Salt creeping during evaporative crystallization is generally associated with hydrophilic substrates, where capillary spreading of the saline solution promotes lateral crystal growth. In contrast, hydrophobic surfaces are typically expected to suppress such spreading. Here, we demonstrate the unexpected and sustained occurrence of salt creeping on both hydrophobic and superhydrophobic substrates during the evaporative crystallization of ternary saline droplets. To establish the generality of this phenomenon, multiple ternary systems are investigated, including water-NaCl-KCl, water-NaCl-KNO3, water-NaCl-NH4Cl, water-KCl-KNO3, water-KCl-NH4Cl, and water-KNO3-NH4Cl, over a broad substrate temperature range of 40-135 °C. Detailed experiments with water-NaCl-KCl mixtures on heated substrates reveal distinct creeping deposition patterns and crystal lifting, accompanied by systematic changes in contact angle, deposit height, and lateral footprint. Morphological analysis shows that ternary crystallization promotes elongated and hopper-type crystals that form interconnected porous networks rather than simple cubic structures. These networks act as efficient capillary pathways, sustaining liquid transport and enabling lateral redistribution of the solution, even on hydrophobic substrates. A local mass-balance between the capillary wicking flux and crystallization indicates that the extent of creeping is governed by the competition between these two processes. At elevated temperatures, this balance shifts, producing a transition from dominant creeping to a mixed regime of creeping and crystal lifting, revealing distinct temperature-dependent deposition behavior. Overall, this work proposes a generalized mechanism based on capillary-driven mass flux to explain salt creeping on hydrophobic substrates and highlights the critical role of crystal morphology in controlling liquid transport and deposit evolution during multicomponent evaporative crystallization.

The dynamics of successive solidification and melting in binary salt systems are governed by intricate interactions between thermal and solutal convection, mushy zone evolution, and density stratification. While prior studies have examined double-diffusive convection in salt-water systems, the successive pathways of solidification and melting-particularly the staged life cycle of double-diffusive layer (DDL) formation during melting-remain unexplored. In this study, the phase-change behavior of water-NH4Cl solutions at hypo-, eutectic-, and hyper-eutectic compositions is investigated using in situ digital single-lens reflex imaging, shadowgraph visualization, thermocouple measurements, concentration sampling, and validated numerical simulations with user-defined functions in ANSYS Fluent. Solidification in the hypo-eutectic regime is dominated by strong thermosolutal convection, leading to mushy zone growth, solute accumulation, and eventual bridging, while melting generates transient DDLs and wavy interfaces. In the eutectic regime, solidification proceeds in an orderly manner without mush formation, governed by stable thermal convection rolls and a prolonged isothermal plateau, whereas melting is rapid, highly convective, and marked by temperature oscillations and vigorous interface variation. The hyper-eutectic regime exhibits dendritic NH4Cl growth, dendrite fragmentation, and preferential eutectic front formation during solidification, while melting proceeds asymmetrically from eutectic-rich top regions, producing stratified DDLs that extend downward in a filling-box mechanism. The staged life cycle of DDL formation-onset, preferential top melting, layered stratification, and eventual merging-is established for the first time in binary melting. This work provides the first mechanistic framework for understanding DDL-driven successive solidification and melting in multicomponent phase-change systems, with implications for desalination, thermal energy storage, and alloy solidification.

