Bubble coalescence phenomenon in electrolytic media finds applications in technologies from mineral flotation to electrochemical energy conversion. However, the underlying governing physics still remains unresolved, with longstanding disagreement over the extent to which Marangoni stresses affect the coalescence time by modulating the interfacial mobility. Here, we show that the thin film morphology governs drainage more strongly than the interfacial boundary conditions. We demonstrate experimentally that thin film drainage during bubble coalescence proceeds through three distinct regimes. An initial visco-capillary stage that exhibits a power-law thinning, followed by an exponential decrease in film thickness with time induced by rim stabilisation. The final regime is governed by disjoining pressure and is marked by an exponential relaxation of the film to the equilibrium thickness. We show that, irrespective of the electrolyte type and concentration, film evolution exhibits universal behavior by collapsing onto a single curve when rescaled with the characteristic film thickness and time scale, demonstrating that electrolyte effects act only to renormalize timescales rather than alter the underlying dynamics.

The levitating Leidenfrost (LF) state of a droplet on a heated substrate is often accompanied by fascinating behaviors such as star-shaped deformations, self-propulsion, bouncing, and trampolining. Here, we report on the intermittent trampolining events at specific drop sizes over the entire lifespan of an evaporating LF droplet. Between the trampolining events, the droplet exhibits decreased amplitude bouncing or quiescent hovering. We show that the reemergence of trampolining behavior of the droplet at specific radii is triggered by parametric resonance between the intervening vapor layer oscillation and the droplet, where the oscillations in the droplet subharmonically excite increased amplitude oscillations in the vapor. The trampolining events are observed when the ratio of the natural frequency of the vapor layer and Rayleigh frequency corresponds to the instability zones on the corresponding Mathieu's chart. This model even explains the observation of trampolining from a quiescent hovering state of the droplet. The proposed mechanism of resonance-driven trampolining of LF droplets is observed to be applicable for different liquids irrespective of the initial volume and substrate temperatures, thus indicating a universality of the behavior.

Droplet impact behavior on solid surfaces is crucial in various industrial applications, including spray coating, food production, printing, and agriculture, where controlling impact dynamics is particularly challenging for non-Newtonian liquids with complex rheology. In agriculture, emulsions used for spraying oil-based pesticides often bounce off hydrophobic plant surfaces, leading to environmental contamination of water sources and soil. This study investigates the impact dynamics of emulsions composed of aqueous polymer (polyvinylpyrrolidone) solutions and hexadecane oil on superhydrophobic substrates. We demonstrate that increasing the polymer concentration from 1 to 10 000 ppm reduces droplet retraction rates by approximately 63% - an effect primarily attributed to the increase in the shear viscosity. In the case of emulsion, the reduction in the retraction rate due to the presence of polymers enables oil imbibition into the surface asperities. We show that incorporating oil into the aqueous polymer emulsion decreases the minimum polymer concentration required to suppress droplet rebound on superhydrophobic surfaces from 10 000 to 100 ppm. The impact dynamics of polymer-oil-water emulsion droplets are influenced by an emerging oil layer on the substrate, which substantially slows the retraction rate. Notably, even at a low oil concentration of 3 vol. %, impinging emulsion droplets adhere to the superhydrophobic substrate regardless of the Weber number. These findings provide new insights into controlling droplet impact behavior and suggest an environmentally friendly and effective strategy for reducing unwanted rebound, particularly in agricultural applications.

