y-shaped wettability-gradient track - laid on a superhydrophobic background - is used to investigate the dynamics of the splitting process. A three-dimensional phase-field Cahn-Hilliard model for interfaces and the Navier-Stokes equations for transport are employed and solved numerically using the finite element method. Numerical results are used to decipher the motion and splitting of droplet at the Y junction using the principle of energy conservation. It is observed that droplet splitting depends on the configuration of the Y junction; droplets split faster for the superhydrophobic wedge angle of 90∘ and the splitting ratio (ratio of the sizes of daughter droplets) depends on the widths of the Y branches. A critical branch-width ratio (w2w1=0.79) is identified below which the droplet does not split and moves towards the branch of higher width and settles there. The present study provides the required theoretical underpinnings to achieve autonomous transport and splitting of droplets on open surfaces, which has clear potential for applications in Lab-on-a-Chip devices.

In the present work, surface wettability modifications were utilized to enhance the phase change heat transfer in a water-charged two-phase flat thermosyphon. A flat thermosyphon's thermal performance with various surface wettability modifications on evaporator and condenser plates was investigated for various heat inputs and filling ratios in the horizontal orientation. The evaporator and condenser's surface wettabilities were varied to superhydrophilic (contact angle of 0-1°) and superhydrophobic (contact angle of 155.4 ± 3°). Changing the evaporator's surface wettability to superhydrophilic nature increased the thermal resistance of thermosyphon due to the high superheat requirement and delay during bubble nucleation. A 43.74% decrease in the thermal resistance was observed for a thermosyphon with a superhydrophobic condenser due to the dropwise condensation and faster condensate return to the evaporator compared to the bare one. A lumped parameter model was used to predict the thermal resistance of flat thermosyphon with a superhydrophobic condenser and hydrophilic evaporator, which is in good agreement with the experimental results. The experimental results encourage research on a two-phase flat thermosyphon with a superbiphilic evaporator and superhydrophobic condenser as it can further improve thermal performance.

In the present work, pool boiling experiments are performed on copper minichannel and flat surfaces at atmospheric pressure with water as the working fluid. The pool boiling experiments are conducted for liquid subcooling of 0K (saturated), 10K, and 20K at atmospheric pressure. Minichannel-1 and Minichannel-2 have a square cross-section with side lengths 1 mm and 2 mm, respectively. The thermal performance of the boiling surface is characterized by the critical heat flux and heat transfer coefficient. The critical heat flux (CHF) is increased by 36-45 % for minichannel-2 and 15-17 % for minichannel-1 compared to a flat surface at all subcoolings. CHF can be increased as high as 287.54 W/cm2 by employing minichannel-2 and 20 K subcooling. Minichannel-1 and minichannel-2 enhanced the heat transfer coefficient as high as 25.21% and 68.59 %, respectively, compared to the flat surface. It is observed that the increase in surface area is the dominant factor in the enhancement of pool boiling heat transfer on minichannel surfaces.

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.

Thermal management is critical in improving the efficiency, lifespan, and reliability of sustainable energy devices such as LEDs, batteries, and solar cells. The compact flat thermosyphon (CFT) is more reliable for the effective thermal management of multiple heat sources in energy-intensive devices. A comprehensive study on the thermal performance of a two-phase CFT is conducted in the present study. The volume of the CFT is 72 × 24 × 15 mm3 and is made up of copper. The thermal performance of the CFT is analyzed by varying the filling ratio (20%, 50%, 80%), working fluid (acetone, ethanol, water), number of heat sources, and condenser surface wettability. The best-performing filling ratio is found to be 50%. Water is the best performing working fluid over a wide range of heat fluxes (10 – 70 W/cm2) due to high heat capacity, latent heat, and low vapor pressure. The CFT can safely cool up to four heat sources of size 12 × 24 mm2 at a heat flux of 30 W/cm2 each. The lifespan of the devices can be increased by 2.49 – 6.62 times when the CFT is used to cool multiple heat sources instead of a cold plate. The superhydrophobic condenser surface further improved the thermal performance of the CFT by 28 – 47% because of the quicker return of condensate and dropwise condensation.

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.