Bubble formation in evaporating saline droplets at subboiling temperatures is a striking and nonintuitive phenomenon that challenges conventional understanding of phase change processes. In this study, the mechanisms of bubble formation are systematically investigated in ternary saline droplets (water + two salts) with different combinations of high- and low-latent heat of solid formation (LHSF) salts, which can potentially be extended to multicomponent systems encompassing all possible combinations. The results reveal that bubble formation is not governed solely by the LHSF but also critically by the rate at which this latent heat is released. Real-time imaging is employed to capture the onset of nucleation, crystal growth, and bubble evolution, while in situ thermocouple measurements provide direct evidence of transient thermal fluctuations. A one-dimensional energy balance at the solid-liquid interface demonstrates that the governing parameter is not LHSF alone but the combined influence of latent heat release and the kinetics of crystal growth. Bubble formation is consistently observed in droplets containing high-LHSF salts as well as in high-high and high-low LHSF combinations, the latter mainly due to the presence of high-LHSF salts. Remarkably, certain low-low LHSF systems also produce bubbles when crystallization proceeds at sufficiently high growth rates. These findings establish that bubble formation arises from the interplay between the thermodynamic magnitude (LHSF) and the kinetic rate (growth), thereby proposing a generalized mechanism for this unusual phenomenon.

Understanding water solidification and melting is crucial for optimizing water-based thermal energy storage systems and designing containers that accommodate ice expansion without structural damage. This study uses experiments and numerical simulations to investigate the shape of the solid-liquid interface, thermal history, velocity distribution, and solid fraction of water during successive solidification and melting processes. PIV (Particle image velocimetry), shadowgraph, and DSLR imaging are used to capture the flow pattern and solidliquid interface during the phase change process. Eight thermocouples were placed inside the cuboidal cell to collect temperature data, which was subsequently compared with results from Fluent simulations. Based on thermal data, the solidification process is divided into three distinct regimes: convective dominant, constant temperature, and solidifying. Flow behaviors are analyzed using the simulation velocity field, experimental velocity field, and thermal Rayleigh number. The simulation reported the maximum height of ice during expansion for different aspect ratios. The melting process revealed complex convective flow patterns, including side convection, Rayleigh-Benard convection near the bottom region, air convection in the upper sections, and ice toppling, causing temperature fluctuations in a zig-zag pattern through experiments. Numerical simulations showed similar trends to experimental results but differed in specific temperature values and behaviors, particularly during melting. The simulation indicated that melting occurs through convective flows, while experimental observations highlighted additional factors such as mixing, melting water, ice movement, and varying convective patterns. This research provides a comprehensive understanding of water's solidification and melting behavior using experiments and numerical simulations, which shows the role of convection.

Freezing-based desalination shows potential with lower energy needs and less environmental impact. However, current freezing methods produce ice with inconsistent salt levels, and new solutions are needed. This paper proposes a novel method inspired by Czochralski's (directional top-cooling solidification) techniques, where motion is introduced during solidification to achieve uniform low salinity in ice. Three distinct setups were employed: setup 1 utilized conventional top-cooling, while setups 2 and 3 incorporated directional top-cooling solidification akin to the Czochralski method. The conventional top-cooling approach often leads to nonuniform salinity distribution within ice structures due to complex convection phenomena, whereas directional top-cooling solidification ensures uniform salinity throughout the ice volume by allowing denser solutes to accumulate at the bottom of the tank. Experimental results demonstrate uniform salinity levels along the lengths of ice, measuring 0.34 +/- 0.04 wt % NaCl for 1.7 wt % NaCl solution and 0.7 +/- 0.05 wt % NaCl for 3.4 wt % NaCl solution. In multistaging desalination, this saline ice again melts and is further desalinized using the same leads to 0.1 +/- 0.05 wt % NaCl, which can be directly used for the drinking stage. Additionally, energy analysis indicates a similar to 5% reduction in consumption compared to conventional top-cooling methods, highlighting the technique's efficiency for sustainable desalination.