The increasing demand for small satellites necessitates development of advanced micro propulsion technologies. The compact size, weight limitations, and restricted power availability of small satellites impose challenges in the integration of propulsion systems. Here we report on the design and experimental characterization of a capillaryfed evaporative microthruster. The microthruster consists of a reservoir, evaporation chamber, and a capillary wick. The capillary wick enables passive transport of the propellant (deionized water) from the reservoir to the evaporation chamber, where the water evaporates upon the application of electric heating. The generated vapor is expelled through a nozzle in the evaporation chamber to generate thrust. Pressure and temperature measurements in the evaporation chamber along with side-view imaging, facilitate real-time monitoring to understand the device behavior at different input power at low pressure conditions (similar to 50 Pa) of the ambient. We present the relevant design parameters of the microthruster device to mitigate issues such as two-phase boiling, water ejection or ice formation, which are detrimental to the performance of vaporizing liquid microthrusters. We show that the evaporative microthruster can achieve thrust ranging from approximately 200 mu N to 820 mu N, with a specific impulse of similar to 100 s, for power input of 0 W to 3 W.

Efficient liquid transport in microstructured evaporators is critical for thermal management and phase-change applications, including electronics cooling, solar-thermal desalination, and heat pipe technologies. Here, we study the effect of evaporation on the capillary wicking in a microchannel subjected to a constant heat flux. A theoretical framework incorporating coupled mass, momentum, and energy conservation equations is developed to examine the influence of geometrical parameters, such as the width and depth of the microchannel, on the wicking length and the evaporation rate. In contrast to the case without evaporation, where the wicking length decreases monotonically as the channel width increases, the presence of evaporation introduces a nonmonotonic behavior in the wicking length with channel width. For a particular depth of microchannels, the width at which the wicking length attains its peak varies with the applied heat flux. The wicking length and the channel width determine the evaporating surface area, an increase of which increases the evaporation rate. For a given width, channels with a greater depth exhibit extended wicking lengths, rendering them more favorable for enhanced evaporation rates. Consequently, a set of geometric parameters are obtained for different heat fluxes that yield peak values of evaporation rates, which can serve as design criteria for efficient evaporators in phase-change applications. Wicking experiments are performed in microchannels of rectangular cross sections with different widths, employing water and ethanol as the working fluids. The experimental results agree well with the theoretically predicted wicking lengths for different surface morphologies and applied heat flux.

A loop thermosyphon is a passive two-phase heat transfer device consisting of an evaporator, riser, condenser, and downcomer, and contains a working fluid. It utilizes the latent heat of vaporization and condensation of the working fluid to transfer heat from the heat source to the heat sink. It is a gravityassisted device (evaporator below condenser), as gravity helps liquid return from the condenser to the evaporator. For modeling of a loop thermosyphon, the general assumptions used are (a) two-phase flow exists in the riser and (b) the downcomer is partially or completely filled. However, it is not well understood in which conditions these assumptions are valid, as they depend on the filling ratio, working fluid, and operating heat loads. The present study experimentally explores flow patterns in the riser and downcomer to verify these assumptions. A stainless-steel loop thermosyphon with horizontal evaporator and condenser is utilized. A portion of the riser and downcomer are made of glass tubes to enable visualization of the flow pattern. The effect of working fluid (methanol and isobutane), filling ratio ($20 \%-50 \%$), and heat load ($100 \mathrm{~W}-500 \mathrm{~W}$) on the flow pattern is investigated. It is found that the flow pattern in the riser is generally two-phase (plug/churn to churn flow) and can be single-phase under certain conditions. In the downcomer, the liquid level is observed to be fluctuating.

A water droplet on a textured superhydrophobic substrate can either rest on top of the pillars (Cassie state) or impale the surface textures (Wenzel state). Here, we report on the Cassie to Wenzel transition and associated dynamics of a water droplet on heated superhydrophobic substrates. Below the saturation temperature of the liquid, the Cassie to Wenzel transition of the droplet occurs at a specific droplet volume which is dependent on the surface morphology and temperature. Near the Leidenfrost temperature of the superhydrophobic substrate (140-170 degrees C), partial impalement into the textures and accompanying increased vapor pressure leads to an explosive out-of-plane lift-off behavior of the droplet. The substrate morphology affects the lubrication pressure due to vapor flow underneath the droplet which dictates the lift-off volume. In addition, the detachment of the droplet from the substrate is also observed to be caused by local bubble nucleation and resulting capillary wave along the liquid-vapor interface. We use the pressure-based analytical transition criteria to predict the volume of drop corresponding to Cassie-Wenzel transition for temperatures lower than the saturation temperature and that for the out-of-plane lift-off at higher temperatures. The predictions agree reasonably well with the experimental observation over the entire range of substrate temperatures and for different surface morphology.