Water is essential for life, yet its scarcity is a growing concern due to population growth, climate change, and pollution. Freeze desalination can give salt-free ice, offering energy efficiency and reduced scaling compared to thermal desalination. This study investigates a top cooling solidification of the binary salt system for desalination. During the top freezing of saline solutions, influenced by thermal and solutal effects, transport phenomena contribute to reduced salinity in the resulting ice. A parametric study, varying initial liquid temperatures, top plate freezing temperatures, and initial solute concentrations, identifies optimal conditions for achieving low salinity and efficient ice formation. Thermal and solutal plumes, solute-rejecting channels, and the solidifying interface during top cooling solidification are observed using experiments (shadowgraph, Digital single-lens reflex (DSLR) imaging, and thermocouple measurement history) and two-dimensional numerical simulation. Real-time liquid salinity, ice salinity (measured after the experiment), and ice generation rate give the overall idea of freeze desalination. A close match between experimental and numerical trends of ice salinity with solid height fraction is obtained. Additionally, the thermal history of the bottom copper plate in a rectangular cavity highlights three distinct time regimes of mass and heat transfer within the bulk liquid. A notable temperature rise in thermal history, indicating restricted localized flow, occurs as the solid interface reaches the bottom plate.

Natural convection is a density-driven flow within the fluid in the presence of gravity. In thermally stable systems, i.e., those with a colder temperature at the bottom, natural convection is primarily driven by dissolved solutes. The phenomenon of freezing mixtures offers an ideal case for studying such flows. The presence of small-scale solid-phase structures further enhances the complexity. The present work shows the evolution and strengthening of natural convection and its effects, such as the fragmentation of growing solids while freezing binary alloys. Furthermore, these fragmented solids remelt in bulk due to composition and temperature differences. The work highlights fundamental aspects of an interplay between the compositional and thermal distributions of freezing mixtures. In the experiment, we used an SCN-15 wt.

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.

This study explores the peculiar phenomenon of bubble formation during evaporative crystallization at temperatures below boiling point, focusing on high latent heat solid-forming salts (Na2CO3, K2CO3, and Na2SO4) compared to lower latent heat solid-forming salts (NaCl, KNO3, and NH4Cl). Droplet evaporative crystallization experiments are conducted at temperatures ranging from 58 degrees C to 110 degrees C. The findings consistently show that salts with high latent heat of solid formation exhibit bubble nucleation and growth during crystallization. A plausible mechanism for this bubble formation is proposed, involving the interplay of thermal gradients and crystallization rates. Crystallization releases a significant amount of latent heat, leading to localized temperature increases near the crystal-liquid interface, often exceeding the boiling point. This can trigger bubble formation even if the substrate is below boiling. Energy balance at the interface during crystallization is used to interpret the local temperature rise at the crystal-liquid interface. The study also examines the influence of various parameters (initial concentration, substrate temperature, surface wettability, and different salts) on bubble nucleation and growth, emphasizing the critical role of latent heat release in these processes. This research offers valuable insights into the mechanisms of bubble formation in evaporative crystallization for the first time in the author's best knowledge.

Recent advancements in the field of material science and robotics have resulted in smart, adaptive, and intelligent systems for in-field applications. Conventional electromagnetism-based actuators contribute significantly to the size and weight of these systems, and hence, they are not suitable for mobile robots. Shape memory alloys (SMA) have emerged as better alternatives due to their unique characteristics, such as high force-to-weight ratio, noiseless operation, and muscle-like motion, with the potential to develop novel actuation for biomedical, space, and robotic applications. SMAs regain their shape at higher temperatures through the shape memory effect. This effect causes the alloy to transform its shape and then fully recover during phase transition. SMA actuators have been thoroughly examined for their potential integration into robotic hands, arms, and manipulators. However, relatively long cooling times to retransform from austenite to martensite state make SMAs unsuitable for fast and rapid cyclic applications. The current research aims to examine the effect of an evaporative (spray) cooling technique using acetone, methanol, and deionized water as cooling agents on the cooling time of SMA. Comparative studies are performed to study the effect of different coolants on a 1-DOF SMA coil actuator. Furthermore, a SMA-based rotary actuator has been developed, demonstrating the feasibility of implementing an acetone-based spray cooling technique. A control circuit is designed to regulate the spraying process over the SMA coils. This novel evaporative technique offers a significant improvement (154%) in the actuation frequency of the SMA-based actuation system compared to free convection. The findings underscore the potential of evaporative cooling methods to enhance the performance of SMA-based actuators, with implications for fast cyclic applications such as robotic systems.