Recent advances in thermal localization-based passive solar desalination provide a great opportunity for the economical generation of freshwater, particularly in regions with insufficient energy and water infrastructure. Yet, the capillary-assisted passive desalination systems with a high-water productivity flux (measured in Lm(-2) h(-1)) suffer from the issue of performance degradation due to salt accumulation and the inability to be scaled up. In this work, we propose siphon-based supply of saline water over the evaporator that enables scale up of the desalination system to a size significantly higher than the capillary rise height of the hydrophilic evaporator while preventing salt accumulation on the evaporator. The composite siphon comprises insulating fabric wick and a metallic grooved surface for localizing heat to evaporate a thin layer of saline liquid over the evaporator. We perform heat and mass transfer analysis to show that the thermal-to-vapor efficiency depends on the inlet mass flow rate and air gap between the evaporator and the condenser. We propose a methodology to passively control the mass flow rate to maximize the thermal to vapor efficiency at different input heat flux and initial concentration of the brine. A grooved condenser avoids mixing of brine and the freshwater, even at air gaps as low as 2 mm. A siphon-assisted 10-stage desalination system with a footprint area 15 cm x 15 cm and an air gap of 2 mm is shown to have a high water productivity flux of similar to 5.73 Lm(-2) h(-1) from 3.5 wt% saline water at an applied heat flux 1000 W/m(2), which increases to a record high distillate flux of similar to 6.23 Lm(-2) h(-1) and thermal to water collection efficiency of similar to 423 % for a 15-stage system. The ability of the desalination system to maintain a high-water productivity flux even when the evaporator area is increased by 4 times demonstrates its scalability to achieve higher desalinated water productivity rate.

PurposeThis study aims to develop a numerical model to simulate the thermochemical energy storage (TCES) process within a porous reactor bed containing potassium carbonate (K2CO3) salt hydrate, focusing on the impact of thermal diffusion and vapor transport length scales on reactant conversion for a prescribed reaction time scale.Design/methodology/approachUsing a finite volume-based numerical technique to solve coupled nonlinear equations governing reaction kinetics, heat transfer and vapor transport, the model identifies critical thermal and vapor transport length scales that influence reactor performance during discharging (hydration) process.FindingsThe results confirm that beyond certain length scales, thermal and vapor transport rates limit reaction rates. Specifically, during the discharging process with a hydration time scale of 1500 s, the critical thermal diffusion (Xcr) and vapor penetration (Ycr) length scales were approximately 10 mm and 45 mm, respectively, for an inlet vapor pressure of 1500 Pa and a side wall temperature of 30 degrees C. Increased inlet vapor pressure significantly extended the critical vapor penetration length scale, aligning with findings in recent literature.Practical implicationsThese insights highlight key design parameters essential for optimizing reactor efficiency and scalability in practical applications of TCES systems.Originality/valueThe present paper highlights the impact of thermal diffusion and vapor transport length scales on reactant conversion within the reactor bed module for a prescribed reaction time scale.