Scaling or mineral fouling occurs due to the presence of dissolved minerals in water. Scaling is problematic in numerous industrial and household plumbing applications where water is used. The current methods of scale removal often utilize harsh chemicals that are not environmentally friendly. The evaporation of a saline droplet provides a platform to study the role of the substrate in the dynamics of crystallization during scaling. In the present work, we show out-of-plane growth of crystal deposits during the evaporation of saline droplets of aqueous potassium chloride on a heated smooth and microtextured hydrophobic substrate. These out-of-plane deposits, termed as "crystal legs", are in minimal contact with the substrate and can be easily removed from the substrate. The out-of-plane evaporative crystallization of saline droplets of different initial volumes and concentrations is observed irrespective of the chemistry of the hydrophobic coating and the crystal habits investigated. We attribute this general behavior of crystal legs to the growth and stacking of smaller crystals (size ∼10 μm) between the primary crystals toward the end of evaporation. We show that the rate at which the crystal legs grow increases with an increase in the substrate temperature. A mass conservation model is applied to predict the leg growth rate, which agrees well with the experiments.

In this work, the effect of solute expansion coefficient on the natural convection and freezing front propagation is investigated by performing three-side cooled solidification experiments. Four different aqueous salt solutions, and different compositions thereof, were employed for experimentation. The mixtures were solidified to analyze the effect of solute expansion coefficients on the convection currents and the composition distribution in the bulk. The initial compositions were chosen such that all cases have the same primary solid fraction at eutectic temperature, for obtaining similar compositional changes in the bulk liquid at various stages. Similar cooling conditions were also maintained to ensure that the variation in convection strength is primarily caused by different solute expansion coefficients. A distinct observation of the free surface freezing before the bulk, termed bridging, is reported in certain cases. Further analysis revealed that the bridging could be attributed to a difference in solute convection caused by the solute expansion coefficient. Numerical simulations were performed to further ascertain the plausible initiation mechanisms for bridging. The predicted compositional and solid fraction distribution revealed lesser solute accumulation near the surface, for the lower solute expansion cases, and the resulting increase in the tendency of freezing at the top. An upper limit for the ratio of solutal to thermal Rayleigh numbers in the experimental conditions has been identified for the occurrence of bridging in high Prandtl number fluids.

In the present work, we investigate the influence of substrate wettability and crystal morphology on the evaporative crystallization of saline droplets. On a superhydrophilic substrate, the evaporative crystals formed during the drying of a saline droplet of aqueous potassium nitrate are observed to be long and needle-shaped, oriented along the substrate. The crystal deposits form a flower-shaped pattern when the initial contact angle of the droplet increases to ∼72°. The orientation of the crystals along the triple contact line of the droplet controls the self-amplifying creeping growth of the salt crystals that eventually determines the overall evaporative patterns. The crystals change from being needle-shaped to globular salt deposits as the volume of liquid available for crystallization reduces. We demonstrate that the arrangement of the crystal with respect to the substrate and the droplet-air interface governs the rate of evaporation, growth, and morphology of the crystals.

Solidifying ternary systems can exhibit complex natural convection phenomena, particularly due to the presence of two porous zones (cotectic and primary mush), and the rejection of two differently dense solutes. The primary objectives of this study are to investigate the following: (i) the natural convection patterns in various compositional regimes of a typical ternary system, and (ii) the role of the combined existence of the microstructure (facets and dendrites) in the porous zone on natural convection, with a motivation to enhance the current understanding of the microstructure–convection relationships. A ternary mixture is chosen such that different compositions of the three primary solidifying components lead to the formation of distinct ice, dendritic and faceted solid structures that cover the complete span of microstructure–convection relationships. The observations of flow in different compositional regimes show convection occurring in the form of plumes, random mixing and double-diffusive layering, as well as combinations of these, which are governed by the type of coexisting microstructures. The study reveals the occurrence of Rayleigh–Taylor instability with varying amounts of the heavier component. The bulk liquid composition showed a tendency to cross the cotectic line, and thus also change the nature of primary solidifying structure from faceted to dendritic in cases where facets and dendrites were present in cotectic mush, and facets in primary mush. These insights are believed to elucidate the complex mechanisms of ternary solidification, as well as provide important real-time data for direct numerical simulations.