Adulteration of milk poses a severe human health hazard. Existing methods for detecting adulterants such as water, urea, ammonium sulfate (AmS), oils, and surfactants in milk are selective, expensive, and often challenging to implement in rural areas. The present work shows the potential of machine learning to detect milk adulterants using patterns of evaporative milk deposits. The final deposit patterns obtained after evaporation of the adulterated milk droplets are used to create an image data set. This data set is used to develop a deep learning model that deploys a convolutional neural network (CNN/ConvNet) to classify the distinct evaporation patterns obtained for different types and concentrations of adulterants. Further, we apply implicit and explicit regularization and compare their accuracies. The models trained with different regularization optimization schemes demonstrate that a CNN can be successfully implemented to detect adulterants in milk. Additionally, we experimentally determine how the type and concentration of milk adulterants, including ammonium sulfate (AmS), urea, oil, and surfactants, affect milk evaporative deposition. Added AmS and urea in milk crystallizes during evaporation to produce recognizable patterns that can be used for their detection. The method is capable of detecting AmS added in excess of 2.4% and urea in excess of 5% in diluted milk (20 wt %) due to the crystallization of AmS and urea, respectively. In the case of milk adulterated with vegetable oil, evaporation leads to the separation and accumulation of oil at the top of the deposit, leading to the detection of oil present in excess of 2% in 20% diluted milk. Furthermore, a minimum individual amount of 5% urea, 2.4% AmS, and 2% oil concentration in diluted milk (20%) is shown to be individually detected by evaporation pattern-based technique when milk is adulterated with all the adulterants (water, urea, AmS, and oil + surfactant) together. When subjected to different regularization optimization schemes, the CNN gives varying degrees of accuracy for successful detection. The use of implicit regularization in the form of data augmentation gives the best results with a testing average accuracy of 98%, showing that a CNN can be successfully deployed to classify and detect adulterants in milk.

Nanosatellites are important for carrying out short-term and cost-effective communication and surveillance missions. Their small size necessitates the need for propulsion systems that are lightweight, compact, and capable of delivering accurate reaction and attitude control while allowing for seamless integration with the satellite. This paper reports on a numerical analysis to determine the performance of a micro-electromechanical system (MEMS)-based vaporizing liquid microthruster that utilizes microtextured substrates for passive feeding of the propellant (water) using capillary force and subsequent thin film evaporation by localized heating. The generated vapor flows through a converging nozzle to produce thrust. The performance of the propulsion device is evaluated in terms of the mass flow rate, thrust, and specific impulse. The model demonstrates a unique way of integrating the evaporation characteristics at the liquid-vapor interface to real nozzle flow dynamics. The evaporation phenomenon at the liquid-vapor interface is captured by utilizing kinetic theory of the gases, and real nozzle flow is analyzed by considering compressible-slip flow through the converging nozzle. It is shown that the microthruster can generate a thrust of similar to 60 mu N and an specific impulse of similar to 67 s with a power input of approximately 3 W. The thrust and specific impulse efficiencies, when compared to quasi-one-dimensional isentropic values, are determined to range between eta thrust similar to 12 and 40% and between eta ISP similar to 55 and 92%, respectively, for a power input of 0.2-3 W.

Thermochemical energy storage (TCES) technology offers a promising avenue for achieving prolonged thermal energy storage through reversible gas-solid reactions. In the present work, a scaling analysis approach is presented to derive the relevant length and time scales for heat charging and discharging processes occurring inside a porous reactor bed of a TCES system. The processes involved are vapour flow through the porous bed, coupled heat and mass transfer, and heat and mass absorption/release in conjunction with reaction kinetics, for the case of water vapour and potassium carbonate as the gas-solid pair. This analysis employs an order-of-magnitude approach on mass, momentum, and energy conservation equations. The scaling analysis of continuity and momentum equations derives the scale for critical vapour penetration depth. It signifies the length scale beyond which pressure drop becomes significant, leading to a significant reduction in the reaction rate. Similarly, a critical thermal diffusion length scale is derived from the scaling analysis of the energy equation. This scale signifies the length beyond which the reaction rate drops significantly due to limited thermal diffusion. The derived scales characterize the performance of the energy storage bed and help in determining bed dimensions for an effective flow of heat and water vapour through the bed.