Adulteration of milk poses a severe health hazard, and it is crucial to develop adulterant-detection techniques that are scalable and easy to use. Water and urea are two of the most common adulterants in commercial milk. Detection of these adulterants is both challenging and costly in urban and rural areas. Here we report on an evaporation-based low-cost technique for the detection of added water and urea in milk. The evaporative deposition is shown to be affected by the presence of adulterants in milk. We observe a specific pattern formation of nonvolatile milk solids deposited at the end of the evaporation of a droplet of unadulterated milk. These patterns alter with the addition of water and urea. The evaporative deposits are dependent on the concentrations of water and urea added. The sensitivity of detection of urea in milk improves with the dilution of milk with water. We show that our method can be used to detect a urea concentration as low as 0.4% in milk. Based on the detection level of urea, we present a regime map that shows the concentration of urea that can be detected at different extents of dilution of milk.

In this study, identical experiments of bottom-cooled solidification fluidic mixtures that exhibit faceted and dendritic microstructures were performed. The strength of compositional convection, created due to the rejection of a lighter solute, was correlated with the solidifying microstructure morphology via separate Rayleigh numbers in the mushy and bulk-fluid zones. While the bulk fluid in dendritic solidification experienced a monotonic decrease in the temperature, solidification of the faceted case revealed an unconventional, anomalous temperature rise in the bulk liquid after the formation of a eutectic solid. Based on the bulk-liquid temperatures, three distinct regimes of heat transfer were observed in the liquid, namely, convection-dominated, transition and conduction-dominated. The observations were analysed and verified with the help of different initial compositions and cooling conditions, as well as other mixtures that form faceted morphology upon freezing. The observed temperature rise was further ascertained by performing an energy balance in an indicative control volume ahead of the solid-liquid interface. The plausible mechanism of permeability-driven flow causing a gain in the temperature of the liquid during freezing was generalized with the help of a semi-analytical investigation of a one-dimensional system comprising solid, porous mush and liquid regions. The analytical scaling relations for fluid velocity and vorticity, for the faceted and dentritic cases, revealed contrasting vorticity values, which are much larger in low permeability (faceted case) and cause enhanced mixing in the bulk. The study sheds new insights into the role of microstructural morphology in governing the transport phenomena in the bulk liquid.

Development and proposition of a numerical model to capture the shrinkage induced flow during directional solidification of a pure substance in a bottom cooled cavity are carried out. A novel numerical scheme involving fixed grid-based volume fraction updating is proposed to track the solid–liquid interface, considering the inclusion of the shrinkage effect. Directional solidification in bottom cooled orientation is of particular interest since shrinkage and buoyancy effects oppose each other. The results from the proposed numerical model indicated the existence of an unprecedented flow reversal phenomenon during the progression of the solidification process, caused by the opposing nature of shrinkage and buoyancy effects. The flow reversal phenomena predicted by the numerical model are validated by conducting experiments involving directional solidification of coconut oil in a bottom cooled cavity. Qualitative and quantitative measurements of the velocity field and interface growth are obtained using the particle image velocimetry technique and compared with three dimensional numerical results. Once the flow reversal phenomena are established through numerical and experimental evidences, case studies are performed, considering varying material properties, cold boundary temperatures, initial temperatures of the melt, and cavity heights to find the effect of each of these parameters on flow reversal phenomena. The parametric study also allowed us to check the robustness and consistency of the proposed model. The proposed model will serve as an important milestone toward the development of numerical models for capturing macro-scale shrinkage defects and prediction of composition heterogeneity during directional alloy solidification.