A thin thermally insulating vapor layer beneath a levitating Leidenfrost droplet adversely affects the heat transfer from the underlying hot substrate during cooling applications. In this work, we present a theoretical model to determine the total heat transfer to a Leidenfrost droplet on microtextured substrates, taking into account the curved shape of the liquid-vapor interface. The profile of the vapor gap and vapor flow field beneath a Leidenfrost droplet on a microtextured surface is shown to be dependent on the substrate morphology. The shape of the liquid-vapor interface beneath the Leidenfrost droplet, in turn, influences the rate of evaporation on a microtextured substrate. We determine the variation of the minimum and maximum vapor gap over micro-pillared surfaces with different substrate permeability for various droplet volumes and wall superheat. The heat transferred to the droplet is via conduction across the vapor gap beneath the droplet and convection from the ambient air around the droplet. We determine the total evaporation time of a Leidenfrost droplet over a micro-textured substrate using the curved interface model and the conventional flat vapor gap interface model and compare with that obtained from experiments. The overprediction of the average rate of evaporation of a Leidenfrost droplet by the flat interface model ranges from-59 % for tall and sparse pillars (marked by high substrate permeability) to -29 % for short pillars (with low substrate permeability). We show that the average rate of evaporation of the LF droplet obtained using the curved interface model agrees reasonably (within 9%-23 %) with that observed in the experiments.

Interaction of a droplet on a heated substrate is a subject of extensive research due to its application in spray cooling of surfaces, thermal management of micro-scale devices, water harvesting etc. Depending on the surface temperature the droplet shows nucleate, transition boiling and Leidenfrost state. The presence of surface textures gives rise to various hydrodynamic outcomes such as droplet atomization, interfacial oscillations, and lift off. When a drop is deposited on a textured surface, it typically exists in two states called Cassie-Baxter and Wenzel depending on the substrate wettability and liquid-solid contact area. In the present work we explore a unique lift off mechanism at temperature range of (140-170 degrees C) attributed to the excessive vapor force generated due to droplet impalement into the gap between the textures followed by evaporation of the imbibed liquid. The presence of microtextures (solid fraction) offers flow resistance to the escaping vapor which leads to increase in vapor pressure underneath the drop. When the vapor force exceeds the pinning force and weight of the drop, an explosive lift off occurs. We develop a force-based model to estimate the droplet volume corresponding to lift off on substrates with different morphologies and temperatures.

Thermochemical energy storage systems (TESS) offer higher volumetric energy density compared to sensible and latent heat storage systems. TESS utilizes endothermic dissociation reaction for thermal energy storage (charging process), and the stored thermal energy is subsequently retrieved by performing a reversible exothermic reaction (discharging process). The reactor of the TESS forms a critical system component as endothermic-exothermic reactions take place in it. The present study focuses on the effect of various reactor configurations on the performance of a potassium carbonate salt hydrate based closed TESS, with the help of numerical studies. The cylindrical and flat type of reactor configurations are commonly used for a closed TESS. The cylindrical reactor configuration offers volumetric compactness but poses challenges such as limited water vapor flow area for penetration into the salt hydrate domain. It is observed that the provision of annular passage for water vapor flow and U-tubes for heat transfer fluid (HTF) flow significantly improve the performance of a cylindrical reactor during the discharging process. Around 69 % of the reactant is consumed by the end of 60 minutes of discharging process with the provision of annular passage for water vapor flow, as opposed to only about 10 % conversion without the provision of the annular passage. On the other hand, a flat reactor configuration provides a larger area for water vapor flow but with higher reactor volume requirement. To reduce the volume requirement, a reactor configuration with multiple stacks of smaller flat reactors is recommended. The addition of metallic fins to the flat reactor results in noticeable improvement. During the discharging process of a flat reactor without fins, complete conversion is achieved in 44 minutes. Whereas in flat reactor with HTF tubes through the salt hydrate domain, complete conversion during the discharging process is achieved within 26 minutes, which is 41 % reduction in the completion time.